Internet Fax Working Group Lloyd McIntyre Internet Draft Stephen Zilles Expires in six months March 20, 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 and proposals for black-and-white and color facsimile. For the most part, existing TIFF constructs and fields are used; new TIFF fields are introduced only when necessary. Black-and-white facsimile is already described by TIFF Class F, and this Draft builds on that to add color fax capability and to establish a structure for future enhancements. Table of Contents 1. INTRODUCTION..................................................3 1.1. Scope....................................................3 1.2. Rationale................................................3 1.3. Organization of This Draft...............................4 2. TIFF and Fax..................................................4 2.1. TIFF Overview............................................4 2.1.1. File Structure.......................................4 2.1.2. Image Structure......................................5 2.2 Fax Requirements..........................................6 2.2.1. TIFF fields required for all fax modes...............6 2.2.2. TIFF fields recommended for all fax modes............7 2.2.3. New TIFF fields for all fax modes....................7 McIntyre & Zilles Expires 9/20/97 [Page 1] Internet Draft File Format for Internet Fax March 20, 1997 3. Base B&W Fax Mode.............................................9 3.1. Overview.................................................9 3.2. Required TIFF Fields.....................................9 3.2.1. Baseline fields.....................................10 3.2.2. Extension fields....................................10 3.2.3. New fields..........................................11 3.3. Recommended TIFF Fields.................................11 3.3.1. Baseline fields.....................................11 3.3.2. Extension fields....................................11 3.3.3. New fields..........................................11 3.4. Base B&W Fax Mode Summary...............................12 4. Base Color Fax Mode..........................................14 4.1. Overview................................................14 4.2. Required TIFF Fields....................................14 4.2.1. Baseline Fields.....................................15 4.2.2. Extension fields....................................16 4.2.3. New fields..........................................18 4.3. Recommended TIFF Fields.................................18 4.4. Base Color Fax Mode Summary.............................18 5. Lossless Color Mode..........................................20 5.1. Overview................................................20 5.1.1. Color Encoding......................................21 5.1.2. JBIG Encoding.......................................21 5.2. Required TIFF Fields....................................21 5.2.1. Baseline fields.....................................22 5.2.2. Extension fields....................................23 5.2.3. New fields..........................................23 5.3. Recommended TIFF Fields.................................23 5.4. Lossless Color Fax Mode Summary.........................24 6. Mixed Raster Content Mode....................................26 6.1 Overview.................................................26 6.1.1. MRC 3-layer model...................................26 6.1.2. A TIFF Representation for the MRC 3-layer model.....27 6.2. Required TIFF Fields....................................29 6.2.1. Baseline fields.....................................29 6.2.2. Extension fields....................................30 6.2.3. New fields..........................................31 6.3. Recommended TIFF Fields.................................32 6.4. Rules and Requirements for Images.......................32 6.5. MRC Fax Mode Summary....................................33 7. Mime Sub-Type................................................36 8. IANA Registration............................................36 9. Security Considerations......................................37 10. References..................................................38 11. Authors' Addresses..........................................39 Annex A: Summary of TIFF Fields for Internet Fax ...............39 McIntyre & Zilles Expires 9/20/97 [Page 2] Internet Draft File Format for Internet Fax March 20, 1997 1. Introduction 1.1 Scope This document defines a TIFF-based file format specification and corresponding IETF 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 content and the structure of the data generated by the suite of existing and developing ITU-T Recommendations for Group 3 facsimile. These standards, and the TIFF fields described here, support four facsimile modes: 1. base black-and-white mode, using binary compression [T.4, T.6, T.82] 2. lossy color and grayscale mode, using JPEG compression [T.42, T.81] 3. lossless color and grayscale mode, using JBIG compression [T.43, T.82] 4. mixed raster content mode [MRC], using a combination of existing compression methods This specification builds on a previously proposed standard [RFC1314] and on work in progress [TIFF-F] for the base black-and- white mode. 