FFV1 Video Coding Format Version 4

Introduction This document describes FFV1, a lossless video encoding format. The design of FFV1 considers the storage of image characteristics, data fixity, and the optimized use of encoding time and storage requirements. FFV1 is designed to support a wide range of lossless video applications such as long-term audiovisual preservation, scientific imaging, screen recording, and other video encoding scenarios that seek to avoid the generational loss of lossy video encodings. This document defines a version 4 of FFV1. Prior versions of FFV1 are defined within . The latest version of this document is available at https://raw.github.com/FFmpeg/FFV1/master/ffv1.md This document assumes familiarity with mathematical and coding concepts such as Range coding and YCbCr color spaces .

Notation and Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in .

Definitions Container: Format that encapsulates Frames (see the section on Frames) and (when required) a Configuration Record into a bitstream. Sample: The smallest addressable representation of a color component or a luma component in a Frame. Examples of Sample are Luma, Blue Chrominance, Red Chrominance, Transparency, Red, Green, and Blue. Plane: A discrete component of a static image comprised of Samples that represent a specific quantification of Samples of that image. Pixel: The smallest addressable representation of a color in a Frame. It is composed of 1 or more Samples. ESC: An ESCape symbol to indicate that the symbol to be stored is too large for normal storage and that an alternate storage method is used. MSB: Most Significant Bit, the bit that can cause the largest change in magnitude of the symbol. RCT: Reversible Color Transform, a near linear, exactly reversible integer transform that converts between RGB and YCbCr representations of a Pixel. VLC: Variable Length Code, a code that maps source symbols to a variable number of bits. RGB: A reference to the method of storing the value of a Pixel by using three numeric values that represent Red, Green, and Blue. YCbCr: A reference to the method of storing the value of a Pixel by using three numeric values that represent the luma of the Pixel (Y) and the chrominance of the Pixel (Cb and Cr). YCbCr word is used for historical reasons and currently references any color space relying on 1 luma Sample and 2 chrominance Samples, e.g. YCbCr, YCgCo or ICtCp. The exact meaning of the three numeric values is unspecified. TBA: To Be Announced. Used in reference to the development of future iterations of the FFV1 specification.

Conventions

Pseudo-code The FFV1 bitstream is described in this document using pseudo-code. Note that the pseudo-code is used for clarity in order to illustrate the structure of FFV1 and not intended to specify any particular implementation. The pseudo-code used is based upon the C programming language and uses its if/else, while and for functions as well as functions defined within this document.

Arithmetic Operators Note: the operators and the order of precedence are the same as used in the C programming language . a + b means a plus b. a - b means a minus b. -a means negation of a. a * b means a multiplied by b. a / b means a divided by b. a ^ b means a raised to the b-th power. a & b means bit-wise "and" of a and b. a | b means bit-wise "or" of a and b. a >> b means arithmetic right shift of two’s complement integer representation of a by b binary digits. a << b means arithmetic left shift of two’s complement integer representation of a by b binary digits.

Assignment Operators a = b means a is assigned b. a++ is equivalent to a is assigned a + 1. a-- is equivalent to a is assigned a - 1. a += b is equivalent to a is assigned a + b. a -= b is equivalent to a is assigned a - b. a *= b is equivalent to a is assigned a * b.

Comparison Operators a > b means a is greater than b. a >= b means a is greater than or equal to b. a < b means a is less than b. a <= b means a is less than or equal b. a == b means a is equal to b. a != b means a is not equal to b. a && b means Boolean logical "and" of a and b. a || b means Boolean logical "or" of a and b. !a means Boolean logical "not" of a. a ? b : c if a is true, then b, otherwise c.

Mathematical Functions floor(a) the largest integer less than or equal to a ceil(a) the smallest integer greater than or equal to a sign(a) extracts the sign of a number, i.e. if a < 0 then -1, else if a > 0 then 1, else 0 abs(a) the absolute value of a, i.e. abs(a) = sign(a)*a log2(a) the base-two logarithm of a min(a,b) the smallest of two values a and b max(a,b) the largest of two values a and b median(a,b,c) the numerical middle value in a data set of a, b, and c, i.e. a+b+c-min(a,b,c)-max(a,b,c) a_{b} the b-th value of a sequence of a a_{b,c} the 'b,c'-th value of a sequence of a

Order of Operation Precedence When order of precedence is not indicated explicitly by use of parentheses, operations are evaluated in the following order (from top to bottom, operations of same precedence being evaluated from left to right). This order of operations is based on the order of operations used in Standard C. a++, a-- !a, -a a ^ b a * b, a / b, a % b a + b, a - b a << b, a >> b a < b, a <= b, a > b, a >= b a == b, a != b a & b a | b a && b a || b a ? b : c a = b, a += b, a -= b, a *= b

Range a...b means any value starting from a to b, inclusive.

NumBytes NumBytes is a non-negative integer that expresses the size in 8-bit octets of a particular FFV1 Configuration Record or Frame. FFV1 relies on its Container to store the NumBytes values, see the section on the Mapping FFV1 into Containers.

Bitstream Functions

remaining_bits_in_bitstream remaining_bits_in_bitstream( ) means the count of remaining bits after the pointer in that Configuration Record or Frame. It is computed from the NumBytes value multiplied by 8 minus the count of bits of that Configuration Record or Frame already read by the bitstream parser.

remaining_symbols_in_syntax remaining_symbols_in_syntax( ) is true as long as the RangeCoder has not consumed all the given input bytes.

byte_aligned byte_aligned( ) is true if remaining_bits_in_bitstream( NumBytes ) is a multiple of 8, otherwise false.

get_bits get_bits( i ) is the action to read the next i bits in the bitstream, from most significant bit to least significant bit, and to return the corresponding value. The pointer is increased by i.

Sample Coding For each Slice (as described in the section on Slices) of a Frame, the Planes, Lines, and Samples are coded in an order determined by the Color Space (see the section on Color Space). Each Sample is predicted by the median predictor as described in the section of the Median Predictor from other Samples within the same Plane and the difference is stored using the method described in Coding of the Sample Difference.

