Internet Draft Lloyd McIntyre Internet Fax Working Group Stephen Zilles Expires in six months July 30, 1997 File Format for Internet Fax Status This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet Drafts as reference material or to cite them other than as a "work in progress." To learn the current status of any Internet-Draft, please check the "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract This Internet Draft describes the TIFF representation of the image data specified by the ITU-T Recommendations for black-and-white and color facsimile. The document provides a standard definition for TIFF-F (also known as TIFF Class F), which is used for a subset of black-and-white facsimile, and standard definitions for the TIFF representation of the ITU-T Recommendations for facsimile, including color facsimile. For the most part, existing TIFF constructs and fields are used; new TIFF fields are introduced as necessary. The document also describes the refinement of the registration of the MIME type image/tiff. Table of Contents 1. INTRODUCTION........................................................3 1.1. Scope..........................................................3 1.2. Approach.......................................................4 1.3. Organization of this draft.....................................4 2. TIFF and Fax........................................................4 2.1. TIFF Overview..................................................4 2.1.1. File Structure.............................................5 2.1.2. Image Structure............................................6 2.1.3. Recommended TIFF File Structure for Fax Applications.......6 2.2 TIFF Fields for Fax Applications................................7 2.2.1. TIFF Fields required for all fax modes.....................8 2.2.2. Additional TIFF Fields required for all fax modes..........9 2.2.3. TIFF Fields recommended for all fax modes..................9 McIntyre & Zilles Expires 1/30/98 [Page 1] Internet Draft File Format for Internet Fax July 30, 1997 2.2.4. New TIFF Fields recommended for fax modes......................10 3. Minimal Black-and-white Mode.......................................11 3.1. Overview......................................................12 3.2. Required TIFF Fields..........................................12 3.2.1 Baseline Fields............................................12 3.2.2 Extension Fields...........................................13 3.2.3 New Fields.................................................14 3.3. Recommended TIFF Fields.......................................14 3.4. End of Line Restrictions......................................14 3.5. Minimal Black-and-White Mode Summary..........................15 4. Extended Black-and-White mode......................................16 4.1. Overview......................................................16 4.2. Required TIFF Fields..........................................17 4.2.1. Baseline Fields...........................................17 4.2.2. Extension Fields..........................................18 4.2.3. New Fields................................................19 4.3. Recommended TIFF Fields.......................................19 4.3.1. Baseline Fields...........................................19 4.3.2. Extension Fields..........................................19 4.3.3. New Fields................................................19 4.4. TIFF Class F Definition.......................................19 4.5. Extended Black-and-White Fax Mode Summary.....................19 5. Base Color Fax Mode................................................22 5.1. Overview......................................................22 5.2. Required TIFF Fields..........................................22 5.2.1. Baseline Fields...........................................22 5.2.2. Extension Fields..........................................23 5.2.3. New Fields................................................24 5.3. Recommended TIFF Fields.......................................24 5.4. Base Color Fax Mode Summary...................................25 6. Lossless Color Mode................................................27 6.1. Overview......................................................27 6.1.1. Color Encoding............................................27 6.1.2. JBIG Encoding.............................................28 6.2. Required TIFF Fields..........................................28 6.2.1. Baseline Fields...........................................28 6.2.2. Extension Fields..........................................29 6.2.3. New Fields................................................29 6.3. Recommended TIFF Fields.......................................29 6.4. Lossless Color Fax Mode Summary...............................29 7. Mixed Raster Content Mode..........................................31 7.1 Overview.......................................................31 7.1.1. MRC 3-layer model.........................................32 7.1.2. A TIFF Representation for the MRC 3-layer model...........33 7.2. Required TIFF Fields..........................................34 7.2.1. Baseline Fields...........................................34 7.2.2. Extension Fields..........................................35 7.2.3. New Fields................................................36 7.3. Recommended TIFF Fields.......................................37 7.4. Rules and Requirements for Images.............................37 7.5. MRC Fax Mode Summary..........................................38 McIntyre & Zilles Expires 1/30/98 [Page 2] Internet Draft File Format for Internet Fax July 30, 1997 8. Mime Sub-Type......................................................41 9. IANA Registration..................................................41 10. Security Considerations...........................................42 11. References........................................................42 12. Authors' Addresses................................................44 Appendix A: Summary of TIFF Fields for Internet Fax ..................44 Appendix B: Use of TIFF-F for Black-and-White Fax.....................49 1. Introduction 1.1 Scope This document defines a TIFF-based file format specification and corresponding MIME image/tiff content type consistent with the charter of the IETF Internet Fax Working Group for enabling standardized messaging-based fax over the Internet. In particular, it describes the fields and parameters required to enable TIFF (Tag Image File Format) to represent the data content and the structure generated by the suite of ITU-T Recommendations for Group 3 facsimile. These standards, and the TIFF fields described here, support five facsimile modes: 1. minimal black-and-white mode, using binary compression [T.4] 2. extended black-and-white mode, using binary compression [T.6, T.82] 3. lossy color and grayscale mode, using JPEG compression [T.42, T.81] 4. lossless color and grayscale mode, using JBIG compression [T.43, T.82] 5. mixed raster content mode [MRC], using a combination of compression methods in 1-4 This specification defines a minimal interchange set of fields that will guarantee that, at least, binary black-and-white images will be supported. Implementations are required to support this minimal interchange set of fields. This specification builds on a previously proposed standard [RFC1314] and on work in progress [TIFF-F] for black-and-white fax to provide a standard definition of TIFF Class F. It extends that work to make it compatible with the relevant ITU-T Recommendations for color and grayscale facsimile. It also proposes a modification to the image/tiff content type that would accommodate the fax modes. Within its defined scope of specifying a file format for Internet Fax, this draft: 1. specifies TIFF structures for facsimile data, 2. defines ITU fax-compatible values for existing TIFF fields, 3. defines new TIFF fields and values for ITU-T compatibility. McIntyre & Zilles Expires 1/30/98 [Page 3] Internet Draft File Format for Internet Fax July 30, 1997 1.2 Approach This specification uses the current TIFF spec [TIFF] as the basis for describing color and grayscale facsimile modes. It takes advantage of TIFF features and structures that bridge the data formats and performance requirements of both legacy fax machines and host-based fax applications. A TIFF-based approach also builds on an established base of users and implementors and ensures backward compatibility with existing TIFF-based IETF proposals and work in progress for Internet fax. The basic approach to using TIFF for facsimile data is to insert the fax data stream in a TIFF file and to use TIFF fields to encode the parameters that describe the properties of the data. TIFF constructs for pages, images, and strips allow a TIFF file to preserve the fax data stream structure, and the performance advantages that come with it. 1.3 Organization of this draft Section 2 describes the general representation of fax data and parameters in a TIFF file, including the TIFF fields that used in all fax modes. Section 3 defines the representation of the minimal black- and-white facsimile mode required of all implementations. Section 4 defines the optional extended black-and-white fax capabilities, including TIFF-F. Section 5 defines the representation of lossy color and grayscale facsimile. Section 6 defines the representation of lossless color and grayscale facsimile and Section 7 defines the representation of mixed raster content facsimile. Sections 8 and 9 describe the MIME content type and IANA registration. The three remaining sections give Security Considerations, References and Authors' Addresses. Appendix A gives a summary of the TIFF fields used or defined in this document. Appendix B summarizes and describes the use of TIFF-F for storing black-and-white images from current implementations. This profile summarizes the work in progress [TIFF-F] for the minimal interchange black-and-white mode. To implement only the minimal interchange black-and-white set of fields and values, one need read only Sections 1, 2, 3, 8, 9, and 10. 2. TIFF and Fax 2.1. TIFF Overview This section is based on the current TIFF specification [TIFF] and selected TIFF Technical Notes [TTN1, TTN2]. Some features of TIFF are: - TIFF is capable of describing bilevel, grayscale, palette-color, and full-color image data in several color spaces. - TIFF includes a number of compression schemes that allow developers to choose the best space or time tradeoff for their applications. McIntyre & Zilles Expires 1/30/98 [Page 4] Internet Draft File Format for Internet Fax July 30, 1997 - TIFF is not tied to specific scanners, printers, or computer display hardware. - TIFF is portable. It does not favor particular operating systems, file systems, compilers, or processors. - TIFF is designed to be extensible-to evolve gracefully as new needs arise. - TIFF allows the inclusion of an unlimited amount of private or special-purpose information. This specification employs these features in using TIFF to represent black-and-white and color facsimile data. 2.1.1. File Structure TIFF is designed for scanned images, which makes it a good match for facsimile documents, which are multi-page scanned raster images. Each raster image consists of a number of rows or scanlines, each of which has the same number of pixels, the unit of sampling. Each pixel has at least one sample (exactly one for black-and-white images). A TIFF file begins with an 8-byte image file header that describes the byte order used within the file, includes a value that identifies the file as a TIFF file, and then gives an offset that points to the first image file directory (IFD). The IFD can be at any location in the file after the header but must begin on a word boundary. An IFD is a sequence of tagged fields, sorted in ascending order by tag value. An IFD consists of a 2-byte count of the number of fields, a sequence of field entries and a 4-byte offset to the next IFD. The fields contain information about the image and pointers to the image data. Each separate raster image in the file is represented by an IFD. Each IFD field entry has 12 bytes, consisting of a 2-byte tag, 2 bytes identifying the field type, 4 bytes giving the field count or number of values, and the 4-byte offset of the field value. If, based on the type and count, the value takes 4 bytes or less, then it replaces the offset in the field entry. Details are given in the TIFF specification [TIFF]. A TIFF file can contain more than one IFD, where each IFD is a subfile whose type is given in the NewSubfileType field. Multiple IFDs can be organized either as a linked list, with the last entry in each IFD pointing to the next IFD (the pointer in the last IFD is 0), or as a tree, using the SubIFDs field in the primary IFD [TTN1]. The SubIFDs field contains an array of pointers to child IFDs of the primary IFD. Child IFDs describe related images, such as reduced resolution versions of the primary IFD image. The same IFD can point both to a next IFD and to child IFDs, and child IFDs can themselves point to other IFDs. A Baseline TIFF reader is not required to read any IFDs beyond the first. McIntyre & Zilles Expires 1/30/98 [Page 5] Internet Draft File Format for Internet Fax July 30, 1997 All fax modes represent a multi-page fax image as a linked list of IFDs, with a NewSubfileType field containing a bit that identifies the IFD as one page of a multi-page document. Each IFD has a PageNumber field, identifying the page number, starting at 0 for the first page. The Mixed Raster Content fax mode, described in Section 7, uses child IFDs also. 2.1.2 Image Structure An IFD stores an image as one or more strips. A strip consists of 1 or more scanlines (rows) of raster image data in compressed form. An image may be stored in a single strip or may be divided into several strips, which would require less memory to buffer. (Baseline TIFF [TIFF] recommends about 8k bytes per strip, but existing FAX usage is typically one strip per image.) Each IFD requires the StripOffsets field, which is an array of pointers to the strip or strips that contain the actual image data. TIFF requires that each strip, except the last, contain the same number of scanlines, which is given in the RowsPerStrip field. This document introduces the new StripRowCounts field that allows a variable number of scanlines per strip, which is required by the Mixed Raster Content fax mode. Image data is stored as uninterpreted, compressed data streams within a strip. The formats of these streams follow the ITU-T Recommendations. The Compression field in the IFD indicates the type of compression, and other TIFF fields in the IFD describe image attributes, such as color encoding and spatial resolution. Compression parameters are stored in the compressed data stream, rather than in TIFF fields. This makes the TIFF representation and compressed data format specification independent of each another. This approach, modeled on [TTN2], allows TIFF to gracefully add new compression schemes as they become available. Some attributes can be specified both in the compressed data stream and within a TIFF field. It is possible that the two values will differ. When this happens for values required to interpret the data stream, then the values in the data stream take precedence. For informational values that are not required to interpret the data stream, such as author name, then the TIFF field value takes precedence. 