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 required for compatibility with ITU fax. 1.2 Rationale This specification uses the current TIFF spec [TIFF] as the basis for describing color and grayscale facsimile modes. By doing so, 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. McIntyre & Zilles Expires 9/20/97 [Page 3] Internet Draft File Format for Internet Fax March 20, 1997 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 would be used in all four fax modes. Sections 3-6 each describe the representation of a single fax mode. Each section starts with an overview of the applicable ITU-T Recommendation and then describes the required and optional TIFF fields for representing the ITU data stream. The fields in each subsection are organized according to whether they are baseline or extensions in the current TIFF specification or that will appear in the next specification, or are new and defined in this document. Sections 7 and 8 describe the MIME content type and IANA registration. The remaining sections are required in all Internet Drafts. Annex A gives a summary of the TIFF fields used or defined in this document. 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]. 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. 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, containing information about the image and pointers to the image data. 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 McIntyre & Zilles Expires 9/20/97 [Page 4] Internet Draft File Format for Internet Fax March 20, 1997 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 a reduced resolution version of the primary IFD image. The same IFD can point to both 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. This specification requires the use of subfiles or multiple IFDs. All four 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. Only the Mixed Raster Content fax mode, described in Section 6, requires child IFDs. 2.1.2 Image Structure An IFD can store an image either as a single strip or split into individually addressable strips, for improved performance at the option of the writer. 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, given in the RowsPerStrip field. This document introduces the new StripRowCounts field that allows a variable number of scanlines per strip. This feature is required by the Mixed Raster Content fax mode. Whether stripped or not, the image data is stored as uninterpreted data streams. The formats of these streams follow the ITU-T Recommendations. The compressed data stream is placed in a strip within an IFD, with the Compression field used to indicate the type of compression, and other TIFF fields used to describe image attributes, such as color encoding and spatial resolution. Compression parameters are stored in the compressed data stream, rather than in additional TIFF fields. This makes the TIFF and compression data format specifications independent of one another. This approach, modeled on [TTN2], allows TIFF to gracefully add new compression schemes as they become available. To reduce overhead in TIFF files with multiple JPEG segments, one can allow JPEG tables to be stored just once in a new JPEGTables field [TTN2]. While this specification does not use this field, this approach can be adapted for use with other compression schemes, when all the data streams refer to the same table and it would reduce file size to store the table once, rather than replicate it throughout the file. McIntyre & Zilles Expires 9/20/97 [Page 5] Internet Draft File Format for Internet Fax March 20, 1997 For attributes that can be specified both places, i.e. in the 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 would take precedence. For informational values that are not required to interpret the data stream, such as author name, then the TIFF field value normally would take precedence. 2.2 Fax requirements 2.2.1. TIFF fields required for all fax modes The TIFF fields listed in this section are common to all fax modes and required by all fax modes. Other TIFF fields are also common to all modes, but the values associated with the field are mode- specific. Such fields are described in the relevant sections below. The pattern for the field descriptions in this draft is FieldName = values. TYPE Count = (omitted if =1) Tag = (if not in current spec but available) Explanation of the field, how its used and the values it can have. ImageWidth. SHORT or LONG Total number of pixels in a scanline. ImageLength. SHORT or LONG Total number of scanlines in image. NewSubFileType = 2. LONG 2 = Bit 1 identifies the image as a single page of a multi-page document. PageNumber. SHORT 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. This field is a TIFF extension, not a Baseline TIFF field. ResolutionUnit = 2, 3. SHORT 2 = inch 3 = centimeter The unit of measure for resolution. McIntyre & Zilles Expires 9/20/97 [Page 6] Internet Draft File Format for Internet Fax March 20, 1997 RowsPerStrip. SHORT or LONG The number of scanlines per TIFF strip. StripByteCounts. SHORT or LONG Count = number of strips For each strip, the number of bytes in that strip. StripOffsets. SHORT or LONG Count = number of strips For each strip, the byte offset from the beginning of the file to that strip. 