Border A border is assumed for each coded Slice for the purpose of the median predictor and context according to the following rules:

one column of Samples to the left of the coded slice is assumed as identical to the Samples of the leftmost column of the coded slice shifted down by one row. The value of the topmost Sample of the column of Samples to the left of the coded slice is assumed to be 0
one column of Samples to the right of the coded slice is assumed as identical to the Samples of the rightmost column of the coded slice
an additional column of Samples to the left of the coded slice and two rows of Samples above the coded slice are assumed to be 0

The following table depicts a slice of 9 Samples a,b,c,d,e,f,g,h,i in a 3x3 arrangement along with its assumed border. +---+---+---+---+---+---+---+---+ | 0 | 0 | | 0 | 0 | 0 | | 0 | +---+---+---+---+---+---+---+---+ | 0 | 0 | | 0 | 0 | 0 | | 0 | +---+---+---+---+---+---+---+---+ | | | | | | | | | +---+---+---+---+---+---+---+---+ | 0 | 0 | | a | b | c | | c | +---+---+---+---+---+---+---+---+ | 0 | a | | d | e | f | | f | +---+---+---+---+---+---+---+---+ | 0 | d | | g | h | i | | i | +---+---+---+---+---+---+---+---+

Samples Relative to any Sample X, six other relatively positioned Samples from the coded Samples and presumed border are identified according to the labels used in the following diagram. The labels for these relatively positioned Samples are used within the median predictor and context. +---+---+---+---+ | | | T | | +---+---+---+---+ | |tl | t |tr | +---+---+---+---+ | L | l | X | | +---+---+---+---+ The labels for these relative Samples are made of the first letters of the words Top, Left and Right.

Median Predictor The prediction for any Sample value at position X may be computed based upon the relative neighboring values of l, t, and tl via this equation: median(l, t, l + t - tl). Note, this prediction template is also used in and . Exception for the median predictor: if colorspace_type == 0 && bits_per_raw_sample == 16 && ( coder_type == 1 || coder_type == 2 ), the following median predictor MUST be used: median(left16s, top16s, left16s + top16s - diag16s) where: left16s = l >= 32768 ? ( l - 65536 ) : l top16s = t >= 32768 ? ( t - 65536 ) : t diag16s = tl >= 32768 ? ( tl - 65536 ) : tl Background: a two's complement signed 16-bit signed integer was used for storing Sample values in all known implementations of FFV1 bitstream. So in some circumstances, the most significant bit was wrongly interpreted (used as a sign bit instead of the 16th bit of an unsigned integer). Note that when the issue is discovered, the only configuration of all known implementations being impacted is 16-bit YCbCr with no Pixel transformation with Range Coder coder, as other potentially impacted configurations (e.g. 15/16-bit JPEG2000-RCT with Range Coder coder, or 16-bit content with Golomb Rice coder) were implemented nowhere . In the meanwhile, 16-bit JPEG2000-RCT with Range Coder coder was implemented without this issue in one implementation and validated by one conformance checker. It is expected (to be confirmed) to remove this exception for the median predictor in the next version of the FFV1 bitstream.

Context Relative to any Sample X, the Quantized Sample Differences L-l, l-tl, tl-t, T-t, and t-tr are used as context: context = Q_{0}[l − tl] + Q_{1}[tl − t] + Q_{2}[t − tr] + Q_{3}[L − l] + Q_{4}[T − t] If context >= 0 then context is used and the difference between the Sample and its predicted value is encoded as is, else -context is used and the difference between the Sample and its predicted value is encoded with a flipped sign.

Quantization Table Sets The FFV1 bitstream contains 1 or more Quantization Table Sets. Each Quantization Table Set contains exactly 5 Quantization Tables with each Quantization Table corresponding to 1 of the 5 Quantized Sample Differences. For each Quantization Table, both the number of quantization steps and their distribution are stored in the FFV1 bitstream; each Quantization Table has exactly 256 entries, and the 8 least significant bits of the Quantized Sample Difference are used as index: Q_{j}[k] = quant_tables[i][j][k&255] In this formula, i is the Quantization Table Set index, j is the Quantized Table index, k the Quantized Sample Difference.

Quantization Table Set Indexes For each Plane of each slice, a Quantization Table Set is selected from an index:

For Y Plane, quant_table_set_index[ 0 ] index is used
For Cb and Cr Planes, quant_table_set_index[ 1 ] index is used
For extra Plane, quant_table_set_index[ (version <= 3 || chroma_planes) ? 2 : 1 ] index is used

Background: in first implementations of FFV1 bitstream, the index for Cb and Cr Planes was stored even if it is not used (chroma_planes set to 0), this index is kept for version <= 3 in order to keep compatibility with FFV1 bitstreams in the wild.

Color spaces FFV1 supports several color spaces. The count of allowed coded planes and the meaning of the extra Plane are determined by the selected color space. The FFV1 bitstream interleaves data in an order determined by the color space. In YCbCr for each Plane, each Line is coded from top to bottom and for each Line, each Sample is coded from left to right. In JPEG2000-RCT for each Line from top to bottom, each Plane is coded and for each Plane, each Sample is encoded from left to right.

YCbCr This color space allows 1 to 4 Planes. The Cb and Cr Planes are optional, but if used then MUST be used together. Omitting the Cb and Cr Planes codes the frames in grayscale without color data. An optional transparency Plane can be used to code transparency data. An FFV1 Frame using YCbCr MUST use one of the following arrangements:

Y
Y, Transparency
Y, Cb, Cr
Y, Cb, Cr, Transparency

The Y Plane MUST be coded first. If the Cb and Cr Planes are used then they MUST be coded after the Y Plane. If a transparency Plane is used, then it MUST be coded last.