2.1.3 Recommended TIFF File Structure for Fax Applications TIFF does not specify the ordering of IFDs, fields values and image data in a file. It is up to individual applications to require or recommend an ordering. In fax applications, it is recommended that the data be ordered in a TIFF file the same way it is ordered in a fax transmission. There would be an IFD for each page in a multi-page fax document, and the IFDs would occur in the same order in the file as the pages occur in the document. The IFD for page 1 would come first, the IFD for page 2 next, and so on. McIntyre & Zilles Expires 1/30/98 [Page 6] Internet Draft File Format for Internet Fax July 30, 1997 It is also recommended that the field values and image data of a given IFD occur in the file between the offset to the next IFD and the start of the next IFD. The fields values would come first, pointed to by the value offsets of the field entries in the IFD, followed by the image data, pointed to by the values of the StripOffsets field. If the image data in the IFD is stored in multiple strips, then the strips would occur in the file in the same order that the data they contain occurs in the facsimile transmission, starting at the top of the page. This specification introduces an optional new GlobalParametersIFD field, defined in Section 2.2.4. This field has type IFD and contains parameters describing the fax session. While it is often possible to obtain these parameters by scanning the file, it is convenient to make them available together in one place for fast and easy access. If the GlobalParametersIFD occurs in a TIFF file, it is recommended that it be located in the first IFD, immediately following the 8-byte image file header. 2.2 TIFF Fields for Fax Applications The TIFF specification is organized as a baseline and several extensions, including technical notes that will be incorporated in the next release of TIFF. The baseline and extensions have required and optional fields. Facsimile applications require (and recommend) a mixture of baseline and extensions fields, as well as new fields that are not part of the TIFF specification and that are defined in this document. Sections 3-7 for each fax mode have sub-sections for required and recommended fields; each sub-section organizes the fields according to whether they are baseline, extension or new fields. The fields required for facsimile have only a few legal values, specified in the ITU-T Recommendations. Of these legal values, some are required and some are optional, just as they are required (mandatory) or optional in fax implementations that conform to the ITU-T Recommendations. The required and optional values are noted in the sections on the different fax modes. This section describes the fields required or recommended by more than one fax mode. The pattern for the description of TIFF fields in this draft is FieldName(TagValueInDecimal) = values. TYPE WhetherRequiredByTIFForTIFFforFAX Count = (omitted if =1) = (if not in current spec but available) Explanation of the field, how it's used, and the values it can have. Default value, if any When a field’s default value is the desired value, that field may be omitted from the relevant IFD unless specifically required by the text of this specification. McIntyre & Zilles Expires 1/30/98 [Page 7] Internet Draft File Format for Internet Fax July 30, 1997 2.2.1. TIFF Fields required for all fax modes The TIFF fields listed in this section are required by all fax modes, but do not have mode-specific values. The next sub-section lists the required fields common to all fax modes and which do have mode-specific values. ImageLength(257). SHORT or LONG RequiredByTIFFBaseline Total number of scanlines in image. No default, must be specified. NewSubFileType(254) = (Bit 1=1). LONG RequiredByTIFFforFAX, RequiredByTIFFBaseline Bit 1 is 1 if the image is a single page of a multi-page document. Default = 0 (no subfile bits on) PageNumber(285) SHORT RequiredByTIFFforFAX, TIFFExtension Count = 2 The first number represents the page number (0 for the first page); the second number is the total number of pages in the document. If the second value is 0, then the total page count is not available. ResolutionUnit(296) = 2, 3. SHORT RequiredByTIFFBaseline The unit of measure for resolution. 2 = inch, 3 = centimeter Default = 2 (field may be omitted if this is the value) RowsPerStrip(278). SHORT or LONG RequiredByTIFFBaseline The number of scanlines per TIFF strip. For a single strip image, this is the same as the value of the ImageLength field. Default = 2**32 - 1 (meaning all scanlines in one strip) StripByteCounts(279). SHORT or LONG RequiredByTIFFBaseline Count = number of strips For each strip, the number of bytes in that strip after compression. No default, must be specified. StripOffsets(273). SHORT or LONG RequiredByTIFFBaseline Count = number of strips For each strip, the byte offset from the beginning of the file to that strip. No default, must be specified. McIntyre & Zilles Expires 1/30/98 [Page 8] Internet Draft File Format for Internet Fax July 30, 1997 2.2.2 Additional TIFF Fields required for all fax modes The TIFF fields listed in this section are required by all fax modes, but the values associated with them depend on the particular fax mode being described. Therefore, only the fields are defined here; the values applicable to a particular fax mode are described in Sections 3-7. BitsPerSample(258) SHORT RequiredByTIFFBaseline Number of bits per image sample Default = 1 (field may be omitted if this is the value) Compression(259) SHORT RequiredByTIFFBaseline Compression method used for strip data Default = 1 (no compression, so may not be omitted for FAX) ImageWidth(256) SHORT or LONG RequiredByTIFFBaseline The number of samples per scanline (row) of the image No default, must be specified. PhotometricInterpretation(262) SHORT RequiredByTIFFBaseline The color space of the image data No default, must be specified SamplesPerPixel(277) SHORT RequiredByTIFFBaseline The number of components per pixel Default =1 (field may be omitted if this is the value) XResolution(282) RATIONAL RequiredByTIFFBaseline The horizontal resolution of the image is expressed in pixels per resolution unit. No default, must be specified YResolution(283) RATIONAL RequiredByTIFFBaseline The vertical resolution of the image is expressed in pixels per resolution unit. No default, must be specified 2.2.3 TIFF Fields recommended for all fax modes The TIFF fields listed in this section are recommended, but not required, for use with all fax modes. Recommended fields that are mode- specific are described in Section 3-7. McIntyre & Zilles Expires 1/30/98 [Page 9] Internet Draft File Format for Internet Fax July 30, 1997 DateTime(306) ASCII OptionalInTIFFBaseline Date/time in the format "YYYY:MM:DD HH:MM:SS" in 24-hour format. No default. DocumentName(269) ASCII OptionalInTIFFExtension The name of the scanned document. This is a TIFF extension field, not a Baseline TIFF field. No default. ImageDescription(270) ASCII OptionalInTIFFBaseline A string describing the contents of the image. No default. Orientation(274) = 1-8 SHORT RequiredByTIFFBaseline 1: 0th row represents the visual top of the image; the 0th column represents the visual left side of the image. See the current TIFF specification [TIFF] for further values. Default = 1 (field may be omitted if this is the value) Software(305) ASCII OptionalInTIFFBaseline The optional name and release number of the software package that created the image. No default. 2.2.4 New TIFF Fields recommended for fax modes The new TIFF fields listed in this section are recommended for all fax modes, but their support is not expected for the minimal black-and-white fax mode described in Section 3. These fields describe "global" parameters of the fax session that created the image data. They are optional, not part of the current TIFF specification, and are defined in this document. The first new field, GlobalParametersIFD, is an IFD that contains global parameters and is located in a Primary IFD. GlobalParametersIFD(TagToBeAssigned). IFD An IFD containing global parameters. It is recommended that a TIFF writer place this field in the first IFD, where a TIFF reader would find it quickly. Each field in the GlobalParametersIFD is a TIFF field that is legal in any IFD. Required baseline fields should not be located in the GlobalParametersIFD, but should be in each image IFD. If a conflict exists between fields in the GlobalParametersIFD and the image IFDs, then the data in the image IFD shall prevail. McIntyre & Zilles Expires 1/30/98 [Page 10] Internet Draft File Format for Internet Fax July 30, 1997 Among the GlobalParametersIFD entries is a new ProfileType field which generally describes information in this IFD and in the TIFF file. ProfileType(TagToBeAssigned) LONG The type of image data stored in this IFD: 0 = Unspecified 1 = Group 3 fax Other values for further study The following new global fields are defined in this document as IFD entries for use with fax applications. ITUFaxMode(TagToBeAssigned) = 0 - 4. BYTE The currently defined values are: 0: does not conform to ITU-T Recommendations for facsimile, 1: minimal black & white lossless (ITU-T Rec. T.4), 2: extended black & white lossless (ITU-T Rec. T.6, T.85), 3: lossy color and grayscale (ITU-T Rec. T.42), 4: lossless color and grayscale (ITU-T Rec. T.43), 5: Mixed Raster Content (ITU-T Draft Rec. T.44). Notes: In a file that uses an ITU encoding with non-ITU parameters, e.g JPEG compression of CMYK data, ITUFaxMode is 0. CodingMethods(TagToBeAssigned). LONG This field indicates which coding methods are used in the file. A bit value of 1 indicates: Bit 0: unspecified compression, Bit 1: 1-dimensional coding, ITU-T Rec. T.4 (MH - Modified Huffman), Bit 2: 2-dimensional coding, ITU-T Rec. T.4 (MR - Modified Read), Bit 3: 2-dimensional coding, ITU-T Rec. T.6 (MMR - Modified MR), Bit 4: ITU-T Rec. T.82 coding, applying ITU-T Rec. T.85 (JBIG), Bit 5: ITU-T Rec. T.81 (Baseline JPEG), Bits 6-31: reserved for future use Notes: There is a limit of 32 coding methods. VersionYear(TagToBeAssigned) BYTE Count: 4 The year of the standard specified by the ITUFaxMode field, given as 4 characters, e.g. '1997'; used in lossy and lossless color modes. VersionNumber(TagToBeAssigned) BYTE The version of the standard specified by the ITUFaxMode field. A value of 0 indicates Version 1.0; used in Mixed Raster Content mode. 3. Minimal Black-and-White Fax Mode Black-and-white mode is the binary fax application most users are familiar with today. This mode is appropriate for black-and-white text and line art. Black-and-white mode is divided into two levels of capability. This section describes the minimal interchange set of TIFF McIntyre & Zilles Expires 1/30/98 [Page 11] Internet Draft File Format for Internet Fax July 30, 1997 fields that must be supported by all implementations in order to assure that some form of image, albeit black-and-white, can be interchanged. Section 4 describes extensions to the minimal interchange set of fields that provide a richer set of black-and-white capabilities. 3.1. Overview The minimal interchange portion of the black-and-white facsimile mode supports the original Group 3 fax resolutions, commonly called "standard" and "fine," and two compression methods: 1-dimensional Modified Huffman (MH) and 2-dimensional Modified READ (MR) lossless bilevel image coding [T.4]. (READ is an acronym for Relative Element Address Designate.) The minimal black-and-white mode corresponds to the original TIFF Class F specification [TIFF-F0]. 3.2. Required TIFF Fields Besides the fields listed in Section 2.2.1, the minimal black-and-white fax mode requires the following fields. The fields listed in Section 2.2.1 and the fields and fax-specific values specified in this sub- section must be supported by all implementations. 3.2.1 Baseline Fields BitsPerSample(258) = 1. SHORT RequiredByTIFFBaseline Binary data only. Default = 1 (field may be omitted if this is the value) Compression(259) = 3. SHORT RequiredByTIFFBaseline The value 3 is not a TIFF Baseline value [TIFF]; it is an extended value. The T4Options field must be specified and its value determines whether the data is encoded using the MH or MR encoding of [T.4]. FillOrder(266) = 1, 2. SHORT RequiredByTIFFBaseline 1 = Pixels are arranged within a byte such that pixels with lower column values are stored in the higher-order bits of the bytes, i.e., most significant bit first (MSB). 