2.2.2 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 the relevant sections below. DateTime. ASCII Date/time in the format "YYYY:MM:DD HH:MM:SS" in 24-hour format. DocumentName. ASCII The name of the scanned document. This field is a TIFF extension, not a Baseline TIFF field. ImageDescription. ASCII A string describing the contents of the image. Orientation = 1-8. SHORT 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 Software. ASCII The optional name and release number of the software package that created the image. 2.2.3. New TIFF fields for all fax modes Almost all the fields recommended in the previous section apply to the collection of images that make up a TIFF-based fax file, rather than to any one specific image in that file. It makes sense to separate these "global" fields from the image-specific fields in an IFD. It would also be useful to have a field, analogous to the Software field, that identifies the fax mode used to create the image data. McIntyre & Zilles Expires 9/20/97 [Page 7] Internet Draft File Format for Internet Fax March 20, 1997 For these reasons, several new fields, which are not part of the current TIFF specification, 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. IFD An IFD containing global parameters. A TIFF writer would likely place this field in the first IFD, where a 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 image IFDs, it is valid to use the data closest to the image data, otherwise the image IFD is ill-formed and should be ignored. Among the GlobalParametersIFD entries is a new ProfileType field which generally describes information in this IFD and in the TIFF file. ProfileType: LONG Tag: to be assigned 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 = 0 - 4. BYTE Tag: to be assigned The currently defined values are: 0: does not conform to ITU-T Recommendations for facsimile, 1: black & white lossless (ITU-T Rec. T.4, T.6), 2: lossy color and grayscale (ITU-T Rec. T.42), 3: lossless color and grayscale (ITU-T Rec. T.43), 4: Mixed Raster Content (ITU-T Draft Rec. T.44). Notes: A file can use an ITU encoding with non-ITU parameters, such as using JPEG to compress a CMYK image. In this case, ITUFaxMode is 0. CodingMethods. LONG Tag: to be assigned This field indicates which coding methods are used in the file. A bit value of 1 indicates: Bit 0: unspecified compression, McIntyre & Zilles Expires 9/20/97 [Page 8] Internet Draft File Format for Internet Fax March 20, 1997 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 Modified Read), 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 compression types to identify standard compression methods. VersionYear. BYTE Tag: to be assigned Count: 4 The year of the standard specified by the ITUFaxMode field, given as 4 characters, e.g. '1997'; used with T.42 and T.43. VersionNumber BYTE Tag: to be assigned The version of the standard specified by the ITUFaxMode field. A value of 0 indicates Version 1.0; used with MRC (Draft T.44). 3. Base B&W Fax Mode 3.1. Overview Base 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. It offers several options for lossless bi-level image coding, including 1-dimensional Modified Huffman (MH) and 2-dimensional Modified Read (MR), both described in [T.4], and Modified Modified Read (MMR), described in [T.6]. (Read is an acronym for Relative Element Address Designate.) 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 is based on the TIFF Class F definition. 3.2. Required TIFF fields This section lists the required fields and the values they must have to be ITU-compatible. McIntyre & Zilles Expires 9/20/97 [Page 9] Internet Draft File Format for Internet Fax March 20, 1997 3.2.1. Baseline fields ImageWidth. SHORT or LONG This mode supports the following fixed page widths: 1728, 2048, 2432, 2592, 3072, 3456, 3648, 4096, 4864. BitsPerSample = 1. SHORT Binary data only. Compression = 2, 3, 4, 9. SHORT 2 = 1-dimensional coding, ITU-T Rec. T.4 (MH - Modified Huffman) 3 = 2-dimensional coding, ITU-T Rec. T.4 (MR - Modified Read) 4 = 2-dimensional coding, ITU-T Rec. T.6 (MMR - Modified Modified Read) 9 = ITU-T Rec. T.82 coding, applying ITU-T Rec. T.85 (JBIG) FillOrder = 1, 2. SHORT 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). Most b&w fax products use LSB first. PhotometricInterpretation = 0, 1. SHORT 0 = pixel value 1 means black 1 = pixel value 1 means white SamplesPerPixel = 1. SHORT 1 = monochrome, bi-level in this case (see BitsPerSample) XResolution. RATIONAL 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. YResolution. RATIONAL 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 3.2.2. Extension fields T4Options = 0, 1, 4, 5. LONG 0: Required if Compression field has a value of 2, indicating McIntyre & Zilles Expires 9/20/97 [Page 10] Internet Draft File Format for Internet Fax March 20, 1997 Modified Huffman encoding, and if fill bits have not been added to insure EOL codes end on a byte boundary. 