RGB This color space allows 3 or 4 Planes. An optional transparency Plane can be used to code transparency data. JPEG2000-RCT is a Reversible Color Transform that codes RGB (red, green, blue) Planes losslessly in a modified YCbCr color space . Reversible Pixel transformations between YCbCr and RGB use the following formulae. Cb=b-g Cr=r-g Y=g+(Cb+Cr)>>2 g=Y-(Cb+Cr)>>2 r=Cr+g b=Cb+g Exception for the JPEG2000-RCT conversion: if bitsperrawsample is between 9 and 15 inclusive and extraplane is 0, the following formulae for reversible conversions between YCbCr and RGB MUST be used instead of the ones above: Cb=g-b Cr=r-b Y=b+(Cb+Cr)>>2 b=Y-(Cb+Cr)>>2 r=Cr+b g=Cb+b Background: At the time of this writing, in all known implementations of FFV1 bitstream, when bitsperrawsample was between 9 and 15 inclusive and extraplane is 0, GBR Planes were used as BGR Planes during both encoding and decoding. In the meanwhile, 16-bit JPEG2000-RCT was implemented without this issue in one implementation and validated by one conformance checker. Methods to address this exception for the transform are under consideration for the next version of the FFV1 bitstream. When FFV1 uses the JPEG2000-RCT, the horizontal Lines are interleaved to improve caching efficiency since it is most likely that the JPEG2000-RCT will immediately be converted to RGB during decoding. The interleaved coding order is also Y, then Cb, then Cr, and then if used transparency. As an example, a Frame that is two Pixels wide and two Pixels high, could be comprised of the following structure: +------------------------+------------------------+ | Pixel[1,1] | Pixel[2,1] | | Y[1,1] Cb[1,1] Cr[1,1] | Y[2,1] Cb[2,1] Cr[2,1] | +------------------------+------------------------+ | Pixel[1,2] | Pixel[2,2] | | Y[1,2] Cb[1,2] Cr[1,2] | Y[2,2] Cb[2,2] Cr[2,2] | +------------------------+------------------------+ In JPEG2000-RCT, the coding order would be left to right and then top to bottom, with values interleaved by Lines and stored in this order: Y[1,1] Y[2,1] Cb[1,1] Cb[2,1] Cr[1,1] Cr[2,1] Y[1,2] Y[2,2] Cb[1,2] Cb[2,2] Cr[1,2] Cr[2,2]

Coding of the Sample Difference Instead of coding the n+1 bits of the Sample Difference with Huffman or Range coding (or n+2 bits, in the case of JPEG2000-RCT), only the n (or n+1, in the case of JPEG2000-RCT) least significant bits are used, since this is sufficient to recover the original Sample. In the equation below, the term "bits" represents bitsperrawsample+1 for JPEG2000-RCT or bitsperrawsample otherwise: coder_input = [(sample_difference + 2^(bits−1)) & (2^bits − 1)] − 2^(bits−1)

Range Coding Mode Early experimental versions of FFV1 used the CABAC Arithmetic coder from H.264 as defined in but due to the uncertain patent/royalty situation, as well as its slightly worse performance, CABAC was replaced by a Range coder based on an algorithm defined by G. Nigel and N. Martin in 1979 .

Range Binary Values To encode binary digits efficiently a Range coder is used. C_{i} is the i-th Context. B_{i} is the i-th byte of the bytestream. b_{i} is the i-th Range coded binary value, S_{0,i} is the i-th initial state. The length of the bytestream encoding n binary symbols is j_{n} bytes. r_{i} = floor( ( R_{i} * S_{i,C_{i}} ) / 2^8 ) S_{i+1,C_{i}} = zero_state_{S_{i,C_{i}}} XOR l_i = L_i XOR t_i = R_i - r_i <== b_i = 0 <==> L_i < R_i - r_i S_{i+1,C_{i}} = one_state_{S_{i,C_{i}}} XOR l_i = L_i - R_i + r_i XOR t_i = r_i <== b_i = 1 <==> L_i >= R_i - r_i S_{i+1,k} = S_{i,k} <== C_i != k R_{i+1} = 2^8 * t_{i} XOR L_{i+1} = 2^8 * l_{i} + B_{j_{i}} XOR j_{i+1} = j_{i} + 1 <== t_{i} < 2^8 R_{i+1} = t_{i} XOR L_{i+1} = l_{i} XOR j_{i+1} = j_{i} <== t_{i} >= 2^8 R_{0} = 65280 L_{0} = 2^8 * B_{0} + B_{1} j_{0} = 2

Termination The range coder can be used in 3 modes.

In Open mode when decoding, every symbol the reader attempts to read is available. In this mode arbitrary data can have been appended without affecting the range coder output. This mode is not used in FFV1.
In Closed mode the length in bytes of the bytestream is provided to the range decoder. Bytes beyond the length are read as 0 by the range decoder. This is generally 1 byte shorter than the open mode.
In Sentinel mode the exact length in bytes is not known and thus the range decoder MAY read into the data that follows the range coded bytestream by one byte. In Sentinel mode, the end of the range coded bytestream is a binary symbol with state 129, which value SHALL be discarded. After reading this symbol, the range decoder will have read one byte beyond the end of the range coded bytestream. This way the byte position of the end can be determined. Bytestreams written in Sentinel mode can be read in Closed mode if the length can be determined, in this case the last (sentinel) symbol will be read non-corrupted and be of value 0.

Above describes the range decoding, encoding is defined as any process which produces a decodable bytestream. There are 3 places where range coder termination is needed in FFV1. First is in the Configuration Record, in this case the size of the range coded bytestream is known and handled as Closed mode. Second is the switch from the Slice Header which is range coded to Golomb coded slices as Sentinel mode. Third is the end of range coded Slices which need to terminate before the CRC at their end. This can be handled as Sentinel mode or as Closed mode if the CRC position has been determined.

Range Non Binary Values To encode scalar integers, it would be possible to encode each bit separately and use the past bits as context. However that would mean 255 contexts per 8-bit symbol that is not only a waste of memory but also requires more past data to reach a reasonably good estimate of the probabilities. Alternatively assuming a Laplacian distribution and only dealing with its variance and mean (as in Huffman coding) would also be possible, however, for maximum flexibility and simplicity, the chosen method uses a single symbol to encode if a number is 0, and if not, encodes the number using its exponent, mantissa and sign. The exact contexts used are best described by the following code, followed by some comments. pseudo-code | type --------------------------------------------------------------|----- void put_symbol(RangeCoder *c, uint8_t *state, int v, int \ | is_signed) { | int i; | put_rac(c, state+0, !v); | if (v) { | int a= abs(v); | int e= log2(a); | | for (i = 0; i < e; i++) { | put_rac(c, state+1+min(i,9), 1); //1..10 | } | | put_rac(c, state+1+min(i,9), 0); | for (i = e-1; i >= 0; i--) { | put_rac(c, state+22+min(i,9), (a>>i)&1); //22..31 | } | | if (is_signed) { | put_rac(c, state+11 + min(e, 10), v < 0); //11..21| } | } | } |

Initial Values for the Context Model At keyframes all Range coder state variables are set to their initial state.