2 = Pixels are arranged within a byte such that pixels with lower column values are stored in the lower-order bits of the bytes, i.e., least significant bit first (LSB). The FillOrder field is used to describe how 1 bit pixels are stored within a byte of compressed image data. There are two possible FillOrders for each byte: 1) the pixels are stored with the lowest numbered pixel in the byte in the Most Significant Bit (MSB) of the byte, or 2) the pixels are stored with the lowest numbered pixel in the byte in the Least Significant Bit (LSB) of the byte. McIntyre & Zilles Expires 1/30/98 [Page 12] Internet Draft File Format for Internet Fax July 30, 1997 NOTE: Baseline TIFF readers are only required to support the first FillOrder where the lowest numbered pixel is stored in the MSB of the byte. However, because many devices, such as modems, transmit the LSB first when converting the data to serial form, it is common for black- and-white FAX products to use the second FillOrder where the lowest numbered pixel is stored in the LSB. ImageWidth(256) = 1728, 2048, 2432. SHORT or LONG RequiredByTIFFBaseline This mode supports the fixed page widths: 1728, 2048, 2432. No default, must be specified. PhotometricInterpretation(262) = 0, 1. SHORT RequiredByTIFFBaseline 0 = pixel value 1 means black, 1 = pixel value 1 means white No default, must be specified SamplesPerPixel(277) = 1. SHORT RequiredByTIFFBaseline The number of components per pixel; 1 for black-and-white Default =1 (field may be omitted if this is the value) XResolution(282). RATIONAL RequiredByTIFFBaseline The horizontal resolution of the image is expressed in pixels per resolution unit. In pixels/inch, the allowed value is 204. No default, must be specified YResolution(283). RATIONAL RequiredByTIFFBaseline The vertical resolution of the image is expressed in pixels per resolution unit. In pixels/inch, the allowed values are 98 and 196. No default, must be specified 3.2.2 Extension Fields T4Options(292) = (Bit 0 = 0 or 1, Bit 1 = 0, Bit 2 = 1) LONG RequiredTIFFExtension (when Compression = 3) Bit 0 = 1 indicates MR encoding, = 0 indicates MH encoding. Bit 1 must be 0 Bit 2 = 1 indicates that fill bits are required, = 0 no fill bits Default is all bits are 0 (therefore field must be specified) The T4Options field is required if the Compression field has a value of 3. The T4Options field specifies which encoding (MH or MR) is used and whether fill bits are required. Fill bits are required in the minimal interchange set. (See Section 3.4). McIntyre & Zilles Expires 1/30/98 [Page 13] Internet Draft File Format for Internet Fax July 30, 1997 3.2.3 New Fields None. 3.3 Recommended TIFF Fields There are three new optional fields for describing black & white fax page quality. They are not defined in the current TIFF specification [TIFF]. BadFaxLines(326). SHORT or LONG The number of "bad" scan lines encountered by the facsimile device during reception. A "bad" scanline is defined as a scanline that, when decoded, comprises an incorrect number of pixels. No default. CleanFaxData(327) = 0, 1, 2. SHORT Indicates if "bad" lines encountered during reception are stored in the data, or if "bad" lines have been replaced by the receiver. 0 = No "bad" lines 1 = "bad" lines exist, but were regenerated by the receiver, 2 = "bad" lines exist, but have not been regenerated. No default. NOTE: Many facsimile devices do not actually output bad lines. Instead, the previous good line is repeated in place of a bad line. Although this substitution, known as line regeneration, results in a visual improvement to the image, the data is nevertheless corrupted. The CleanFaxData field describes the error content of the data. That is, when the BadFaxLines and ImageLength fields indicate that the facsimile device encountered lines with an incorrect number of pixels during reception, the CleanFaxData field indicates whether these lines are actually in the data or if the receiving facsimile device replaced them with regenerated lines. ConsecutiveBadFaxLines(328). SHORT or LONG Maximum number of consecutive "bad" scanlines received. The BadFaxLines field indicates only the quantity of bad lines. The ConsecutiveBadFaxLines field is an indicator of the distribution of bad lines and may therefore be a better general indicator of perceived image quality. No Default. 3.4 End of Line Restrictions The minimal interchange set of fields and values for black-and-white fax requires all EOLs in compressed image data must be byte-aligned. An EOL is said to be byte-aligned when fill bits have been added as necessary before EOL codes such that EOL always ends on a byte boundary, thus ensuring an EOL-sequence of a one byte preceded by a zero nibble: xxxx0000 00000001. McIntyre & Zilles Expires 1/30/98 [Page 14] Internet Draft File Format for Internet Fax July 30, 1997 Recall that Huffman encoding encodes bits, not bytes. This means that the end-of-line token may end in the middle of a byte. In byte alignment, extra zero bits (Fill) are added so that the first bit of data following an EOL begins on a byte boundary. In effect, byte alignment relieves application software of the burden of bit-shifting every byte while parsing scan lines for line-oriented image manipulation (such as writing a TIFF file). 3.5 Minimal Black-and-White Fax Mode Summary The table below summarizes the TIFF fields that comprise the minimal interchange set for black-and-white facsimile. The Baseline and Extension fields must be supported by all implementations. For convenience in the table, certain fields which have a value that is a sequence of flag bits are shown taking integer values that correspond to the flags that are set. An implementation should test the setting of the relevant flag bits individually, however, to allow extensions to the sequence of flag bits to be appropriately ignored. (See, for example, T4Options below.) Fields suffixed with an asterisk, "*", are optional fields and need not be specified. +---------------------------+--------------------------------+ | Baseline Fields | Values | +---------------------------+--------------------------------+ | BitsPerSample | 1 | +---------------------------+--------------------------------+ | Compression | 3: 1D Modified Huffman coding | | | set T4Options = 4 | | | 3: 2D Modified Read coding | | | set T4Options = 5 | +---------------------------+--------------------------------+ | FillOrder | 1: most significant bit first | | | 2: least significant bit first | +---------------------------+--------------------------------+ | ImageWidth | 1728, 2048, 2432 | +---------------------------+--------------------------------+ | ImageLength | n: total number of scanlines | | | in image | +---------------------------+--------------------------------+ | NewSubFileType | 2: Bit 1 identifies single | | | page of a multi-page document | +---------------------------+--------------------------------+ | PageNumber | n,m: page number n followed by | | | total page count m | +---------------------------+--------------------------------+ | PhotometricInterpretation | 0: pixel value 1 means black | | | 1: pixel value 1 means white | +---------------------------+--------------------------------+ McIntyre & Zilles Expires 1/30/98 [Page 15] Internet Draft File Format for Internet Fax July 30, 1997 +---------------------------+--------------------------------+ | ResolutionUnit | 2: inch | | | 3: centimeter | +---------------------------+--------------------------------+ | RowsPerStrip | n: number of scanlines per | | | TIFF strip | +---------------------------+--------------------------------+ | SamplesPerPixel | 1 | +---------------------------+--------------------------------+ | StripByteCounts | : number or bytes in TIFF | | | strip | +---------------------------+--------------------------------+ | StripOffsets | : offset from beginning of | | | file to each TIFF strip | +---------------------------+--------------------------------+ | XResolution | 204 (pixels/inch) | +---------------------------+--------------------------------+ | YResolution | 98, 196 (pixels/inch) | +---------------------------+--------------------------------+ | Extension Fields | +---------------------------+--------------------------------+ | T4Options | 4: Modified Huffman, fill bits | | | required | | | 5: 2D Modified Read, fill bits | | | required | +---------------------------+--------------------------------+ | New Fields | +---------------------------+--------------------------------+ | BadFaxLines* | number of "bad" scanlines | | | encountered during reception | +---------------------------+--------------------------------+ | CleanFaxData* | 0: no "bad" lines | | | 1: "bad" lines exist, but were | | | regenerated by receiver | | | 2: "bad" lines exist, but have | | | not been regenerated | +---------------------------+--------------------------------+ | ConsecutiveBadFaxLines* | Max number of consecutive | | | "bad" lines received | +---------------------------+--------------------------------+ 4. Extended Black-and-white fax mode 4.1 Overview The previous section described the minimal interchange set of TIFF fields that must be supported by all implementations in order to assure that some form of image, albeit black-and-white, can be interchanged. McIntyre & Zilles Expires 1/30/98 [Page 16] Internet Draft File Format for Internet Fax July 30, 1997 This section describes extensions to the minimal interchange set of fields that provide a richer set of black-and-white capabilities. In addition to the MH and MR encodings, Modified Modified READ (MMR) as described in [T.6] is supported. Also available is the single- progression sequential mode of JBIG [T.82], used in accordance with the application rules given in ITU-T Rec. T.85 [T.85]. JBIG coding offers improved compression for halftoned originals. This section also provides a standard, updated definition of TIFF-F, also known as TIFF Class F,equivalent to that found in [TIFF-F]. It has been proposed to make [TIFF-F] an informational RFC. 4.2. Required TIFF Fields This section lists the required fields and the values they must have to be ITU-compatible. Besides the fields listed in Section 2.2.1, the extended black-and-white fax mode requires the following fields. 4.2.1. Baseline Fields BitsPerSample(258) = 1. SHORT RequiredByTIFFBaseline Binary data only. Default = 1 (field may be omitted if this is the value) Compression(259) = 3, 4, 9. SHORT RequiredByTIFFBaseline 3 = 1- or 2- dimensional coding, must have T4Options field This is a TIFF Extension value [TIFF]. 4 = 2-dimensional coding, ITU-T Rec. T.6 (MMR - Modified Modified Read, must have T6Options field)) This is a TIFF Extension value. 9 = ITU-T Rec. T.82 coding, applying ITU-T Rec. T.85 (JBIG) This is a TIFF Extension value. Default = 1 (and is not applicable; field must be specified) ImageWidth(256) SHORT or LONG RequiredByTIFFBaseline This mode supports the following fixed page widths: 1728, 2048, 2432, 2592, 3072, 3456, 3648, 4096, 4864. No default; must be specified PhotometricInterpretation(262) = 0, 1. SHORT RequiredByTIFFBaseline 0 = pixel value 1 means black 1 = pixel value 1 means white No default, must be specified SamplesPerPixel(277) = 1. SHORT RequiredByTIFFBaseline 1 = monochrome, bilevel in this case (see BitsPerSample) Default =1 (field may be omitted if this is the value) McIntyre & Zilles Expires 1/30/98 [Page 17] Internet Draft File Format for Internet Fax July 30, 1997 XResolution(282) RATIONAL RequiredByTIFFBaseline The horizontal resolution of the image is expressed in pixels per resolution unit. In pixels/inch, the allowed values are: 200, 204, 300, 400, and 406. No default, must be specified YResolution(283) RATIONAL RequiredByTIFFBaseline The vertical resolution of the image is expressed in pixels per resolution unit. In pixels/inch, the allowed values are: 98, 100, 196, 200, 300, 392, and 400 pixels/inch No default, must be specified NOTE: Not all combinations of XResolution, YResolution and ImageWidth are legal. The following table gives the legal combinations for inch- based resolutions and corresponding paper sizes [T.30]. +--------------+-----------------+---------------------------+ | XResolution x YResolution | ImageWidth | +--------------+-----------------+---------------------------+ | 200x100,200x200,200x400 | 1728 | 2048 | 2432 | +--------------+-----------------+---------------------------+ | 300 x 300 | 2592 | 3072 | 3648 | +--------------+-----------------+---------------------------+ | 400 x 400 | 3456 | 4096 | 4864 | +--------------+-----------------+---------------------------+ |Letter,A4| B4 | A3 | +---------+--------+--------+ | Paper Size | +---------------------------+ FillOrder(266) = 1, 2. SHORT See 3.2.1 above 4.2.2. Extension Fields T4Options(292) = (Bit 0 = 0 or 1, Bit 1 = 0, Bit 2 = 0 or 1) LONG RequiredTIFFExtension (when Compression = 3) T4Options was known as Group3Options in a prior version of [TIFF]. Bit 0 = 1 indicates MR encoding, = 0 indicates MH encoding. Bit 1 must be 0 Bit 2 = 1 indicates that fill bits are required, = 0 no fill bits Default is all bits are 0 (field must be specified for fax,see 3.2.2.) McIntyre & Zilles Expires 1/30/98 [Page 18] Internet Draft File Format for Internet Fax July 30, 1997 T6Options(293) = (Bit 0 = 0, Bit 1 = 0). LONG RequiredTIFFExtension (when Compression = 4) Used to indicate parameterization of 2D Modified Modified Read compression. Bit 0 must be 0. Bit 1 = 0 indicates uncompressed data mode is not allow; = 1 indicates uncompressed data is allowed (see [TIFF]). Default is all bits 0. For FAX, the field must be present and have the value 0. The use of uncompressed data where compression would expand the data size is not allowed for FAX. 4.2.3. New Fields None. 4.3. Recommended TIFF Fields 4.3.1. Baseline Fields See Section 2.2.3. 4.3.2. Extension Fields See Section 2.2.3. 4.3.3. New Fields See Sections 2.2.4 and 3.3. 