1: Required if Compression field has a value of 3, indicating Modified Read encoding, and if fill bits have not been added to insure EOL codes end on a byte boundary. 4: Required if Compression field has a value of 2, indicating Modified Huffman encoding, and if fill bits have been added to insure EOL codes end on a byte boundary. 5: Required if Compression field has a value of 3, indicating Modified Read encoding, and if fill bits have been added to insure EOL codes end on a byte boundary. T6Options = 0. LONG Required if Compression field has a value of 4, indicating 2D Modified Modified Read. 3.2.3. New fields No new extensions are required for black & white fax. 3.3 Recommended TIFF fields 3.3.1. Baseline fields See Section 2.2.2. 3.3.2. Extension fields See Section 2.2.2. 3.3.3. New fields There are three new optional fields for describing black & white fax page quality. They are not defined in the current TIFF specification. BadFaxLines. SHORT or LONG The number of "bad" scan lines encountered by the facsimile during reception. A "bad" scanline is defined as a scanline that, when decoded, comprises an incorrect number of pixels. CleanFaxData = 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. McIntyre & Zilles Expires 9/20/97 [Page 11] Internet Draft File Format for Internet Fax March 20, 1997 ConsecutiveBadFaxLines. SHORT or LONG Maximum number of consecutive "bad" scanlines received. 3.4. Base B&W Fax Mode Summary Recommended fields are shown with an asterisk * +---------------------------+--------------------------------+ | Baseline Fields | Values | +---------------------------+--------------------------------+ | BitsPerSample | 1 | +---------------------------+--------------------------------+ | Compression | 2: 1D Modified Huffman coding | | | 3: 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 | +---------------------------+--------------------------------+ McIntyre & Zilles Expires 9/20/97 [Page 12] Internet Draft File Format for Internet Fax March 20, 1997 +---------------------------+--------------------------------+ | 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 | +---------------------------+--------------------------------+ | 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 | +---------------------------+--------------------------------+ McIntyre & Zilles Expires 9/20/97 [Page 13] Internet Draft File Format for Internet Fax March 20, 1997 +---------------------------+--------------------------------+ | 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 | +---------------------------+--------------------------------+ | VersionNumber* | n: version of ITU fax standard | +---------------------------+--------------------------------+ | VersionYear* | byte sequence with year of | | | ITU fax standard | +---------------------------+--------------------------------+ 4. Base Color Fax Mode 4.1. 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 CIELAB 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 CIELAB encoding specified in [T.42]. Compared to the other two CIELAB-based values for this field, ITU-T Rec. T.42 specifies a different default range for the a* and b* components, based on a comprehensive evaluation of existing hardcopy output. It optionally allows a selectable range for the L*, a* and b* components. 4.2. Required TIFF fields This section lists the required fields and the applications they must support to be compatible with ITU-T Rec. T.42 and Annex E in ITU-T Rec. T.4. McIntyre & Zilles Expires 9/20/97 [Page 14] Internet Draft File Format for Internet Fax March 20, 1997 4.2.1. Baseline Fields ImageWidth. SHORT or LONG This mode supports the following fixed page widths: 864, 1024, 1216, 1728, 2048, 2432, 2592, 3072, 3456, 3648, 4096, 4864. BitsPerSample = 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 = 7. SHORT Base color fax mode uses Baseline JPEG compression. Value 7 represents JPEG compression as specified in [TTN2], which also defines a new extension field JPEGTables that is not used here. PhotometricInterpretation = 10. SHORT Base color fax mode requires pixel values to be stored as CIELAB, specified by the PhotometricInterpretation value 10, referred to as ITULAB and which is based on the sample value encoding defined in ITU-T Rec. T.42. 