State Transition Table one_state_{i} = default_state_transition_{i} + state_transition_delta_{i} zero_state_{i} = 256 - one_state_{256-i}

default_state_transition 0, 0, 0, 0, 0, 0, 0, 0, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 94, 95, 96, 97, 98, 99,100,101,102,103, 104,105,106,107,108,109,110,111,112,113,114,114,115,116,117,118, 119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,133, 134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149, 150,151,152,152,153,154,155,156,157,158,159,160,161,162,163,164, 165,166,167,168,169,170,171,171,172,173,174,175,176,177,178,179, 180,181,182,183,184,185,186,187,188,189,190,190,191,192,194,194, 195,196,197,198,199,200,201,202,202,204,205,206,207,208,209,209, 210,211,212,213,215,215,216,217,218,219,220,220,222,223,224,225, 226,227,227,229,229,230,231,232,234,234,235,236,237,238,239,240, 241,242,243,244,245,246,247,248,248, 0, 0, 0, 0, 0, 0, 0,

Alternative State Transition Table The alternative state transition table has been built using iterative minimization of frame sizes and generally performs better than the default. To use it, the coder_type (see the section on coder_type) MUST be set to 2 and the difference to the default MUST be stored in the Parameters, see the section on Parameters. The reference implementation of FFV1 in FFmpeg uses this table by default at the time of this writing when Range coding is used. 0, 10, 10, 10, 10, 16, 16, 16, 28, 16, 16, 29, 42, 49, 20, 49, 59, 25, 26, 26, 27, 31, 33, 33, 33, 34, 34, 37, 67, 38, 39, 39, 40, 40, 41, 79, 43, 44, 45, 45, 48, 48, 64, 50, 51, 52, 88, 52, 53, 74, 55, 57, 58, 58, 74, 60,101, 61, 62, 84, 66, 66, 68, 69, 87, 82, 71, 97, 73, 73, 82, 75,111, 77, 94, 78, 87, 81, 83, 97, 85, 83, 94, 86, 99, 89, 90, 99,111, 92, 93,134, 95, 98,105, 98, 105,110,102,108,102,118,103,106,106,113,109,112,114,112,116,125, 115,116,117,117,126,119,125,121,121,123,145,124,126,131,127,129, 165,130,132,138,133,135,145,136,137,139,146,141,143,142,144,148, 147,155,151,149,151,150,152,157,153,154,156,168,158,162,161,160, 172,163,169,164,166,184,167,170,177,174,171,173,182,176,180,178, 175,189,179,181,186,183,192,185,200,187,191,188,190,197,193,196, 197,194,195,196,198,202,199,201,210,203,207,204,205,206,208,214, 209,211,221,212,213,215,224,216,217,218,219,220,222,228,223,225, 226,224,227,229,240,230,231,232,233,234,235,236,238,239,237,242, 241,243,242,244,245,246,247,248,249,250,251,252,252,253,254,255,

Golomb Rice Mode The end of the bitstream of the Frame is filled with 0-bits until that the bitstream contains a multiple of 8 bits.

Signed Golomb Rice Codes This coding mode uses Golomb Rice codes. The VLC is split into 2 parts, the prefix stores the most significant bits and the suffix stores the k least significant bits or stores the whole number in the ESC case. pseudo-code | type --------------------------------------------------------------|----- int get_ur_golomb(k) { | for (prefix = 0; prefix < 12; prefix++) { | if (get_bits(1)) { | return get_bits(k) + (prefix << k) | } | } | return get_bits(bits) + 11 | } | | int get_sr_golomb(k) { | v = get_ur_golomb(k); | if (v & 1) return - (v >> 1) - 1; | else return (v >> 1); | }

Prefix

bits	value
1	0
01	1
...	...
0000 0000 0001	11
0000 0000 0000	ESC

Suffix


non ESC	the k least significant bits MSB first
ESC	the value - 11, in MSB first order, ESC may only be used if the value cannot be coded as non ESC

Examples

k	bits	value
0	`1`	0
0	`001`	2
2	`1 00`	0
2	`1 10`	2
2	`01 01`	5
any	`000000000000 10000000`	139

Run Mode Run mode is entered when the context is 0 and left as soon as a non-0 difference is found. The level is identical to the predicted one. The run and the first different level are coded.

Run Length Coding The run value is encoded in 2 parts, the prefix part stores the more significant part of the run as well as adjusting the run_index that determines the number of bits in the less significant part of the run. The 2nd part of the value stores the less significant part of the run as it is. The run_index is reset for each Plane and slice to 0. pseudo-code | type --------------------------------------------------------------|----- log2_run[41]={ | 0, 0, 0, 0, 1, 1, 1, 1, | 2, 2, 2, 2, 3, 3, 3, 3, | 4, 4, 5, 5, 6, 6, 7, 7, | 8, 9,10,11,12,13,14,15, | 16,17,18,19,20,21,22,23, | 24, | }; | | if (run_count == 0 && run_mode == 1) { | if (get_bits(1)) { | run_count = 1 << log2_run[run_index]; | if (x + run_count <= w) { | run_index++; | } | } else { | if (log2_run[run_index]) { | run_count = get_bits(log2_run[run_index]); | } else { | run_count = 0; | } | if (run_index) { | run_index--; | } | run_mode = 2; | } | } | The log2_run function is also used within .

Level Coding Level coding is identical to the normal difference coding with the exception that the 0 value is removed as it cannot occur: diff = get_vlc_symbol(context_state); if (diff >= 0) { diff++; } Note, this is different from JPEG-LS, which doesn’t use prediction in run mode and uses a different encoding and context model for the last difference On a small set of test Samples the use of prediction slightly improved the compression rate.