4.4. TIFF-F Definition TIFF-F, also known as TIFF Class F, was originally defined in 1989 [TIFF-F0]. Recent discussions and work have proposed updating it to include MMR compression and higher resolutions, which the ITU-T has approved for use with Group 3 facsimile. If JBIG compression (Compression=9) and the global parameters are excluded, then the extended black-and-white mode described in this section corresponds to the most recent proposal for TIFF-F [TIFF-F]. Therefore, this section provides a standard definition of TIFF-F. 4.5. Extended Black-and-white Fax Mode Summary Recommended fields are shown with an asterisk *. Required fields or values are shown with a double asterisk **. If the double asterisk is on the field name, then all the listed values are required of implementations; if the double asterisks are in the Values column, then only the values suffixed with a double asterisk are required of implementations. McIntyre & Zilles Expires 1/30/98 [Page 19] Internet Draft File Format for Internet Fax July 30, 1997 +---------------------------+--------------------------------+ | Baseline Fields | Values | +---------------------------+--------------------------------+ | BitsPerSample | 1** | +---------------------------+--------------------------------+ | Compression | 3**: 1D Modified Huffman and | | | 2D Modified Read coding | | | 4: 2D Modified Modified Read | | | coding | | | 9: JBIG | +---------------------------+--------------------------------+ | DateTime* | {ASCII}: date/time in the | | | 24-hour format | | | "YYYY:MM:DD HH:MM:SS" | +---------------------------+--------------------------------+ | FillOrder** | 1: most significant bit first | | | 2: least significant bit first | +---------------------------+--------------------------------+ | ImageDescription* | {ASCII}: A string describing | | | the contents of the image. | +---------------------------+--------------------------------+ | ImageWidth | 1728**, 2048**, 2432**, 2592, | | | 3072, 3456, 3648, 4096, 4864 | +---------------------------+--------------------------------+ | ImageLength** | n: total number of scanlines | | | in image | +---------------------------+--------------------------------+ | NewSubFileType** | 2: Bit 1 identifies single | | | page of a multi-page document | +---------------------------+--------------------------------+ | Orientation* | 1-8, Default 1 | +---------------------------+--------------------------------+ | PhotometricInterpretation | 0: pixel value 1 means black | | ** | 1: pixel value 1 means white | +---------------------------+--------------------------------+ | ResolutionUnit** | 2: inch | | | 3: centimeter | +---------------------------+--------------------------------+ | RowsPerStrip** | n: number of scanlines per | | | TIFF strip | +---------------------------+--------------------------------+ | SamplesPerPixel** | 1 | +---------------------------+--------------------------------+ | Software* | {ASCII}: name & release | | | number of creator software | +---------------------------+--------------------------------+ | StripByteCounts** | : number or bytes in TIFF | | | strip | +---------------------------+--------------------------------+ | StripOffsets** | : offset from beginning of | | | file to each TIFF strip | +---------------------------+--------------------------------+ McIntyre & Zilles Expires 1/30/98 [Page 20] Internet Draft File Format for Internet Fax July 30, 1997 +---------------------------+--------------------------------+ | XResolution | 200, 204**, 300, 400, 406 | | | (written in pixels/inch) | +---------------------------+--------------------------------+ | YResolution | 98**, 196**, 100, | | | 200, 300, 392, 400 | | | (written in pixels/inch) | +---------------------------+--------------------------------+ | Extension Fields | +---------------------------+--------------------------------+ | T4Options | 0: required if Compression is | | | Modified Huffman, no fill bits | | | 1: required if Compression is | | | 2D Modified Read, no fill bits | | | 4**: required if Compression | | | is Modified Huffman, fill bits | | | 5**: required if Compression | | | is 2D Modified Read, fill bits | +---------------------------+--------------------------------+ | T6Options | 0: required if Compression is | | | 2D Modified Modified Read | +---------------------------+--------------------------------+ | DocumentName* | {ASCII}: name of scanned | | | document | +---------------------------+--------------------------------+ | PageNumber** | n,m: page number followed by | | | total page count | +---------------------------+--------------------------------+ | New Fields | +---------------------------+--------------------------------+ | BadFaxLines* | number of "bad" scanlines | | | encountered during reception | +---------------------------+--------------------------------+ | CleanFaxData* | 0: no "bad" lines | | | 1: "bad" lines exist, but were | | | regenerated by receiver | | | 2: "bad" lines exist, but have | | | not been regenerated | +---------------------------+--------------------------------+ | ConsecutiveBadFaxLines* | Max number of consecutive | | | "bad" lines received | +---------------------------+--------------------------------+ | GlobalParametersIFD* | : global parameters IFD | +---------------------------+--------------------------------+ | ProfileType* | n: type of data stored in | | | TIFF file | +---------------------------+--------------------------------+ | ITUFaxMode* | n: ITU-compatible fax mode | +---------------------------+--------------------------------+ | CodingMethods* | n: compression algorithms used | | | in file | +---------------------------+--------------------------------+ McIntyre & Zilles Expires 1/30/98 [Page 21] Internet Draft File Format for Internet Fax July 30, 1997 5. Base Color Fax Mode 5.1. Overview This is the base mode for color and grayscale facsimile, which means that all applications that support color fax must support this mode. The basic approach is the lossy JPEG compression [T.81] of L*a*b* color data [T.42]. Grayscale applications use the L* lightness component; color applications use the L*, a* and b* components. This mode uses a new PhotometricInterpretation field value to describe the L*a*b* encoding specified in [T.42]. This encoding differs in two ways from the other L*a*b* encodings used in TIFF [TIFF, TTN1]: it specifies a different default range for the a* and b* components, based on a comprehensive evaluation of existing hardcopy output, and it optionally allows selectable range for the L*, a* and b* components. 5.2. Required TIFF Fields This section lists the required fields, in addition to those given in Section 2.2.1, and the values they must support to be compatible with ITU-T Rec. T.42 and Annex E in ITU-T Rec. T.4. 5.2.1. Baseline Fields ImageWidth(256). SHORT or LONG This mode supports the following fixed page widths: 864, 1024, 1216, 1728, 2048, 2432, 2592, 3072, 3456, 3648, 4096, 4864. BitsPerSample(258) = 8, 12. SHORT Count = SamplesPerPixel The base color fax mode requires 8 bits per sample, with 12 as an option. 12 bits per sample is not baseline TIFF. Compression(259) = 7. SHORT Base color fax mode uses Baseline JPEG compression. Value 7 represents JPEG compression as specified in [TTN2]. PhotometricInterpretation(262) = 10. SHORT Base color fax mode requires pixel values to be stored using the CIE L*a*b* encoding defined in ITU-T Rec. T.42. This encoding is indicated by the PhotometricInterpretation value 10, referred to as ITULAB. With this encoding, the minimum sample value is mapped to 0 and the maximum sample value is mapped to (2^n - 1), i.e. the maximum value, where n is the BitsPerSample value. The conversion from unsigned ITULAB-encoded samples values to signed CIE L*a*b* values is determined by the Decode field; see Sec. 5.2.2 Note: PhotometricInterpretation values 8 and 9 specify encodings for CIE L*a*b* [TIFF] and ICC L*a*b* [TTN1], but they are fixed encodings, which use different minimum and maximum samples than the T.42 default encoding. As currently defined, they are not able to represent fax-encoded L*a*b* data. McIntyre & Zilles Expires 1/30/98 [Page 22] Internet Draft File Format for Internet Fax July 30, 1997 SamplesPerPixel(277) = 1, 3. SHORT 1: L* component only 3: L*, a*, b* components Encoded according to PhotometricInterpretation field XResolution(282). RATIONAL YResolution(283). RATIONAL The resolution of the image is expressed in pixels per resolution unit. In pixels per inch, allowed XResolution values are: 100, 200, 300, and 400. The base color fax mode requires the pixels to be square, hence YResolution must equal XResolution. 5.2.2 Extension Fields The JPEG compression standard allows for the a*b* chroma components of an image to be subsampled relative to the L* lightness component. The extension fields ChromaSubSampling and ChromaPositioning define the subsampling. They are the same as YCbCrSubSampling and YCbCrPositioning, which have been renamed to reflect their applicability to other color spaces. ChromaSubSampling(530). SHORT Count = 2 Specifies the subsampling factors for the chroma components of a L*a*b* image. The two subfields of this field, ChromaSubsampleHoriz and ChromaSubsampleVert, specify the horizontal and vertical subsampling factors respectively. SHORT 0: ChromaSubsampleHoriz = 1, 2, 4. 1: equal numbers of lightness and chroma samples horizontally, 2: twice as many lightness samples as chroma samples horizontally, 4: four times as many lightness samples as chroma samples horizontally. SHORT 1: ChromaSubsampleVert = 1, 2, 4. 1: equal numbers of lightness and chroma samples vertically, 2: twice as many lightness samples as chroma samples vertically, 4: four times as many lightness samples as chroma samples vertically. The default value for ChromaSubSampling is (2,2), which is the default for chroma subsampling in color fax [T.42]. T.42 supports no chroma subsampling, i.e. ChromaSubSampling = (1,1), as an option. ChromaPositioning(531) = 1, 2. SHORT Specifies the spatial positioning of chroma components relative to the lightness component. See the current TIFF specification under YCbCrPositioning for further information. 1: centered, 2: cosited. Default = 1, which is what ITU-T T.4, Annex E specifies. McIntyre & Zilles Expires 1/30/98 [Page 23] Internet Draft File Format for Internet Fax July 30, 1997 Decode(TagToBeAssigned). SHORT Count = 2 * SamplesPerPixel Describes how to map image sample values into the range of values appropriate for the current color space. In general, the values are taken in pairs and specify the minimum and maximum output value for each color component. For the base color fax mode, Decode maps from the ITULAB encoding to L*a*b* and has a count of 6 values. In this case, the input is an unsigned ITULAB-encoded value and the output is a signed L*a*b* value. L* = Decode[0] + Lsample x (Decode[1]-Decode[0])/(2^n -1) a* = Decode[2] + asample x (Decode[3]-Decode[2])/(2^n -1) b* = Decode[4] + bsample x (Decode[5]-Decode[4])/(2^n -1) where Decode[0] is the minimum value for L*, Decode[1] is the maximum value, etc. and n is the BitsPerSample field value, either 8 or 12. ITU-T Rec. T.42 specifies the ITULAB encoding in terms of a range and offset for each component, which are related to the Decode field values as follows: Decode[0] = - (Lrange x Loffset) / 2^n Decode[1] = Decode[0] + Lrange NOTE: The following table gives the default range and offsets, as specified in ITU-T Rec. T.42, and the corresponding minimum and maximum L*a*b* component and ITULAB encoded values. +-----------------------------------------------------------+ | Component | Rec. T.42 | L*a*b* | ITULAB | | | defaults | values | encoding | +-----------+------------------+------------+---------------+ | | Range | Offset | Min | Max | Min | Max | +-----------+--------+---------+------+-----+-----+---------+ | L* | 100 | 0 | 0 | 100 | 0 | 2^n - 1 | +-----------+--------+---------+------+-----+-----+---------+ | a* | 170 | 128 | -85 | 85 | 0 | 2^n - 1 | +-----------+--------+---------+------+-----+-----+---------+ | b* | 200 | 96 | -75 | 125 | 0 | 2^n - 1 | +-----------+--------+---------+------+-----+-----+---------+ The default value for Decode is (0, 100, -85, 85, -75, 125), which is based on the ITULAB encoding, with PhotometricInterpretation 10, SamplesPerPixel 3, and BitsPerSample 8. 5.2.3. New Fields None. 5.3. Recommended TIFF Fields See Sections 2.2.3. and 2.2.4. McIntyre & Zilles Expires 1/30/98 [Page 24] Internet Draft File Format for Internet Fax July 30, 1997 5.4 Base Color Fax Mode Summary Recommended fields are shown with an asterisk * +---------------------------+--------------------------------+ | Baseline Fields | Values | +---------------------------+--------------------------------+ | BitsPerSample | 8: 8 bits per color sample | | | 12: optional 12 bits/sample | +---------------------------+--------------------------------+ | Compression | 7: JPEG | +---------------------------+--------------------------------+ | DateTime* | {ASCII}: date/time in the | | | 24-hour format | | | "YYYY:MM:DD HH:MM:SS" | +---------------------------+--------------------------------+ | ImageDescription* | {ASCII}: A string describing | | | the contents of the image. | +---------------------------+--------------------------------+ | ImageWidth | 864, 1024, 1216, 1728, 2048, | | | 2432, 2592, 3072, 3456, 3648, | | | 4096, 4864 | +---------------------------+--------------------------------+ | ImageLength | n: total number of scanlines | | | in image | +---------------------------+--------------------------------+ | NewSubFileType | 2: Bit 1 identifies single page| | | of a multi-page document | +---------------------------+--------------------------------+ | Orientation* | 1-8, Default 1 | +---------------------------+--------------------------------+ | PhotometricInterpretation | 10: ITULAB | +---------------------------+--------------------------------+ | ResolutionUnit | 2: inch | | | 3: centimeter | +---------------------------+--------------------------------+ | RowsPerStrip | n: number of scanlines per | | | TIFF strip | +---------------------------+--------------------------------+ | SamplesPerPixel | 1: L* (lightness) | | | 3: LAB | +---------------------------+--------------------------------+ | Software* | {ASCII}: name & release number | | | of creator software | +---------------------------+--------------------------------+ | StripByteCounts | : number or bytes in | | | TIFF strip | +---------------------------+--------------------------------+ | StripOffsets | : offset from beginning | | | of file to each TIFF strip | +---------------------------+--------------------------------+ McIntyre & Zilles Expires 1/30/98 [Page 25] Internet Draft File Format for Internet Fax July 30, 1997 +---------------------------+--------------------------------+ | XResolution | 100, 200, 300, 400 (written | | | in pixels/inch) | +---------------------------+--------------------------------+ | YResolution | equal to XResolution (pixels | | | must be square) | +---------------------------+--------------------------------+ | Extension Fields | +---------------------------+--------------------------------+ | DocumentName* | {ASCII}: name of scanned | | | document | +---------------------------+--------------------------------+ | PageNumber | n,m: page number followed by | | | total page count | +---------------------------+--------------------------------+ | Decode | minL, maxL, mina, maxa, minb, | | | maxb: minimum and maximum | | | values for L*a*b* | +---------------------------+--------------------------------+ | ChromaSubSampling | ChromaSubsampleHoriz: | | | 1: equal numbers of lightness | | | and chroma samples | | | 2: twice as many lightness | | | samples as chroma samples | | | 4: four times as many | | | lightness samples as chroma | | | samples | | | ChromaSubsampleVert: | | | 1: equal numbers of lightness | | | and chroma samples | | | 2: twice as many lightness | | | samples as chroma samples | | | 4: four times as many lightness| | | samples as chroma samples | +---------------------------+--------------------------------+ | ChromaPositioning | 1: centered | | | 2: cosited | +---------------------------+--------------------------------+ | New Fields | +---------------------------+--------------------------------+ | GlobalParametersIFD* | : IFD containing | | | global parameters | +---------------------------+--------------------------------+ | ProfileType* | n: type of data stored in | | | TIFF file | +---------------------------+--------------------------------+ | ITUFaxMode* | n: ITU-compatible fax mode | +---------------------------+--------------------------------+ | CodingMethods* | n: compression algorithms | | | used in file | +---------------------------+--------------------------------+ McIntyre & Zilles Expires 1/30/98 [Page 26] Internet Draft File Format for Internet Fax July 30, 1997 +---------------------------+--------------------------------+ | VersionNumber* | n: version of ITU fax standard | +---------------------------+--------------------------------+ | VersionYear* | byte sequence with year of | | | ITU fax standard | +---------------------------+--------------------------------+ 6. Lossless Color Mode 6.1. Overview This mode, defined in [T.43], uses JBIG to losslessly code three types of color and grayscale images: one bit per color CMY, CMYK and RGB images; a palettized (i.e. mapped) color image; and continuous tone color and grayscale images. The last two are multi-level and use the L*a*b* encoding specified in [T.42]. 6.1.1. Color Encoding While under development, this mode was called T.Palette, as one of its major additions was palette or mapped color images. Baseline TIFF only allows RGB color maps, but ITU-T Rec. T.43 requires L*a*b* color maps, using the encoding specified in ITU-T Rec. T.42. Palette color images are expressed with indices (bits per sample) of 12 bits or less, or optionally 13 to 16 bits. Enabling T.43 color maps in TIFF requires the extension field Indexed, defined in [TTN1], and the PhotometricInterpretation field value 10, defined in Section 5.2.1. The following table shows how T.43 image data is interpreted, using the PhotometricInterpretation, SamplesPerPixel, BitsPerSample and Indexed fields. +----------------------------------------------------------+ | Image Type | PhotoInter- | Samples | Bits Per | Indexed | | | pretation | PerPixel | Sample | | |------------+-------------+----------+----------+---------| | RGB | 2=RGB | 3 | 1 | 0 | +----------------------------------------------------------+ | CMY | 5=CMYK | 3 | 1 | 0 | +------------+-------------+----------+----------+---------+ | CMYK | 5=CMYK | 4 | 1 | 0 | +------------+-------------+----------+----------+---------+ | Palette | 10=ITULAB | 1 | n | 1 | +------------+-------------+----------+----------+---------+ | Grayscale | 10=ITULAB | 1 | 8, 12 | 0 | +------------+-------------+----------+----------+---------+ | Color | 10=ITULAB | 3 | 8, 12 | 0 | +------------+-------------+----------+----------+---------+ McIntyre & Zilles Expires 1/30/98 [Page 27] Internet Draft File Format for Internet Fax July 30, 1997 6.1.2. JBIG Encoding T.43 uses the single-progression sequential mode of JBIG, defined in ITU-T Rec. T.82, in accordance with the application rules described in ITU-T Rec. T.85. To code multi-level images using JBIG, which is a bi-level compression method, an image is resolved into a set of bit-planes using Gray code conversion, and each bit-plane is then JBIG compressed. The Gray code conversion is part of the data stream encoding, and is therefore invisible to TIFF. 6.2. Required TIFF Fields This section lists the required fields, in addition to those in Section 2.2.1, and the values they must have to be compatible with ITU-T Rec. T.43. 6.2.1. Baseline Fields ImageWidth(256). SHORT or LONG Same page widths as the base color mode; see Section 5.2.1. BitsPerSample(258) = 1, 8, 9-16. SHORT Count = SamplesPerPixel RGB, CMY, CMYK: 1 bit per sample Continuous tone (L*a*b*): 8 bits per sample, 12 bits optional Palette color: 12 or fewer bits per sample, 13-16 bits optional Note: More than 8 bits per sample is not baseline TIFF. ColorMap(320). SHORT Count = 3 * (2**BitsPerSample) Lossless color fax mode supports palette-color (indexed) images where the single component value is used as an index into a full color lookup table stored in the ColorMap field. With lossless color fax mode, only the ITULAB encoding with 8 bits per sample is supported for palette-color images. To utilize a color map, the TIFF Indexed field must be present. TIFF orders the color map values so that all the L* values come first, followed by all the a* values and then all the b* values. Because ITU-T Rec. T.43 specifies a "chunky" ordering with the L*a*b*components of the first value, followed by those of the second value, and so on, transferring color map values between a TIFF file and fax data stream requires reordering values. Compression(259) = 9. SHORT 9: ITU-T Rec. T.82 coding, applying ITU-T Rec. T.85 (JBIG) FillOrder(266) = 1, 2. SHORT This field is only relevant for 1 bit per sample color (RGB, CMY, CMYK); see Section 3.2.1 for further information. PhotometricInterpretation(262) = 2, 5, 10. SHORT 2: RGB 5: CMYK, including CMY McIntyre & Zilles Expires 1/30/98 [Page 28] Internet Draft File Format for Internet Fax July 30, 1997 10: ITULAB Image data may also be stored as palette color images, where pixel values are represented by a single component that is an index into a color map using the ITULAB encoding. This color map is specified by the ColorMap field. To use palette color images, set the PhotometricInterpretation to 10,SamplesPerPixel to 1, and Indexed to 1. The color map is stored in the ColorMap field. See Section 6.1.1 for further discussion on the color encoding. SamplesPerPixel(277) = 1, 3, 4. SHORT 1: Palette color image, or L*-only if Indexed = 0 and PhotometricInterpretation is 10 (ITULAB). 3: RGB, or L*a*b*, or CMY if PhotometricInterpretation is 5 (CMYK). 4: CMYK. XResolution(282). RATIONAL YResolution(283). RATIONAL The resolution of the image is expressed in pixels per resolution unit. In pixels per inch, allowed XResolution values are: 100, 200, 300, and 400. The lossless color fax mode requires the pixels to be square, hence YResolution must equal XResolution. 6.2.2. Extension Fields Indexed(364) = 0, 1. SHORT 0: not a palette-color image 1: palette-color image This field is used to indicate that the sample values are an index into an array of color values specified in the ColorMap field. Lossless color fax mode supports palette-color images with the ITULAB encoding. The SamplesPerPixel value must be 1. Decode(TagToBeAssigned) SHORT Decode is used in connection with the ITULAB encoding of image data; see Section 5.2.2. 6.2.3. New Fields None. 6.3. Recommended TIFF Fields See Sections 2.2.3. and 2.2.4. 6.4. Lossless Color Fax Mode Summary Recommended fields are shown with an asterisk * McIntyre & Zilles Expires 1/30/98 [Page 29] Internet Draft File Format for Internet Fax July 30, 1997 +--------------------|--------------------------------------+ | Baseline Fields | Values | +--------------------+--------------------------------------+ | BitsPerSample | 1: Binary RGB, CMY(K) | | | 8: 8 bits per color sample | | | 9-16: optional | +--------------------+--------------------------------------+ | ColorMap | n: LAB color map | +--------------------+--------------------------------------+ | Compression | 9: JBIG | +--------------------+--------------------------------------+ | DateTime* | {ASCII}: date/time in the 24-hour | | | format "YYYY:MM:DD HH:MM:SS" | +--------------------+--------------------------------------+ | FillOrder | Applies only to 1 bit/sample | | | encodings | | | 1: Most significant bit first | | | 2: Least significant bit first | +--------------------+--------------------------------------+ | ImageDescription* | {ASCII}: A string describing the | | | contents of the image. | +--------------------+--------------------------------------+ | ImageWidth | 864, 1024, 1216, 1728, 2048, 2432, | | | 2592, 3072, 3456, 3648, 4096, 4864 | +--------------------+--------------------------------------+ | ImageLength | n: total number of scanlines in image| +--------------------+--------------------------------------+ | NewSubFileType | 2: Bit 1 identifies single page of a | | | multi-page document | +--------------------+--------------------------------------+ | Orientation* | 1-8, Default 1 | +--------------------+--------------------------------------+ | PhotometricInter- | 2: RGB | | pretation | 5: CMYK | | | 10: ITULAB | +--------------------+--------------------------------------+ | ResolutionUnit | 2: inch | | | 3: centimeter | +--------------------+--------------------------------------+ | RowsPerStrip | n: number of scanlines per TIFF strip| +--------------------+--------------------------------------+ | SamplesPerPixel | 1: L* (lightness) | | | 3: LAB, RGB, CMY | | | 4: CMYK | +--------------------+--------------------------------------+ | Software* | {ASCII}: name & release number of | | | creator software | +--------------------+--------------------------------------+ | StripByteCounts | : number or bytes in TIFF strip | +--------------------+--------------------------------------+ | StripOffsets | : offset from beginning of file to| | | each TIFF strip | +--------------------+--------------------------------------+ McIntyre & Zilles Expires 1/30/98 [Page 30] Internet Draft File Format for Internet Fax July 30, 1997 +--------------------+--------------------------------------+ | XResolution | 100, 200, 300, 400 (written in | | | pixels/inch) | | YResolution | equal to XResolution (pixels must be | | | square) | +--------------------+--------------------------------------+ | Extension Fields | +--------------------+--------------------------------------+ | Decode | minL, maxL, mina, maxa, minb, maxb: | | |minimum and maximum values for L*a*b* | +--------------------+--------------------------------------+ | DocumentName* | {ASCII}: name of scanned document | +--------------------+--------------------------------------+ | PageNumber | n,m: page number followed by total | | | page count | +--------------------+--------------------------------------+ | Indexed | 0: not a palette-color image | | | 1: palette-color image | +--------------------+--------------------------------------+ | New Fields | +--------------------+--------------------------------------| | GlobalParameters | : global parameters IFD | | IFD* | | +--------------------+--------------------------------------+ | ProfileType* | n: type of data stored in TIFF file | +--------------------+--------------------------------------+ | ITUFaxMode* | n: ITU-compatible fax mode | +--------------------+--------------------------------------+ | CodingMethods* | n:compression algorithms used in | | | file | +--------------------+--------------------------------------+ | VersionNumber* | n: version of ITU fax standard | +--------------------+--------------------------------------+ | VersionYear* | byte sequence with year of ITU fax | | | standard | +--------------------+--------------------------------------+ 7. Mixed Raster Content Mode 7.1. Overview Unlike previous fax modes, which use a single coding method and spatial resolution for an entire fax page, the Mixed Raster Content mode [MRC] enables different coding methods and resolutions within a single page. For example, consider a page that contains black-and-white text, which is best coded with MMR or JBIG, a color bar chart, best coded with JBIG, and a scanned color image, best coded with JPEG. Similarly, while spatial resolution of 400 pixels per inch may be best for the black-and- white text, 200 pixel per inch is usually sufficient for a color image. McIntyre & Zilles Expires 1/30/98 [Page 31] Internet Draft File Format for Internet Fax July 30, 1997 Rather than applying one coding method and resolution to all elements, MRC allows multiple coders and resolutions within a page. By itself, MRC does not define any new coding methods or resolutions. Instead it defines a 3-layer model for structuring and combining the scanned image data. The ITU MRC document is currently a draft. However, its technical content is stable, and editorial changes only are permitted between now and its scheduled approval in October 1997. Upon approval, the MRC document will become ITU-T Recommendation T.44. 