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 BitsPerSample. The conversion from unsigned ITULAB-encoded samples values to signed CIELAB values is determined by the Decode field; see Section 4.2.2 PhotometricInterpretation values 8 and 9 also specify CIELAB encodings, but with different, fixed ranges than ITU-T Rec. T.42, and so are not used for color fax. Note: The intent of PhotometricInterpretation 10 is to broaden the scope of the CIELAB parameter space and not just support color fax. Therefore, the name and default Decode values are still under discussion. SamplesPerPixel = 1, 3. SHORT 1: L* component only 3: L*, a*, b* components Encoded according to PhotometricInterpretation field XResolution. RATIONAL YResolution. RATIONAL The resolution of the image is expressed in pixels per resolution unit. In pixels per inch, allowed XResolution McIntyre & Zilles Expires 9/20/97 [Page 15] Internet Draft File Format for Internet Fax March 20, 1997 values are: 100, 200, 300, and 400. The base color fax mode requires the pixels to be square, hence YResolution must equal XResolution. 4.2.2 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. SHORT Count = 2 Specifies the subsampling factors for the chroma components of a CIELAB 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 (1,1) for LAB-based images, which is an option for color fax. The color fax default is (2,2), which must be specified explicitly in the IFD. ChromaPositioning = 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 9/20/97 [Page 16] Internet Draft File Format for Internet Fax March 20, 1997 Decode. SSHORT 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 CIELAB and has a count of 6 values. In this case, the input is an unsigned ITULAB-encoded value and the output is a signed CIELAB 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 The following table gives the default range and offsets, as specified in ITU-T Rec. T.42, and the corresponding minimum and maximum CIELAB component and ITULAB encoded values. +-----------------------------------------------------------+ | Component | Rec. T.42 | CIELAB | 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 | +-----------+--------+---------+------+-----+-----+---------+ McIntyre & Zilles Expires 9/20/97 [Page 17] Internet Draft File Format for Internet Fax March 20, 1997 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. Note: The intent of PhotometricInterpretation 10 is to broaden the scope of the CIELAB parameter space and not just support color fax. Therefore, the name and default Decode values are still under discussion. 4.2.3. New fields None. 4.3. Recommended TIFF fields See Sections 2.2.2. and 2.2.3. 4.4 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 | +---------------------------+--------------------------------+ McIntyre & Zilles Expires 9/20/97 [Page 18] Internet Draft File Format for Internet Fax March 20, 1997 +---------------------------+--------------------------------+ | 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 | +---------------------------+--------------------------------+ | 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 CIELAB | +---------------------------+--------------------------------+ | ChromaSubSampling | ChromaSubsampleHoriz: | | | 0: equal numbers of lightness | | | and chroma samples | | | 1: twice as many lightness | | | samples as chroma samples | McIntyre & Zilles Expires 9/20/97 [Page 19] Internet Draft File Format for Internet Fax March 20, 1997 | | 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 | +---------------------------+--------------------------------+ | 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 | +---------------------------+--------------------------------+ | VersionNumber* | n: version of ITU fax standard | +---------------------------+--------------------------------+ | VersionYear* | byte sequence with year of | | | ITU fax standard | +---------------------------+--------------------------------+ 5. Lossless Color Mode 5.1. 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 CIELAB encoding specified in [T.42]. McIntyre & Zilles Expires 9/20/97 [Page 20] Internet Draft File Format for Internet Fax March 20, 1997 5.1.1. 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 CIELAB 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. To enable T.43 color maps in TIFF requires the extension field Indexed, defined in [TTN1], and the PhotometricInterpretation field value 10, defined in Section 4.2.1. 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 | +------------+-------------+----------+----------+---------+ 5.1.2. 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. 