Scalar Mode Each difference is coded with the per context mean prediction removed and a per context value for k. get_vlc_symbol(state) { i = state->count; k = 0; while (i < state->error_sum) { k++; i += i; } v = get_sr_golomb(k); if (2 * state->drift < -state->count) { v = -1 - v; } ret = sign_extend(v + state->bias, bits); state->error_sum += abs(v); state->drift += v; if (state->count == 128) { state->count >>= 1; state->drift >>= 1; state->error_sum >>= 1; } state->count++; if (state->drift <= -state->count) { state->bias = max(state->bias - 1, -128); state->drift = max(state->drift + state->count, -state->count + 1); } else if (state->drift > 0) { state->bias = min(state->bias + 1, 127); state->drift = min(state->drift - state->count, 0); } return ret; }

Initial Values for the VLC context state At keyframes all coder state variables are set to their initial state. drift = 0; error_sum = 4; bias = 0; count = 1;

Bitstream An FFV1 bitstream is composed of a series of 1 or more Frames and (when required) a Configuration Record. Within the following sub-sections, pseudo-code is used to explain the structure of each FFV1 bitstream component, as described in the section on Pseudo-Code. The following table lists symbols used to annotate that pseudo-code in order to define the storage of the data referenced in that line of pseudo-code.

Symbol	Definition
u(n)	unsigned big endian integer using n bits
sg	Golomb Rice coded signed scalar symbol coded with the method described in Signed Golomb Rice Codes
br	Range coded Boolean (1-bit) symbol with the method described in Range binary values
ur	Range coded unsigned scalar symbol coded with the method described in Range non binary values
sr	Range coded signed scalar symbol coded with the method described in Range non binary values

The same context that is initialized to 128 is used for all fields in the header. The following MUST be provided by external means during initialization of the decoder: frame_pixel_width is defined as Frame width in Pixels. frame_pixel_height is defined as Frame height in Pixels. Default values at the decoder initialization phase: ConfigurationRecordIsPresent is set to 0.

Parameters The Parameters section contains significant characteristics about the decoding configuration used for all instances of Frame (in FFV1 version 0 and 1) or the whole FFV1 bitstream (other versions), including the stream version, color configuration, and quantization tables. The pseudo-code below describes the contents of the bitstream. pseudo-code | type --------------------------------------------------------------|----- Parameters( ) { | version | ur if (version >= 3) { | micro_version | ur } | coder_type | ur if (coder_type > 1) { | for (i = 1; i < 256; i++) { | state_transition_delta[ i ] | sr } | } | colorspace_type | ur if (version >= 1) { | bits_per_raw_sample | ur } | chroma_planes | br log2_h_chroma_subsample | ur log2_v_chroma_subsample | ur extra_plane | br if (version >= 3) { | num_h_slices - 1 | ur num_v_slices - 1 | ur quant_table_set_count | ur } | for (i = 0; i < quant_table_set_count; i++) { | QuantizationTableSet( i ) | } | if (version >= 3) { | for (i = 0; i < quant_table_set_count; i++) { | states_coded | br if (states_coded) { | for (j = 0; j < context_count[ i ]; j++) { | for (k = 0; k < CONTEXT_SIZE; k++) { | initial_state_delta[ i ][ j ][ k ] | sr } | } | } | } | ec | ur intra | ur } | } |

version version specifies the version of the FFV1 bitstream. Each version is incompatible with other versions: decoders SHOULD reject a file due to an unknown version. Decoders SHOULD reject a file with version <= 1 && ConfigurationRecordIsPresent == 1. Decoders SHOULD reject a file with version >= 3 && ConfigurationRecordIsPresent == 0.

value	version
0	FFV1 version 0
1	FFV1 version 1
2	reserved*
3	FFV1 version 3
4	FFV1 version 4
Other	reserved for future use

* Version 2 was never enabled in the encoder thus version 2 files SHOULD NOT exist, and this document does not describe them to keep the text simpler.

micro_version micro_version specifies the micro-version of the FFV1 bitstream. After a version is considered stable (a micro-version value is assigned to be the first stable variant of a specific version), each new micro-version after this first stable variant is compatible with the previous micro-version: decoders SHOULD NOT reject a file due to an unknown micro-version equal or above the micro-version considered as stable. Meaning of micro_version for version 3:

value	micro_version
0...3	reserved*
4	first stable variant
Other	reserved for future use

* development versions may be incompatible with the stable variants. Meaning of micro_version for version 4 (note: at the time of writing of this specification, version 4 is not considered stable so the first stable version value is to be announced in the future):

value	micro_version
0...TBA	reserved*
TBA	first stable variant
Other	reserved for future use

* development versions which may be incompatible with the stable variants.

coder_type coder_type specifies the coder used.

value	coder used
0	Golomb Rice
1	Range Coder with default state transition table
2	Range Coder with custom state transition table
Other	reserved for future use

state_transition_delta state_transition_delta specifies the Range coder custom state transition table. If statetransitiondelta is not present in the FFV1 bitstream, all Range coder custom state transition table elements are assumed to be 0.

colorspace_type colorspace_type specifies the color space encoded, the pixel transformation used by the encoder, the extra plane content, as well as interleave method.

value	color space encoded	pixel transformation	extra plane content	interleave method
0	YCbCr	None	Transparency	`Plane` then `Line`
1	RGB	JPEG2000-RCT	Transparency	`Line` then `Plane`
Other	reserved for future use	reserved for future use	reserved for future use	reserved for future use

Restrictions: If colorspace_type is 1, then chroma_planes MUST be 1, log2_h_chroma_subsample MUST be 0, and log2_v_chroma_subsample MUST be 0.

chroma_planes chroma_planes indicates if chroma (color) Planes are present.

value	presence
0	chroma `Planes` are not present
1	chroma `Planes` are present

bits_per_raw_sample bits_per_raw_sample indicates the number of bits for each Sample. Inferred to be 8 if not present.