7.1.1. MRC 3-layer model The 3 layers of the MRC model are Foreground and Background, which are both multi-level, and Mask, which is bi-level. Each layer may appear only once on a page and is coded independently of the other two. In our earlier example, the black-and-white text could be in the Mask layer, the color chart in the Foreground layer, and the color image in the Background layer. The distribution of content among layers is determined by the writer, as is the choice of compression method, color encoding and spatial resolution for a layer. The final image is obtained by using the Mask layer to select pixels from the other two layers. When the Mask layer pixel value is 1, the corresponding pixel from the Foreground layer is selected; when it is 0, the corresponding pixel from the Background layer is selected. Details are given in Appendix 1 of [MRC]. Not all pages, and not all parts of a page, require 3 layers. If there is only one layer present, then that layer is the primary image and must be page size. If there is more than one layer, then the Mask must be one of the layers, in which case it is the primary image and must be page size. MRC allows a page to be split into strips, with a variable number of scanlines in a strip. A strip can have 1, 2 or 3 layers. A single, stripped layer may be stored as a single, stripped image in an IFD, e.g., all strips associated with the Background layer may be treated as a single image. Alternatively, each strip associated with a layer may be stored as a separate image or IFD, e.g., the Background layer can be composed of several images that are offset vertically with respect to the page. In this case, there can be no overlap between images associated with a single layer. Furthermore, color fax also requires the spatial resolutions of Background and Foreground images to be integer factors of the Mask image resolution. For example, if the Mask Layer is 400 pixels per inch, then the Background layer can be 100, 200 or 400 pixels per inch. McIntyre & Zilles Expires 1/30/98 [Page 32] Internet Draft File Format for Internet Fax July 30, 1997 7.1.2. A TIFF Representation for the MRC 3-layer model In the TIFF representation of the 3-layer MRC model, each page is represented by a single IFD, called the Primary IFD, that represents the Mask layer (unless the Foreground or Background is the single layer present), and a set of child IFDs that are referenced through the SubIFDs extension field. The structure of SubIFDs is described in [TTN1]. To distinguish MRC-specific SubIFDs from other SubIFDs, the NewSubFileType field will have Bit 4 ON, indicating an MRC-related IFD. A new ImageLayer field is also introduced that consists of two values which identify the layer (Foreground, Background, or Mask) and the order within the layer (first, second, ... image of the layer); see Section 7.2.3. Because MRC allows strips with variable numbers of scanlines, a new StripRowCounts field is introduced to replace the RowsPerStrip field in this mode. The StripRowCounts field allows each layer, with a variable number of scanlines in each strip, to be represented by a single IFD. Alternatively, each strip in the Foreground and Background layers can be represented by a single IFD at the option of the writer. In all cases, the Mask layer is required to be represented by a single IFD. The use of SubIFDs to store child IFDs is described in [TTN1]. An example is shown graphically below. The Primary IFD associated with page 1 (PrimaryIFD 0) points to page 2 (PrimaryIFD 1) with the nextIFD offset. The Primary IFD, corresponding to the Mask layer (ImageLayer=[2,1]), contains a SubIFDs field that points to a list of child IFDs. The first child IFD represents one image of the Background layer, i.e., ImageLayer=[1,1]. This child IFD points to the second child IFD via the nextIFD offset. This child represents the second Background layer image, ImageLayer=[1,2]. Finally, the second child points to the third child, which corresponds to the single Foreground layer image, ImageLayer=[3,1]. The next IFD offset associated with this Foreground image is 0, indicating no more child IFDs exist. Each primary IFD has the NewSubFileType set to 18, indicating the IFD is MRC-specific (bit 4) and that it is a single page of a multi-page document (bit 1). Each child IFD has the NewSubFileType set to 16, indicating the IFD is MRC-specific. Note: the 'V' character should be read as a down-pointing arrow. (nextIFD) PRIMARY IFD 0 ------------> PRIMARY IFD 1--> ... ImageLayer = [2,1] NewSubFileType = 18 SubIFDs | V Child IFD ImageLayer = [1,1] McIntyre & Zilles Expires 1/30/98 [Page 33] Internet Draft File Format for Internet Fax July 30, 1997 NewSubFileType = 16 | |(nextIFD) | V Child IFD ImageLayer = [1,2] NewSubFileType = 16 | |(nextIFD) | V Child IFD ImageLayer = [3,1] NewSubFileType = 16 | |(nextIFD) | V 0 In the example above, the SubIFDs field of the Primary IFD points to the first IFD in a list of child IFDs. TIFF allows the SubIFDs field to point to an array of IFDs, each of which can be the first of a list of IFDs. An MRC-enabled TIFF reader must scan all available child IFDs to locate and identify IFDs associated with MRC layers. In the case where the Background or Foreground layers are described with multiple IFDs, the XPosition and YPosition TIFF fields specify the offset to the upper-left corner of the IFD with respect to the Mask layer; see Section 7.2.2. When there is only a single layer (Mask, Foreground, or Background), it is stored as the Primary IFD. 7.2. Required TIFF Fields This section describes the TIFF fields required, in addition to those in Section 2.2.1, to store MRC mode fax images. Since MRC mode stores the fax data as a collection of images corresponding to layers or parts of layers, the compression mechanisms, color encodings and spatial resolutions used by previous modes apply to MRC. Therefore, the descriptions here will typically reference the appropriate earlier section. Fields and values specific to MRC mode are pointed out. 7.2.1. Baseline Fields ImageWidth(256). SHORT or LONG Same page widths as the base color mode; see Section 5.2.1. BitsPerSample(258) = 1, 8, 9-16 SHORT Compression(259) = 3, 4, 7, 9. SHORT SamplesPerPixel(277) = 1, 3, 4. SHORT FillOrder(266) = 1, 2. SHORT McIntyre & Zilles Expires 1/30/98 [Page 34] Internet Draft File Format for Internet Fax July 30, 1997 PhotometricInterpretation(262) = 0, 1, 2, 5, 10. SHORT For Mask layer, see Section 3.2.1. For Foreground and Background layers, see Sections 5.2.1 and 6.2.1. ColorMap(320). SHORT Count = 3 * (2**BitsPerSample) Used when Foreground or Background layer is a palette-color image; see Section 6.2.1. NewSubFileType(254) = 16, 18. LONG For MRC fax mode, the NewSubFileType field has two bits that are required. Bit 1 indicates a single page of a multi-page document and must be set for the Primary IFDs; Bit 4 indicates MRC imaging model as described in ITU-T Recommendation Draft T.44 [MRC], and must be set for Primary IFDs and all MRC-specific child IFDs. StripRowCounts(559). LONG Count = number of strips The number of scanlines stored in a strip. MRC allows each fax strip to store a different number of scanlines, up to a specified maximum strip size, which is either 256 scanlines or the page length in scanlines, depending on the implementation. This field replaces RowsPerStrip for IFDs with variable-sized strips. Only one of the two fields, StripRowCounts and RowsPerStrip, may be used in an IFD. XResolution(282). RATIONAL YResolution(283). RATIONAL The resolution of the image is expressed in pixels per resolution unit. In pixels per inch, allowed XResolution values are: 100, 200, 300, and 400. MRC color fax mode requires the pixels to be square, hence YResolution must equal XResolution. 7.2.2. Extension Fields ChromaSubSampling(530). SHORT ChromaPositioning(531). SHORT Decode(TagToBeAssigned). SHORT For Foreground and Background layers, see Section 5.2.2. Indexed(346) = 0, 1. SHORT For Foreground and Background layers: 1 indicates a palette-color image, see Section 6.2.2. T4Options(292) = 0, 1, 4, 5. SHORT T6Options(293) = 0. SHORT For Mask layer, see Section 4.2.2. McIntyre & Zilles Expires 1/30/98 [Page 35] Internet Draft File Format for Internet Fax July 30, 1997 DefaultImageColor(TagToBeAssigned). SHORT or LONG Count = SamplesPerPixel In areas where no image data is available, a default color is needed to specify the color value. If the StripByteCounts value for a strip is 0, then the color for that strip must be defined by a default image color. The DefaultImageColor field uses the same encoding as the image data, and its value is therefore interpreted using the PhotometricInterpretation, SamplesPerPixel, BitsPerSample, and Indexed fields. If the fax data stream requires a different encoding, then transferring the default color value between a TIFF file and fax data stream requires a color conversion. The default value for the DefaultImageColor field is white. SubIFDs(330). IFD Count = number of child IFDs Each value is an offset from the beginning of the TIFF file to a child IFD [TTN1]. XPosition(286). RATIONAL YPosition(287). RATIONAL Specifies the horizontal and vertical offsets of the top-left of the IFD from the top-left of the Primary IFD in page resolution units. For example, if the Primary IFD is at 400 pixels per inch, and a foreground layer IFD is at 200 pixels per inch and located at pixel coordinate (345, 678) with respect to the Primary IFD, the XPosition value is 345/400 and the YPosition value is 678/400. Color fax does not currently allow overlap of any component images within a single layer. Default values for XPosition and YPosition are 0. 7.2.3. New Fields The Mixed Raster Content mode requires one new field: ImageLayer. ImageLayer (34732). SHORT or LONG. Count = 2 Image layers are defined such that layer 1 is the Background layer, layer 3 is the Foreground layer, and layer 2 is the Mask layer that selects pixels from the Background and Foreground layers. The ImageLayer tag contains two values, describing the layer to which the image belongs and the order in which it is imaged. ImageLayer[0]: 1, 2, 3. 1: Image is a Background image, i.e., the image that will appear whenever the Mask contains a value of 0. Background images typically contain low-resolution, continuous-tone imagery. 2: Image is the Mask layer. In MRC, if the Mask layer is present, it must be the Primary IFD and be full page in extent (no gaps.) McIntyre & Zilles Expires 1/30/98 [Page 36] Internet Draft File Format for Internet Fax July 30, 1997 3: Image is a Foreground image, i.e., the image that will appear whenever the Mask contains a value of 1. The Foreground image generally defines the color of text or lines, but may also contain high-resolution imagery. ImageLayer[1]: 1: first image to be imaged in this layer (e.g., first fax strip), 2: second image to be imaged in this layer (e.g., second fax strip), 3: ... Value describing the image order. In MRC, this may be considered the strip number. Since MRC mode currently does not allow overlap between images within a layer, the order value does not have any visual effect. In MRC fax mode, it is possible that only a single layer is transmitted. For example, if a page contains only a single continuous-tone photograph, then only the Background layer may be transmitted. In this case, the Background layer will be full page and stored as the Primary IFD. ImageLayer[0] will be 1 indicating Background; ImageLayer[1] will be 1 since the layer must be full page and there can be no other IFDs associated with that layer. No Mask layer will exist. 7.3. Recommended TIFF Fields See Sections 2.2.3. and 2.2.4. 7.4. Rules and Requirements for Images The MRC mode defines a fundamental set of rules for images in the 3-layer representation. 1. If more than one layer exists, then the binary Mask layer must be present and it is the primary image. If only one layer exists, then the image corresponding to that layer is the primary image. 2. The Primary image defines and extends to the entire page boundary; all attached model images cannot extend beyond the Primary image. Resolution differences may cause some pixels to "hang over" the page boundary, but no new pixels should exist completely beyond the page extent 3. The Background and Foreground images may use any color encoding defined in Sections 5 and 6, and may optionally cover only a portion of the region defined by the Primary image, unless one of them is the primary image. 4. Each Primary IFD and each MRC-specific SubIFD must have an ImageLayer field to specify which layer the IFD belongs to, and the imaging order of that IFD within the layer. McIntyre & Zilles Expires 1/30/98 [Page 37] Internet Draft File Format for Internet Fax July 30, 1997 5. Each Primary IFD must have a NewSubFileType field value set to 18, indicating a single page of a multi-page document (bit 1) and MRC mode (bit 4). 6. Each MRC-specific child IFD must have a NewSubFileType field value set to 16, indicating MRC mode (bit 4). 7. In MRC mode, each layer is transmitted as a sequence of strips. It is possible that each strip of each layer can be stored as a separate IFD. In this case, the SubIFDs structure pointed to by the Primary IFD will contain several IFDs that have an ImageLayer field with the layer identified as either Background (layer 1) or Foreground (layer 3). There may be no overlap in the vertical direction between IFDs associated with a single layer, although there may be a gap from one of these images to the next. The TIFF XPosition and YPosition fields are used to indicate the placement of these images with respect to the primary image. 