5.2. Required TIFF Fields This section lists the required fields and the values they must have to be compatible with ITU-T Rec. T.43. McIntyre & Zilles Expires 9/20/97 [Page 21] Internet Draft File Format for Internet Fax March 20, 1997 5.2.1. Baseline fields ImageWidth. SHORT or LONG Same page widths as the base color mode; see Section 4.2.1. BitsPerSample = 1, 8, 9-16. SHORT Count = SamplesPerPixel RGB, CMY, CMYK: 1 bit per sample Continuous tone (CIELAB): 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. 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 = 9. SHORT 9: ITU-T Rec. T.82 coding, applying ITU-T Rec. T.85 (JBIG) FillOrder = 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 = 2, 5, 10. SHORT 2: RGB 5: CMYK, including CMY 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, McIntyre & Zilles Expires 9/20/97 [Page 22] Internet Draft File Format for Internet Fax March 20, 1997 SamplesPerPixel to 1, and Indexed to 1. The color map is stored in the ColorMap field. See Section 5.1.1 for further discussion on the color encoding. SamplesPerPixel = 1, 3, 4. SHORT 1: Palette color image, or L*-only if Indexed = 0 and PhotometricInterpretation is 10 (ITULAB). 3: RGB, or CIELAB, or CMY if PhotometricInterpretation is 5 (CMYK). 4: CMYK. XResolution. RATIONAL YResolution. 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. 5.2.2. Extension fields Indexed = 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 SHORT Decode is used in connection with the ITULAB encoding of image data; see Section 4.2.2. 5.2.3. New Fields None. 5.3. Recommended TIFF fields See Sections 2.2.2. and 2.2.3. McIntyre & Zilles Expires 9/20/97 [Page 23] Internet Draft File Format for Internet Fax March 20, 1997 5.4. Lossless Color Fax Mode Summary Recommended fields are shown with an asterisk * +--------------------|--------------------------------------+ | 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 | +--------------------+--------------------------------------+ McIntyre & Zilles Expires 9/20/97 [Page 24] Internet Draft File Format for Internet Fax March 20, 1997 +--------------------+--------------------------------------+ | 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 | +--------------------+--------------------------------------+ | Decode | minL, maxL, mina, maxa, minb, maxb: | | |minimum and maximum values for CIELAB | +--------------------+--------------------------------------+ | 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 | +--------------------+--------------------------------------+ McIntyre & Zilles Expires 9/20/97 [Page 25] Internet Draft File Format for Internet Fax March 20, 1997 6. Mixed Raster Content Mode 6.1. 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. 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. 6.1.1. 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]. McIntyre & Zilles Expires 9/20/97 [Page 26] Internet Draft File Format for Internet Fax March 20, 1997 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. 6.1.2. 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 6.2.3. 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. McIntyre & Zilles Expires 9/20/97 [Page 27] Internet Draft File Format for Internet Fax March 20, 1997 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] NewSubFileType = 16 | |(nextIFD) | V Child IFD ImageLayer = [1,2] NewSubFileType = 16 | |(nextIFD) | V Child IFD ImageLayer = [3,1] NewSubFileType = 16 | |(nextIFD) | V 0 McIntyre & Zilles Expires 9/20/97 [Page 28] Internet Draft File Format for Internet Fax March 20, 1997 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 6.2.2. When there is only a single layer (Mask, Foreground, or Background), it is stored as the Primary IFD. 6.2. Required TIFF Fields This section describes the TIFF fields required 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. 6.2.1. Baseline fields ImageWidth. SHORT or LONG Same page widths as the base color mode; see Section 4.2.1. BitsPerSample = 1, 8, 9-16 SHORT Compression = 2, 3, 4, 7, 9. SHORT SamplesPerPixel = 1, 3, 4. SHORT FillOrder = 1, 2. SHORT PhotometricInterpretation = 0, 1, 2, 5, 10. SHORT For Mask layer, see Section 3.2.1. For Foreground and Background layers, see Sections 4.2.1 and 5.2.1. ColorMap. SHORT Count = 3 * (2**BitsPerSample) Used when Foreground or Background layer is a palette-color image; see Section 5.2.1. NewSubFileType = 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 McIntyre & Zilles Expires 9/20/97 [Page 29] Internet Draft File Format for Internet Fax March 20, 1997 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. LONG Tag = 559 (0x022F) 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. RATIONAL YResolution. 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. 6.2.2. Extension fields ChromaSubSampling. SHORT ChromaPositioning. SHORT Decode. SHORT For Foreground and Background layers, see Section 4.2.2. Indexed = 0, 1. SHORT For Foreground and Background layers: 1 indicates a palette- color image, see Section 5.2.2. T4Options = 0, 1, 4, 5. SHORT T6Options = 0. SHORT For Mask layer, see Section 3.2.2. DefaultImageColor. 