value	bits for each sample
0	reserved*
Other	the actual bits for each `Sample`

* Encoders MUST NOT store bitsperrawsample = 0 Decoders SHOULD accept and interpret bitsperrawsample = 0 as 8.

log2_h_chroma_subsample log2_h_chroma_subsample indicates the subsample factor, stored in powers to which the number 2 must be raised, between luma and chroma width (chroma_width = 2^(-log2_h_chroma_subsample) * luma_width).

log2_v_chroma_subsample log2_v_chroma_subsample indicates the subsample factor, stored in powers to which the number 2 must be raised, between luma and chroma height (chroma_height=2^(-log2_v_chroma_subsample) * luma_height).

extra_plane extra_plane indicates if an extra Plane is present.

value	presence
0	extra `Plane` is not present
1	extra `Plane` is present

num_h_slices num_h_slices indicates the number of horizontal elements of the slice raster. Inferred to be 1 if not present.

num_v_slices num_v_slices indicates the number of vertical elements of the slice raster. Inferred to be 1 if not present.

quant_table_set_count quant_table_set_count indicates the number of Quantization Table Sets. Inferred to be 1 if not present. MUST NOT be 0.

states_coded states_coded indicates if the respective Quantization Table Set has the initial states coded. Inferred to be 0 if not present.

value	initial states
0	initial states are not present and are assumed to be all 128
1	initial states are present

initial_state_delta initial_state_delta[ i ][ j ][ k ] indicates the initial Range coder state, it is encoded using k as context index and pred = j ? initial_states[ i ][j - 1][ k ] : 128 initial_state[ i ][ j ][ k ] = ( pred + initial_state_delta[ i ][ j ][ k ] ) & 255

ec ec indicates the error detection/correction type.

value	error detection/correction type
0	32-bit CRC on the global header
1	32-bit CRC per slice and the global header
Other	reserved for future use

intra intra indicates the relationship between the instances of Frame. Inferred to be 0 if not present.

value	relationship
0	Frames are independent or dependent (keyframes and non keyframes)
1	Frames are independent (keyframes only)
Other	reserved for future use

Configuration Record In the case of a FFV1 bitstream with version >= 3, a Configuration Record is stored in the underlying Container, at the track header level. It contains the Parameters used for all instances of Frame. The size of the Configuration Record, NumBytes, is supplied by the underlying Container. pseudo-code | type --------------------------------------------------------------|----- ConfigurationRecord( NumBytes ) { | ConfigurationRecordIsPresent = 1 | Parameters( ) | while (remaining_symbols_in_syntax(NumBytes - 4)) { | reserved_for_future_use | br/ur/sr } | configuration_record_crc_parity | u(32) } |

reserved_for_future_use reserved_for_future_use has semantics that are reserved for future use. Encoders conforming to this version of this specification SHALL NOT write this value. Decoders conforming to this version of this specification SHALL ignore its value.

configuration_record_crc_parity configuration_record_crc_parity 32 bits that are chosen so that the Configuration Record as a whole has a crc remainder of 0. This is equivalent to storing the crc remainder in the 32-bit parity. The CRC generator polynomial used is the standard IEEE CRC polynomial (0x104C11DB7) with initial value 0.

Mapping FFV1 into Containers This Configuration Record can be placed in any file format supporting Configuration Records, fitting as much as possible with how the file format uses to store Configuration Records. The Configuration Record storage place and NumBytes are currently defined and supported by this version of this specification for the following formats:

AVI File Format The Configuration Record extends the stream format chunk ("AVI ", "hdlr", "strl", "strf") with the ConfigurationRecord bitstream. See for more information about chunks. NumBytes is defined as the size, in bytes, of the strf chunk indicated in the chunk header minus the size of the stream format structure.

ISO Base Media File Format The Configuration Record extends the sample description box ("moov", "trak", "mdia", "minf", "stbl", "stsd") with a "glbl" box that contains the ConfigurationRecord bitstream. See for more information about boxes. NumBytes is defined as the size, in bytes, of the "glbl" box indicated in the box header minus the size of the box header.

NUT File Format The codec_specific_data element (in "stream_header" packet) contains the ConfigurationRecord bitstream. See for more information about elements. NumBytes is defined as the size, in bytes, of the codec_specific_data element as indicated in the "length" field of codec_specific_data

Matroska File Format FFV1 SHOULD use V_FFV1 as the Matroska Codec ID. For FFV1 versions 2 or less, the Matroska CodecPrivate Element SHOULD NOT be used. For FFV1 versions 3 or greater, the Matroska CodecPrivate Element MUST contain the FFV1 Configuration Record structure and no other data. See for more information about elements. NumBytes is defined as the Element Data Size of the CodecPrivate Element.

Frame A Frame is an encoded representation of a complete static image. The whole Frame is provided by the underlaying container. A Frame consists of the keyframe field, Parameters (if version <=1), and a sequence of independent slices. The pseudo-code below describes the contents of a Frame. pseudo-code | type --------------------------------------------------------------|----- Frame( NumBytes ) { | keyframe | br if (keyframe && !ConfigurationRecordIsPresent { | Parameters( ) | } | while (remaining_bits_in_bitstream( NumBytes )) { | Slice( ) | } | } | Architecture overview of slices in a Frame:


first slice header
first slice content
first slice footer
---------------------------------------------------------------
second slice header
second slice content
second slice footer
---------------------------------------------------------------
...
---------------------------------------------------------------
last slice header
last slice content
last slice footer

Slice A Slice is an independent spatial sub-section of a Frame that is encoded separately from an other region of the same Frame. The use of more than one Slice per Frame can be useful for taking advantage of the opportunities of multithreaded encoding and decoding. A Slice consists of a Slice Header (when relevant), a Slice Content, and a Slice Footer (when relevant). The pseudo-code below describes the contents of a Slice. pseudo-code | type --------------------------------------------------------------|----- Slice( ) { | if (version >= 3) { | SliceHeader( ) | } | SliceContent( ) | if (coder_type == 0) { | while (!byte_aligned()) { | padding | u(1) } | } | if (version <= 1) { | while (remaining_bits_in_bitstream( NumBytes ) != 0) {| reserved | u(1) } | } | if (version >= 3) { | SliceFooter( ) | } | } | padding specifies a bit without any significance and used only for byte alignment. MUST be 0. reserved specifies a bit without any significance in this revision of the specification and may have a significance in a later revision of this specification. Encoders SHOULD NOT fill these bits. Decoders SHOULD ignore these bits. Note in case these bits are used in a later revision of this specification: any revision of this specification SHOULD care about avoiding to add 40 bits of content after SliceContent for version 0 and 1 of the bitstream. Background: due to some non conforming encoders, some bitstreams where found with 40 extra bits corresponding to error_status and slice_crc_parity, a decoder conforming to the revised specification could not do the difference between a revised bitstream and a buggy bitstream.