8. The resolution of Background and Foreground images must each be an integer factor of the Primary image. For example, if the Primary image is 400 pixels/inch, then the model images may be at 400 pixels/inch (400 / 1), 200 pixels/inch (400 / 2), 100 pixels/inch (400 / 4), etc. 7.5. MRC Fax Mode Summary Recommended fields are shown with an asterisk * +------------------+-----------------------------------------+ | Baseline Fields | Values | |------------------|-----------------------------------------| | BitsPerSample | 1: binary mask | | | 8: 8 bits per color sample | | | 9-16: optional 12 bits/sample | +------------------+-----------------------------------------+ | ColorMap | n: LAB color map | +------------------+-----------------------------------------+ | Compression | 3: Modified Huffman and Modified Read | | | 4: Modified Modified Read | | | 7: JPEG | | | 9: JBIG | +------------------+-----------------------------------------+ | DateTime* | {ASCII): date/time in the 24-hour format| | | "YYYY:MM:DD HH:MM:SS" | +------------------+-----------------------------------------| | FillOrder | Applies only to 1 bit/sample encodings | | | 1: Most significant bit first | | | 2: Least significant bit first | +------------------+-----------------------------------------+ McIntyre & Zilles Expires 1/30/98 [Page 38] Internet Draft File Format for Internet Fax July 30, 1997 +------------------+-----------------------------------------| | ImageDescription*| {ASCII}: A string describing the | | | contents of the image. | +------------------+-----------------------------------------+ | ImageWidth | 864, 1024, 1216, 1728, 2048, 2432, 2592,| | | 3072, 3456, 3648, 4096, 4864 | +------------------+-----------------------------------------+ | ImageLength | n: total number of scanlines in image | +------------------+-----------------------------------------| | NewSubFileType | 16, 18: | | | Bit 1 indicates single page of a multi- | | | page document on Primary IFD | | | Bit 4 indicates MRC model | +------------------+-----------------------------------------+ | Orientation* | 1-8, Default 1 | +------------------+-----------------------------------------+ | PhotometricInter | 0: WhiteIsZero | | pretation | 1: BlackIsZero | | | 2: RGB | | | 5: CMYK | | | 10: ITULAB | +------------------+-----------------------------------------+ | ResolutionUnit | 2: inch | | | 3: centimeter | +------------------+-----------------------------------------+ | RowsPerStrip | n: number of scanlines in each strip | +------------------+-----------------------------------------+ | SamplesPerPixel | 1: L* (lightness) | | | 3: RGB, LAB, CMY | | | 4: CMYK | +------------------+-----------------------------------------+ | Software* | {ASCII}: name & release number of | | | creator software | +------------------+-----------------------------------------+ | StripByteCounts | : number or bytes in TIFF strip | +------------------+-----------------------------------------+ | StripOffsets | : offset from beginning of file to | | | each TIFF strip | +------------------+-----------------------------------------| | XResolution | 100, 200, 300, 400 (written in | | | pixels/inch) | +------------------+-----------------------------------------| | YResolution | equal to XResolution (pixels must be | | | square) | +------------------+-----------------------------------------+ | Extension Fields | +------------------+-----------------------------------------+ McIntyre & Zilles Expires 1/30/98 [Page 39] Internet Draft File Format for Internet Fax July 30, 1997 +------------------+-----------------------------------------+ | T4Options | 0: required if Compression is | | | Modified Huffman, no fill bits | | | 1: required if Compression is 2D | | | Modified Read, no fill bits | | | 4: required if Compression is Modified | | | Huffman, fill bits | | | 5: required if Compression is 2D | | | Modified Read, fill bits | +------------------+-----------------------------------------+ | T6Options | 0: required if Compression is 2D | | | Modified Modified Read | +------------------+-----------------------------------------+ | DocumentName* | {ASCII}: name of scanned document | +------------------+-----------------------------------------+ | PageNumber | n,m: page number followed by total page | | | count | +------------------+-----------------------------------------+ | Decode | minL, maxL, mina, maxa, minb, maxb: | | | minimum and maximum values for L*a*b* | +------------------+-----------------------------------------+ | ChromaSubSampling| ChromaSubsampleHoriz: | | | 0: equal numbers of lightness and chroma| | | samples | | | 1: twice as many lightness samples as | | | chroma samples | | | 2: four times as many lightness samples | | | as chroma samples | | | ChromaSubsampleVert: | | | 0: equal numbers of lightness and chroma| | | samples | | | 1: twice as many lightness samples as | | | chroma samples | | | 2: four times as many lightness samples | | | as chroma samples | +------------------+-----------------------------------------+ | ChromaPositioning| 1: centered | | | 2: cosited | +------------------+-----------------------------------------+ | DefaultImageColor| : background color | +------------------+-----------------------------------------+ | Indexed | 0: not a palette-color image | | | 1: palette-color image | +------------------+-----------------------------------------+ | SubIFDs | : byte offset to fg/bg IFDs | +------------------+-----------------------------------------+ | XPosition | horizontal offset in primary IFD | | | coordinates | +------------------+-----------------------------------------+ | YPosition | vertical offset in primary IFD | | | coordinates | +------------------+-----------------------------------------+ McIntyre & Zilles Expires 1/30/98 [Page 40] Internet Draft File Format for Internet Fax July 30, 1997 +------------------+-----------------------------------------+ | New Fields | +------------------+-----------------------------------------+ | ImageLayer | n, m: layer number, imaging sequence | | | (e.g., strip number) | +------------------+-----------------------------------------+ | GlobalParameters | : global parameters IFD | | IFD* | | +------------------+-----------------------------------------+ | ProfileType* | n: type of data stored in TIFF file | +------------------+-----------------------------------------+ | ITUFaxMode* | n: ITU-compatible fax mode | +------------------+-----------------------------------------+ | CodingMethods* | n: compression algorithms used in file | +------------------+-----------------------------------------+ | VersionNumber* | n: version of ITU fax standard | +------------------+-----------------------------------------+ | VersionYear* | byte sequence with year of ITU fax | | | standard | +------------------+-----------------------------------------+ 8. MIME sub-type The draft proposes to redefine the image/tiff sub-type to correspond to the current TIFF specification, Revision 6.0, dated June 3, 1992, with the addition of the new fields defined here. Further, an optional application parameter is proposed that would distinguish subsets of the image/tiff subtype corresponding to the TIFF representation of the fax modes defined in this document. 9. IANA Registration To: ietf-types@iana.org Subject: Registration of Standard MIME media type image/tiff MIME media type name: image MIME subtype name: tiff Required parameters: none Optional parameters: application The value of the application parameter of image/tiff is denoted by a single letter. Six values are proposed. McIntyre & Zilles Expires 1/30/98 [Page 41] Internet Draft File Format for Internet Fax July 30, 1997 B - minimal black-and-white fax mode (defined in Section 3). F - extended black-and-white fax mode (defined in Section 4, excluding Compression=9) J - extended black-and-white fax mode (defined in Section 4, including Compression=9) C - lossy color mode (defined in Section 5) P - lossless color mode (defined in Section 6) M - Mixed Raster Content mode (defined in Section 7) There is no default value for application, and in the absence of the application parameter, the user should assume baseline TIFF. Encoding considerations: binary or base64 generally preferred Security considerations: The security issues associated with this type have not been assessed. Interoperability considerations: These application parameter values for image/tiff are intended for interoperability between ITU and Internet fax data formats. Published specification: TIFF Revision 6.0 is available at: ftp:// ftp.adobe.com/pub/adobe/devrelations/devtechnotes/pdffiles/tiff6.pdf, http://www.adobe.com/supportservice/devrelations/PDFS/TN/TIFF6.pdf Person & e-mail address to contact for further information: Lloyd McIntyre: lmcintyre@adoc.xerox.com Steve Zilles: szilles@adobe.com Intended usage: Common 10. Security Considerations Security issues have not been assessed in this document. 11. References [MRC] TD1018, "Revision of D25, forming the basis for Draft Rec. T.44 (MRC-Colour)", ITU-T Study Group 8, February 1997 currently available at http://www.xerox.com/xis/mrc/ [RFC1314] Katz, A., and Cohen, D., "A File Format for the Exchange of Images in the Internet", RFC 1314, USC Information Sciences Institute, April 1992 McIntyre & Zilles Expires 1/30/98 [Page 42] Internet Draft File Format for Internet Fax July 30, 1997 [RFC1528] Malamud, C., and Rose, M., Principles of Operation for the TPC.INT Subdomain: Remote Printing -- Technical Procedures, RFC 1528, October 1993; also Rose, M., Registration of new MIME content- type/subtype, July 31, 1993, ftp://ftp.isi.edu/in-notes/iana/ assignments/media-types/image/tiff [T.4] ITU-T Recommendation T.4, Standardization of group 3 facsimile apparatus for document transmission, July 1996 [T.6] ITU-T Recommendation T.6, Facsimile coding schemes and coding control functions for group 4 facsimile apparatus, November 1988 [T.42] ITU-T Recommendation T.42, Continuous-tone colour representation method for facsimile, February 1996 [T.43] ITU-T Recommendation T.43, Colour and gray-scale image representations using lossless coding scheme for facsimile, February 1997 [T.81] ITU-T Recommendation T.81, Information technology - Digital compression and coding of continuous-tone still images - Requirements and guidelines, September 1992 [T.82] ITU-T Recommendation T.82, Information technology - Coded representation of picture and audio information - Progressive bi-level image compression, March 1995 [T.85] ITU-T Recommendation T.85, Application profile for Recommendation T.82 - Progressive bi-level image compression (JBIG coding scheme) for facsimile apparatus, August 1995 [TIFF] Tag Image File Format, Revision 6.0, Adobe Developers Association, June 3, 1992, ftp://ftp.adobe.com/pub/adobe/devrelations/ devtechnotes/pdffiles/tiff6.pdf [TIFF-F] work in progress, Tag Image File Format (TIFF) - Class F, May 30, 1997, ftp://ftp.ietf.org/internet-drafts/draft-ietf-fax-tiff-02.txt [TIFF-F0] TIFF Class F specification, Apr 28, 1990, ftp://ftp.faximum.com/pub/documents/tiff_f.txt [TTN1] Adobe PageMaker 6.0 TIFF Technical Notes, Sept. 14, 1995, http://www.adobe.com/supportservice/devrelations/PDFS/TN/TIFFPM6.pdf [TTN2] Draft TIFF Technical Note 2, Replacement TIFF/JPEG specification, March 17, 1995, ftp://ftp.sgi.com/graphics/tiff/TTN2.draft.txt The ITU-T Recommendations are available at http://www.itu.ch. [MRC] is currently available at http://www.xerox.com/xis/mrc/ McIntyre & Zilles Expires 1/30/98 [Page 43] Internet Draft File Format for Internet Fax July 30, 1997 12. Authors' Addresses Lloyd McIntyre Xerox Corporation Mailstop PAHV-305 3400 Hillview Ave. Palo Alto, CA 94304 USA Voice: +1-415-813-6762 Fax: +1-415-813-6792 Email: lmcintyre@adoc.xerox.com Stephen Zilles Adobe Systems Inc. Mailstop W14 345 Park Avenue San Jose, CA 95110-2704 Voice: +1-408-536-4766 Fax: +1-408-536-4042 Email: szilles@adobe.com Appendix A: Summary of TIFF Fields for Internet Fax This annex includes tables which list by mode the TIFF fields used in the proposed fax file format. The fields are organized into 3 categories: 1) TIFF Baseline Fields 2) TIFF Extension Fields 3) New Fields. The tables include the allowed values for each fax mode. Entries other than explicit numbers are described by: n - single number n, m - 2 numbers a, b, c - 3 numbers r - rational number - array of numbers - byte sequence {ASCII} - string - array of IFD byte offsets A blank entry in the table indicates the field is not used by that particular fax mode. McIntyre & Zilles Expires 1/30/98 [Page 44] Internet Draft File Format for Internet Fax July 30, 1997 Table A.1 TIFF Baseline Fields +----------------------------------------------------------+ | Fax Mode | +----------+----------------------------------------------------------| | Field | B&W | B&W | Color | Color | Mixed Raster| | | Minimal | Extended | Lossy | Lossless | Content | +----------+---------------------+----------+-----------+-------------+ | BitsPer | | | | | | | Sample | 1 | 1 | 8, 12 | 1, 8, 9-16| 1, 8, 9-16 | +----------+---------------------+----------+-----------|-------------| | ColorMap | | | | | | +----------+---------------------+----------+-----------|-------------| | Compres- | | | | | | | sion | 3 | 3, 4 | 7 | 9 | 3, 4, 7, 9 | +----------+---------------------+----------+-----------+-------------+ | DateTime | {ASCII} | {ASCII} | {ASCII} | {ASCII} |{ASCII} | +----------+---------------------+--------- +-----------|-------------| | FillOrder| 1, 2 | 1, 2 | | 1, 2 | 1, 2 | +----------+---------------------+----------+-----------+-------------+ | ImageDes-| {ASCII} | {ASCII} | {ASCII} | {ASCII} | {ASCII} | | cription | | | | | | +----------+---------------------+----------+-----------+-------------| | Image- | 1728, |1728, 2048 |864, 1024 |864, 1024 |864, 1024 | | Width | 2048, |2432, 2592 |1216, 1728|1216, 1728 |1216, 1728 | | | 2432 |3072, 3456 |2048, 2432|2048, 2432 |2048, 2432 | | | |3648, 4096 |2592, 3072|2592, 3072 |2592, 3072 | | | |4864 |3456, 3648|3456, 3648 |3456, 3648 | | | | |4096, 4864|4096, 4864 |4096, 4864 | +----------+---------------------+----------+-----------+-------------+ | Image- | | | | | | | Length | n | n | n | n | n | +----------+---------------------+----------+-----------+-------------+ | NewSub- | | | | | | | FileType | 2 | 2 | 2 | 2 | 16, 18 | +----------+---------------------+----------+-----------+-------------+ | Orien- | | | | | | | tation | 1 | 1 | 1 |1 | 1 | +----------+---------------------+----------+-----------+-------------+ | Photo- | | | | | | | metric- | | | | | | | Interp- | | | | | | | retation | 0, 1 | 0, 1 | 10 | 2, 5, 10 | 0,1,2,5,10 | +----------+---------------------+----------+-----------+-------------+ | Resolu- | | | | | | | tionUnit | 2, 3 | 2, 3 | 2, 3 | 2, 3 | 2, 3 | +----------+---------------------+----------+-----------+-------------+ | RowsPer- | | | | | | | Strip | n | n | n | n | | +----------+---------------------+----------+-----------+-------------+ McIntyre & Zilles Expires 1/30/98 [Page 45] Internet Draft File Format for Internet Fax July 30, 1997 +----------+---------------------+----------+-----------+-------------+ | Samples- | | | | | | | PerPixel | 1 | 1 | 1, 3 | 1, 3, 4 | 1, 3, 4 | +----------+---------------------+----------+-----------+-------------+ | Software | {ASCII} | {ASCII} | {ASCII} | {ASCII} | {ASCII} | +----------+---------------------+----------+-----------+-------------+ | Strip- | | | | | | | Byte- | | | | | | | Counts | | | | | | +----------+---------------------+----------+-----------+-------------+ | Strip- | | | | | | | Offsets | | | | | | +----------+---------------------+----------+-----------+-------------+ | XResolu- | 204 | 200, 204, | 100, 200,| 100, 200, | 100, 200, | | tion | | 300, 400, | 300, 400 | 300, 400 | 300, 400 | | | | 406 | | | | +----------+---------------------+----------+-----------+-------------+ | YResolu- | 98, 196 | 98, 196, | 100, 200,| 100, 200, | 100, 200, | | tion | | 100, 200, | 300, 400 | 