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 McIntyre & Zilles Expires 9/20/97 [Page 30] Internet Draft File Format for Internet Fax March 20, 1997 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. XPosition. RATIONAL YPosition. 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. 6.2.3. New fields MRC Color fax mode requires one new field: ImageLayer. ImageLayer. SHORT or LONG. Count = 2 Tag = 0x87ac 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.) 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. McIntyre & Zilles Expires 9/20/97 [Page 31] Internet Draft File Format for Internet Fax March 20, 1997 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. 6.3. Recommended TIFF fields See Sections 2.2.2. and 2.2.3. 6.4. 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 4 and 5, 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. 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). McIntyre & Zilles Expires 9/20/97 [Page 32] Internet Draft File Format for Internet Fax March 20, 1997 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. 6.5. 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 | 2: Modified Huffman | | | 3: 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 | +------------------+-----------------------------------------+ | ImageDescription*| {ASCII}: A string describing the | | | contents of the image. | +------------------+-----------------------------------------+ McIntyre & Zilles Expires 9/20/97 [Page 33] Internet Draft File Format for Internet Fax March 20, 1997 +------------------+-----------------------------------------+ | 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 | +------------------+-----------------------------------------+ | T4Options | 0: required if Compression is | | | Modified Huffman, no fill bits | | | 1: required if Compression is 2D | | | Modified Read, no fill bits | McIntyre & Zilles Expires 9/20/97 [Page 34] Internet Draft File Format for Internet Fax March 20, 1997 | | 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 CIELAB | +------------------+-----------------------------------------+ | 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 9/20/97 [Page 35] Internet Draft File Format for Internet Fax March 20, 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 | +------------------+-----------------------------------------+ 7. 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. 8. 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 or a pair of letters. Three applications are proposed. McIntyre & Zilles Expires 9/20/97 [Page 36] Internet Draft File Format for Internet Fax March 20, 1997 B - multi-page bi-level monochrome images [RFC1314]. F - the current reference for the image/tiff subtype [RFC1528], with adds from work in progress [TIFF-F] FX - this specification, with ITU-fax-compatible parameter values 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: image/tiff, class=FX is 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 Person & e-mail address to contact for further information: Lloyd McIntyre Steve Zilles Intended usage: Common 9. Security Considerations Security issues have not been assessed in this document. 10. 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 9/20/97 [Page 37] Internet Draft File Format for Internet Fax March 20, 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 Draft Recommendation T.43, T.Palette-Colour - 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 [TIFF-F] work in progress, Tag Image File Format (TIFF) - Class F, Jan. 1997, ftp://ftp.ietf.org/internet-drafts/draft-ietf- fax-tiff-00.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 McIntyre & Zilles Expires 9/20/97 [Page 38] Internet Draft File Format for Internet Fax March 20, 1997 T.4, T.6, T.42, T.81, T.82 and T.85 are available at http://www.itu.ch. [MRC] is currently available at http://www.xerox.com/xis/mrc/ 11. 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 Annex A: Summary of TIFF Fields for Internet Fax This annex includes tables which list all relevant 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 relevant to 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 McIntyre & Zilles Expires 9/20/97 [Page 39] Internet Draft File Format for Internet Fax March 20, 1997 The absence of an entry indicates the field is not part of the current proposal. Table A.1 TIFF Baseline Fields +----------+-------------------------------------------------+ | | Fax Modes | +----------+-------------------------------------------------| | Field | Bi-level | Color | Color | Mixed Raster| | | | (Lossy) | (Lossless) | Content | +----------+-----------+----------+------------+-------------+ | BitsPer | | | | | | Sample | 1 | 8, 12 | 1, 8, 9-16 | 1, 8, 9-16 | +----------+-----------+----------+------------|-------------| | ColorMap | | | | | +----------+-----------+----------+------------|-------------| | Compres- | | | | | | sion | 2, 3, 4 | 7 | 9 |2, 3, 4, 