Slice Header A Slice Header provides information about the decoding configuration of the Slice, such as its spatial position, size, and aspect ratio. The pseudo-code below describes the contents of the Slice Header. pseudo-code | type --------------------------------------------------------------|----- SliceHeader( ) { | slice_x | ur slice_y | ur slice_width - 1 | ur slice_height - 1 | ur for (i = 0; i < quant_table_set_index_count; i++) { | quant_table_set_index[ i ] | ur } | picture_structure | ur sar_num | ur sar_den | ur if (version >= 4) { | reset_contexts | br slice_coding_mode | ur } | } |

slice_x slice_x indicates the x position on the slice raster formed by numhslices. Inferred to be 0 if not present.

slice_y slice_y indicates the y position on the slice raster formed by numvslices. Inferred to be 0 if not present.

slice_width slice_width indicates the width on the slice raster formed by numhslices. Inferred to be 1 if not present.

slice_height slice_height indicates the height on the slice raster formed by numvslices. Inferred to be 1 if not present.

quant_table_set_index_count quant_table_set_index_count is defined as 1 + ( ( chroma_planes || version <= 3 ) ? 1 : 0 ) + ( extra_plane ? 1 : 0 ).

quant_table_set_index quant_table_set_index indicates the Quantization Table Set index to select the Quantization Table Set and the initial states for the slice. Inferred to be 0 if not present.

picture_structure picture_structure specifies the temporal and spatial relationship of each Line of the Frame. Inferred to be 0 if not present.

value	picture structure used
0	unknown
1	top field first
2	bottom field first
3	progressive
Other	reserved for future use

sar_num sar_num specifies the Sample aspect ratio numerator. Inferred to be 0 if not present. A value of 0 means that aspect ratio is unknown. Encoders MUST write 0 if Sample aspect ratio is unknown. If sar_den is 0, decoders SHOULD ignore the encoded value and consider that sar_num is 0.

sar_den sar_den specifies the Sample aspect ratio denominator. Inferred to be 0 if not present. A value of 0 means that aspect ratio is unknown. Encoders MUST write 0 if Sample aspect ratio is unknown. If sar_num is 0, decoders SHOULD ignore the encoded value and consider that sar_den is 0.

reset_contexts reset_contexts indicates if slice contexts must be reset. Inferred to be 0 if not present.

slice_coding_mode slice_coding_mode indicates the slice coding mode. Inferred to be 0 if not present.

value	slice coding mode
0	Range Coding or Golomb Rice
1	raw PCM
Other	reserved for future use

Slice Content A Slice Content contains all Line elements part of the Slice. Depending on the configuration, Line elements are ordered by Plane then by row (YCbCr) or by row then by Plane (RGB). pseudo-code | type --------------------------------------------------------------|----- SliceContent( ) { | if (colorspace_type == 0) { | for (p = 0; p < primary_color_count; p++) { | for (y = 0; y < plane_pixel_height[ p ]; y++) { | Line( p, y ) | } | } | } else if (colorspace_type == 1) { | for (y = 0; y < slice_pixel_height; y++) { | for (p = 0; p < primary_color_count; p++) { | Line( p, y ) | } | } | } | } |

primary_color_count primary_color_count is defined as 1 + ( chroma_planes ? 2 : 0 ) + ( extra_plane ? 1 : 0 ).

plane_pixel_height plane_pixel_height[ p ] is the height in pixels of plane p of the slice. plane_pixel_height[ 0 ] and plane_pixel_height[ 1 + ( chroma_planes ? 2 : 0 ) ] value is slice_pixel_height. If chroma_planes is set to 1, plane_pixel_height[ 1 ] and plane_pixel_height[ 2 ] value is ceil(slice_pixel_height / log2_v_chroma_subsample).

slice_pixel_height slice_pixel_height is the height in pixels of the slice. Its value is floor(( slice_y + slice_height ) * slice_pixel_height / num_v_slices) - slice_pixel_y.

slice_pixel_y slice_pixel_y is the slice vertical position in pixels. Its value is floor(slice_y * frame_pixel_height / num_v_slices).

Line A Line is a list of the sample differences (relative to the predictor) of primary color components. The pseudo-code below describes the contents of the Line. pseudo-code | type --------------------------------------------------------------|----- Line( p, y ) { | if (colorspace_type == 0) { | for (x = 0; x < plane_pixel_width[ p ]; x++) { | sample_difference[ p ][ y ][ x ] | } | } else if (colorspace_type == 1) { | for (x = 0; x < slice_pixel_width; x++) { | sample_difference[ p ][ y ][ x ] | } | } | } |

plane_pixel_width plane_pixel_width[ p ] is the width in Pixels of Plane p of the slice. plane_pixel_width[ 0 ] and plane_pixel_width[ 1 + ( chroma_planes ? 2 : 0 ) ] value is slice_pixel_width. If chroma_planes is set to 1, plane_pixel_width[ 1 ] and plane_pixel_width[ 2 ] value is ceil(slice_pixel_width / (1 << log2_h_chroma_subsample)).

slice_pixel_width slice_pixel_width is the width in Pixels of the slice. Its value is floor(( slice_x + slice_width ) * slice_pixel_width / num_h_slices) - slice_pixel_x.

slice_pixel_x slice_pixel_x is the slice horizontal position in Pixels. Its value is floor(slice_x * frame_pixel_width / num_h_slices).

sample_difference sample_difference[ p ][ y ][ x ] is the sample difference for Sample at Plane p, y position y, and x position x. The Sample value is computed based on median predictor and context described in the section on Samples.