300, 400 | 300, 400 | | | | 300, 392, | | | | | | | 400 | | | | +----------+---------------------+----------+-----------+-------------+ Table A.2 TIFF Extension Fields +---------------------------------------------------------+ | Fax Mode | +-----------+---------------------------------------------------------| | Field | B&W | B&W | Color | Color |Mixed Raster| | | Minimal| Extended | Lossy | Lossless | Content | +-----------+--------+-----------+----------+----- ------+------------+ | T4Options | 4, 5 | 0, 1, 4, 5| | | 0, 1, 4, 5 | +-----------+--------+-----------+----------+------------+------------+ | T6Options | | 0 | | | 0 | +-----------+--------+-----------+----------+------------+------------+ | Document- | | | | | | | Name | {ASCII}| {ASCII} | {ASCII} | {ASCII} | {ASCII} | +-----------+--------+-----------+----------+------------+------------+ | Page- | | | | | | | Number | n, m | n, m | n, m | n, m | n, m | +-----------+--------+-----------+----------+------------+------------+ | XPosition | | | | | r | +-----------+--------+-----------+----------+------------+------------+ | YPosition | | | | | r | +-----------+--------+-----------+----------+------------+------------+ | SubIFDs | | | | | | +-----------+--------+-----------+----------+------------+------------+ | Indexed | | | | 0, 1 | 0, 1 | +-----------+--------+-----------+----------+------------+------------+ McIntyre & Zilles Expires 1/30/98 [Page 46] Internet Draft File Format for Internet Fax July 30, 1997 +-----------+--------+-----------+----------+------------+------------+ | Chroma- | | | | | | | SubSampl- | | | | | | | ing | | | n, m | | n, m | +-----------+--------+-----------+----------+------------+------------+ | Chroma- | | | | | | | Position- | | | | | | | ing | | | 1, 2 | | 1,2 | +-----------+--------+-----------+----------+------------+------------+ | Decode | | | | | | +-----------+--------+-----------+----------+------------+------------+ | Default- | | | | | | | ImageColor| | | | | | +-----------+--------+-----------+----------+------------+------------+ | Strip- | | | | | | | RowCounts | | | | | | +-----------+--------+-----------+----------+------------+------------+ Table A.3 New Fields +---------------------------------------------------------+ | Fax Mode | +-----------+---------------------------------------------------------| | Field | B&W | B&W | Color | Color |Mixed Raster| | | Minimal| Extended | Lossy | Lossless | Content | +-----------+--------+-----------+----------+----- ------+------------+ | BadFax- | | | | | | | Lines | n | n | | | | +-----------+--------+-----------+----------+------------+------------+ | CleanFax- | | | | | | | Data | 0,1,2 | 0,1,2 | | | | +-----------+--------+-----------+----------+------------+------------+ | Consecu- | | | | | | | tiveBad- | | | | | | | FaxLines | n | n | | | | +-----------+--------+-----------+----------+------------+------------+ | Image- | | | | | | | Layer | | | | | n, m | +-----------+--------+-----------+----------+------------+------------+ | Global- | | | | | | | Parame- | | | | | | | tersIFD | | | | | | +-----------+--------+-----------+----------+------------+------------+ | Profile- | | | | | | | Type | | n | n | n | n | +-----------+--------+-----------+----------+------------+------------+ | ITUFax- | | | | | | | Mode | | n | n | n | n | +-----------+--------+-----------+----------+------------+------------+ McIntyre & Zilles Expires 1/30/98 [Page 47] Internet Draft File Format for Internet Fax July 30, 1997 +-----------+--------+-----------+----------+------------+------------+ | Coding- | | | | | | | Method | | n | n | n | n | +-----------+--------+-----------+----------+------------+------------+ | Version- | | | | | | | Number | | n | n | n | n | +-----------+--------+-----------+----------+------------+------------+ | Version- | | | | | | | Year | | | | | | +-----------+--------+-----------+----------+------------+------------+ McIntyre & Zilles Expires 1/30/98 [Page 48] Internet Draft File Format for Internet Fax July 30, 1997 Appendix B: Use of TIFF-F for Black-and-White Fax B.1 Overview Section 4 provides the standard definition of image/tiff, application=F, known commonly as TIFF-F and historically as TIFF Class F (see Section 4.4). Although Section 4 is a necessary and sufficient definition of TIFF-F, there is much lore associated with TIFF-F. This appendix, based on [TIFF-F], captures this lore and provides background information for implementors. B.2 TIFF-F Fields The following table summarizes the required and optional (or recommended) fields for TIFF-F (See Section 4.). The optional fields are shown with an asterisk * For convenience, a minimum subset of values is also shown; these values should provide maximum portability with historical TIFF-F readers. Field | Values | Minimum | Comment ------------------|-------------|--------------|---------------------- BadFaxLines* |>=0 | |number of bad fax lines BitsPerSample | 1 | 1 |one bit per sample CleanFaxData* | 0,1,2 | 0,1,2 |status of bad lines Compression | 3,4 | 3 |3 for T.4 (MH, MR) | | |4 for T.6 (MMR) ConsecutiveBad- |>=0 | |max number of bad lines Lines* | | | DateTime* | ASCII | |"YYYY:MM:DD:HH:MM:SS" DocumentName* | ASCII | |informational field FillOrder | 2,1 | 2 |LSB first or MSB first ImageDescription* | ASCII | |informational field ImageWidth | 1728, 2048, | 1728, 2048 |depends on XResolution | 2432, 2592, | | | 3072, 3648, | | | 3456, 4096, | | | 4864 | | ImageLength | >0 | |required NewSubFileType | 2 | 2 |single page of | | |multipage file Orientation* | 1 | 1 |1st row=top left, | | | 1st col=top PageNumber | X/X | 0/1 |pg/tot, 0 base, | | | tot in 1st IFD PhotometricInterp | 0,1 | 0 |0 is white ResolutionUnit | 2,3 | 2 |inches (default) RowsPerStrip |=ImageLength |=ImageLength |required | or other | | McIntyre & Zilles Expires 1/30/98 [Page 49] Internet Draft File Format for Internet Fax July 30, 1997 SamplesPerPixel | 1 | 1 |one sample per pixel Software* | ASCII | |informational field StripByteCounts | >0 | |required StripOffsets | >0 | |required T4Options | 4,5 | 4 |MH, MR: incl. if not MMR T6Options | 0 | |MMR: incl. only if MMR XResolution | 204,200,300 | 204 |If unit is per inch | 400,406 | | | 77 | | If unit is per cm YResolution | 196,98,100, | 196,98 |If unit is per inch | 200,300,392,| | | 400 | | | 77,38.5 | | If unit is per cm ------------------|-------------|--------------|-------------------- B.3 Implementation Notes B.3.1 Strips In general, TIFF files divide an image into "strips," also known as "bands." Each strip contains a few scanlines of the image. By using strips, a TIFF reader need not load the entire image into memory, thus enabling it to fetch and decompress small random portions of the image as necessary. The number of scanlines in a strip is described by the RowsPerStrip value and the number of bytes in the strip after compression by the StripByteCount value. The location in the TIFF file of each strip is given by the StripOffsets values. Strip size is application dependent. Existing TIFF-F usage is typically one strip per page in multi-page TIFF-F. See Sections 2.1.2 and 2.1.3. B.3.2 Bit Order The current TIFF specification [TIFF] does not require a Baseline TIFF reader to support FillOrder=2, i.e. lowest numbered 1-bit pixel in the least significant bit of a byte. It further recommends that FillOrder=2 be used only in special purpose applications. Facsimile data appears on the phone line in bit-reversed order relative to its description in ITU-T Recommendation T.4. Therefore, a wide majority of facsimile applications choose this natural order for data in a file. Nevertheless, TIFF F readers must be able to read data in both bit orders and support FillOrder values of 1 and 2. See Section 3.2.1. McIntyre & Zilles Expires 1/30/98 [Page 50] Internet Draft File Format for Internet Fax July 30, 1997 B.3.3. Multi-Page Files Many existing applications already read TIFF-F-like files, but do not support multiples pages using a linked list of IFDs. A Baseline TIFF reader is not required to read any IFDs beyond the first one. Since a multi-page format greatly simplifies file management in fax application software, TIFF-F specifies multi-page documents and uses NewSubfileType = 2. B.3.4. Compression Group 3 facsimile uses three compression methods. The ITU-T T.4 Recommendation [T.4] defines a one-dimensional compression method known as Modified Huffman (MH) and a two-dimensional method known as Modified READ (MR) (READ is short for Relative Element Address Designate). In 1984, a somewhat more efficient compression method known as Modified Modified READ (MMR) was defined in the ITU-T T.6 Recommendation [T.6]. MMR was originally defined for use with Group 4 facsimile, so that this compression method has been commonly called Group 4 compression. In 1991, the MMR method was approved for use in Group 3 facsimile and has since been widely utilized. TIFF-F supports these three compression methods. The most common practice is the one-dimensional Modified Huffman (MH) compression method. This is specified by setting the Compression field value to 3 and then setting bit 0 of the T4Options field to 0. Alternatively, the two dimensional Modified READ (MR) method, which is much less frequently used in historical TIFF-F implementations, may be selected by setting bit 0 of the T4Options field to 1. The value of Bit 2 in this field is determined by the use of fill bits. Depending upon the application, the more efficient two-dimensional Modified Modified Read (MMR)compression method from T.6 may be selected by setting the Compression field value to 4 and then setting the first two bits (and all unused bits) of the T6Options field to 0. Baseline TIFF also permits use of Compression=2 to specify Modified Huffman compression, but the data is presented in a form that is not byte aligned. As a result, TIFF-F uses Compression=3 instead of Compression=2 to specify Modified Huffman compression. Uncompressed data is not supported in TIFF-F. Therefore, Bit 1 in both the T4Options and T6Options fields must always be 0. Since two-dimensional encoding is not required for Group 3 compatibility, some historic TIFF F readers have not been able to read such files. However, for maximum efficiency, images should be compressed using T.6 MMR compression when possible. For maximum portability, applications also need to be able to read and write one- dimensional (Modified Huffman) files. Some TIFF-F readers will also support two-dimensional Modified READ files. Applications that wish to have the maximum flexibility in reading TIFF F files should support all three of the supported compression methods. McIntyre & Zilles Expires 1/30/98 [Page 51] Internet Draft File Format for Internet Fax July 30, 1997 B.3.5. Example Use of Page-quality Fields Here are examples for writing the CleanFaxData, BadFaxLines, and ConsecutiveBadFaxLines fields: 1. Facsimile hardware does not provide page quality information: Do not write page-quality fields. 2. Facsimile hardware provides page quality information, but reports no bad lines. Write only BadFaxLines = 0. 3. Facsimile hardware provides page quality information, and reports bad lines. Write both BadFaxLines and ConsecutiveBadFaxLines. Also write CleanFaxData = 1 or 2 if the hardware's regeneration capability is known. 4. Computer generated file: write CleanFaxData = 0. 5. Source image data stream is error-corrected or otherwise guaranteed to be error-free: Do not write page-quality fields. B.3.6 Resolution Almost all facsimile products support both "standard" (98 dpi) resolution and "fine" (196 dpi) vertical resolution. Therefore, fine- resolution files are quite portable in the real world. In 1993, the ITU-T added support for higher resolutions in the T.30 Recommendation, including 200 x 200, 300 x 300, 400 x 400 in dots per inch based units. At the same time, support was added for metric dimensions which are equivalent to the following inch based resolutions: 392v x 203h and 392v x 406h. Therefore, the full set of inch-based equivalents of the new resolutions are supported in the TIFF-F, since they may appear in some image data streams received from Group 3 facsimile devices. However, many facsimile terminals and older versions of TIFF-F readers are likely to not support the use of these higher resolutions. It is permissible for applications to treat the following XResolution values as being equivalent: <204,200> and <400,406>. In a similar respect, the following YResolution values may also be treated as being equivalent: <98, 100>, <196, 200>, and <392, 400>. These equivalencies were allowed by [T.30] to permit conversions between inch and metric based facsimile terminals. In a similar respect, the optional support of metric based resolutions in the TIFF-F reader (i.e. 77 x 38.5 cm) is included for completeness, since they are used in some legacy TIFF-F applications, but this use is not recommended for the creation of TIFF-F files by a writer. Historical TIFF-F documents also included the following values related to A5 and A6 widths: 816 and 1216. Per the most recent version of [T.4], A5 and A6 documents are no longer supported in Group 3 facsimile, so the related width values are now obsolete. McIntyre & Zilles Expires 1/30/98 [Page 52] Internet Draft File Format for Internet Fax July 30, 1997 B.3.7. EOL As illustrated in FIGURE 1/T.4 in [T.4], facsimile documents encoded with Modified Huffman begin with an EOL, which in TIFF-F is byte- aligned. The last line of the image is not terminated by an EOL. In a similar respect, images encoded with Modified READ two dimensional encoding begin with an EOL, followed by a tag bit. The EOL/tag bit combination is byte aligned in TIFF-F. See Section 3.4. B.3.8. RTC Exclusion Aside from EOLs, TIFF F files contain only image data. This means that the Return To Control sequence (RTC) is specifically excluded. B.3.9. Use of EOFB for T.6 Compressed Images TIFF-F pages which are encoded with the T.6 Modified Modified READ compression method should include an "end-of-facsimile-block" (EOFB) code at the end of each coded strip. Per [TIFF], the EOFB code is followed by pad bits as needed to align on a byte boundary. TIFF readers should ignore any bits other than pad bits beyond the EOFB. McIntyre & Zilles Expires 1/30/98 [Page 53]