7, 9| +----------+-----------+----------+------------+-------------+ | DateTime | {ASCII} | {ASCII} | {ASCII} |{ASCII} | +----------+-----------+--------- +------------|-------------| | FillOrder| 1, 2 | | 1, 2 | 1, 2 | +----------+-----------+----------+------------+-------------+ | ImageDes-| (ASCII} | {ASCII} | {ASCII} | {ASCII} | | cription | | | | | +----------+-----------+----------+------------+-------------| | Image- |1728, 2048 |864, 1024 |864, 1024 |864, 1024 | | Width |2432, 2592 |1216, 1728|1216, 1728 |1216, 1728 | | |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 | +----------+-----------+----------+------------+-------------+ | NewSub- | | | | | | FileType | 2 | 2 | 2 | 16, 18 | +----------+-----------+----------+------------+-------------+ | Orien- | | | | | | tation | 1 | 1 |1 | 1 | +----------+-----------+----------+------------+-------------+ | Photo- | | | | | | metric- | | | | | | Interp- | | | | | | retation | 0, 1 | 10 | 2, 5, 10 | 0,1,2,5,10 | +----------+-----------+----------+------------+-------------+ | Resolu- | | | | | | tionUnit | 2, 3 | 2, 3 | 2, 3 | 2, 3 | +----------+-----------+----------+------------+-------------+ McIntyre & Zilles Expires 9/20/97 [Page 40] Internet Draft File Format for Internet Fax March 20, 1997 +----------+-----------+----------+------------+-------------+ | RowsPer- | | | | | | Strip |n | n | n | | +----------+-----------+----------+------------+-------------+ | Samples- | | | | | | PerPixel | 1 | 1, 3 | 1, 3, 4 | 1, 3, 4 | +----------+-----------+----------+------------+-------------+ | Software | {ASCII} | {ASCII} | {ASCII} | {ASCII} | +----------+-----------+----------+------------+-------------+ | Strip- | | | | | | Byte- | | | | | | Counts | | | | | +----------+-----------+----------+------------+-------------+ | Strip- | | | | | | Offsets | | | | | +----------+-----------+----------+------------+-------------+ | XResolu- | 200, 204, | 100, 200,| 100, 200, | 100, 200, | | tion | 300, 400, | 300, 400 | 300, 400 | 300, 400 | | | 406 | | | | +----------+-----------+----------+------------+-------------+ | YResolu- | 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 Modes | +-----------+------------------------------------------------| | Field | Bi-level | Color | Color | Mixed Raster| | | | (Lossy) | (Lossless) | Content | +-----------+-----------+----------+----- ------+------------+ | T4Options | 0, 1, 4, 5| | | 0, 1, 4, 5 | +-----------+-----------+----------+------------+------------+ | T6Options | 0 | | | 0 | +-----------+-----------+----------+------------+------------+ | Document- | | | | | | Name | {ASCII} | {ASCII} | {ASCII} | {ASCII} | +-----------+-----------+----------+------------+------------+ | Page- | | | | | | Number | n, m | n, m | n, m | n, m | +-----------+-----------+----------+------------+------------+ | XPosition | | | | r | +-----------+-----------+----------+------------+------------+ | YPosition | | | | r | +-----------+-----------+----------+------------+------------+ McIntyre & Zilles Expires 9/20/97 [Page 41] Internet Draft File Format for Internet Fax March 20, 1997 +-----------+-----------+----------+------------+------------+ | SubIFDs | | | | | +-----------+-----------+----------+------------+------------+ | Indexed | | | 0, 1 | 0, 1 | +-----------+-----------+----------+------------+------------+ | 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 Modes | +-----------+------------------------------------------------| | Field | Bi-level | Color | Color | Mixed Raster| | | | (Lossy | (Lossless) | Content | +-----------+-----------+---------+------------+-------------+ | Global- | | | | | | Parame- | | | | | | tersIFD | | | | | +-----------+-----------+---------+------------+-------------+ | Profile- | | | | | | Type | n | n | n | n | +-----------+-----------+---------+------------+-------------+ | ITUFax- | | | | | | Mode | n | n | n | n | +-----------+-----------+---------+------------+-------------+ | Coding- | | | | | | Methods | n | n | n | n | +-----------+-----------+---------+------------+-------------+ | Version- | | | | | | Number | n | n |n | n | +-----------+-----------+---------+------------+-------------+ | Version- | | | | | | Year | | | | | +-----------+-----------+---------+------------+-------------+ McIntyre & Zilles Expires 9/20/97 [Page 42] Internet Draft File Format for Internet Fax March 20, 1997 +-----------+-----------+---------+------------+-------------+ | BadFax- | | | | | | Lines | n | | | | +-----------+----------------------------------+-------------+ | CleanFax- | | | | | | Data | 0, 1, 2 | | | | +-----------+-----------+---------+------------+-------------+ | Consecu- | | | | | | tiveBad- | | | | | | FaxLines | n | | | | +-----------+-----------+---------+------------+-------------+ | ImageLayer| | | | n, m | +-----------+-----------+---------+------------+-------------+ McIntyre & Zilles Expires 9/20/97 [Page 43]