Slice Footer A Slice Footer provides information about slice size and (optionally) parity. The pseudo-code below describes the contents of the Slice Footer. Note: Slice Footer is always byte aligned. pseudo-code | type --------------------------------------------------------------|----- SliceFooter( ) { | slice_size | u(24) if (ec) { | error_status | u(8) slice_crc_parity | u(32) } | } |

slice_size slice_size indicates the size of the slice in bytes. Note: this allows finding the start of slices before previous slices have been fully decoded, and allows parallel decoding as well as error resilience.

error_status error_status specifies the error status.

value	error status
0	no error
1	slice contains a correctable error
2	slice contains a uncorrectable error
Other	reserved for future use

slice_crc_parity slice_crc_parity 32 bits that are chosen so that the slice as a whole has a crc remainder of 0. This is equivalent to storing the crc remainder in the 32-bit parity. The CRC generator polynomial used is the standard IEEE CRC polynomial (0x104C11DB7) with initial value 0.

Quantization Table Set The Quantization Table Sets are stored by storing the number of equal entries -1 of the first half of the table (represented as len - 1 in the pseudo-code below) using the method described in Range Non Binary Values. The second half doesn’t need to be stored as it is identical to the first with flipped sign. scale and len_count[ i ][ j ] are temporary values used for the computing of context_count[ i ] and are not used outside Quantization Table Set pseudo-code. Example: Table: 0 0 1 1 1 1 2 2 -2 -2 -2 -1 -1 -1 -1 0 Stored values: 1, 3, 1 pseudo-code | type --------------------------------------------------------------|----- QuantizationTableSet( i ) { | scale = 1 | for (j = 0; j < MAX_CONTEXT_INPUTS; j++) { | QuantizationTable( i, j, scale ) | scale *= 2 * len_count[ i ][ j ] - 1 | } | context_count[ i ] = ceil( scale / 2 ) | } | MAX_CONTEXT_INPUTS is 5. pseudo-code | type --------------------------------------------------------------|----- QuantizationTable(i, j, scale) { | v = 0 | for (k = 0; k < 128;) { | len - 1 | ur for (a = 0; a < len; a++) { | quant_tables[ i ][ j ][ k ] = scale * v | k++ | } | v++ | } | for (k = 1; k < 128; k++) { | quant_tables[ i ][ j ][ 256 - k ] = \ | -quant_tables[ i ][ j ][ k ] | } | quant_tables[ i ][ j ][ 128 ] = \ | -quant_tables[ i ][ j ][ 127 ] | len_count[ i ][ j ] = v | } |

quant_tables quant_tables[ i ][ j ][ k ] indicates the quantification table value of the Quantized Sample Difference k of the Quantization Table j of the Set Quantization Table Set i.

context_count context_count[ i ] indicates the count of contexts for Quantization Table Set i.

Restrictions To ensure that fast multithreaded decoding is possible, starting with version 3 and if frame_pixel_width * frame_pixel_height is more than 101376, slice_width * slice_height MUST be less or equal to num_h_slices * num_v_slices / 4. Note: 101376 is the frame size in Pixels of a 352x288 frame also known as CIF ("Common Intermediate Format") frame size format. For each Frame, each position in the slice raster MUST be filled by one and only one slice of the Frame (no missing slice position, no slice overlapping). For each Frame with keyframe value of 0, each slice MUST have the same value of slice_x, slice_y, slice_width, slice_height as a slice in the previous Frame, except if reset_contexts is 1.

Security Considerations Like any other codec, (such as ), FFV1 should not be used with insecure ciphers or cipher-modes that are vulnerable to known plaintext attacks. Some of the header bits as well as the padding are easily predictable. Implementations of the FFV1 codec need to take appropriate security considerations into account, as outlined in . It is extremely important for the decoder to be robust against malicious payloads. Malicious payloads must not cause the decoder to overrun its allocated memory or to take an excessive amount of resources to decode. The same applies to the encoder, even though problems in encoders are typically rarer. Malicious video streams must not cause the encoder to misbehave because this would allow an attacker to attack transcoding gateways. A frequent security problem in image and video codecs is also to not check for integer overflows in Pixel count computations, that is to allocate width * height without considering that the multiplication result may have overflowed the arithmetic types range. The range coder could, if implemented naively, read one byte over the end. The implementation must ensure that no read outside allocated and initialized memory occurs. The reference implementation contains no known buffer overflow or cases where a specially crafted packet or video segment could cause a significant increase in CPU load. The reference implementation was validated in the following conditions:

Sending the decoder valid packets generated by the reference encoder and verifying that the decoder's output matches the encoder's input.
Sending the decoder packets generated by the reference encoder and then subjected to random corruption.
Sending the decoder random packets that are not FFV1.

In all of the conditions above, the decoder and encoder was run inside the memory debugger as well as clangs address sanitizer , which track reads and writes to invalid memory regions as well as the use of uninitialized memory. There were no errors reported on any of the tested conditions.

Media Type Definition This registration is done using the template defined in and following . Type name: video Subtype name: FFV1 Required parameters: None. Optional parameters: This parameter is used to signal the capabilities of a receiver implementation. This parameter MUST NOT be used for any other purpose. version: The version of the FFV1 encoding as defined by the section on version. micro_version: The micro_version of the FFV1 encoding as defined by the section on micro_version. coder_type: The coder_type of the FFV1 encoding as defined by the section on coder_type. colorspace_type: The colorspace_type of the FFV1 encoding as defined by the section on colorspace_type. bits_per_raw_sample: The bits_per_raw_sample of the FFV1 encoding as defined by the section on bits_per_raw_sample. max-slices: The value of max-slices is an integer indicating the maximum count of slices with a frames of the FFV1 encoding. Encoding considerations: This media type is defined for encapsulation in several audiovisual container formats and contains binary data; see the section on "Mapping FFV1 into Containers". This media type is framed binary data Section 4.8 of . Security considerations: See the "Security Considerations" section of this document. Interoperability considerations: None. Published specification: and RFC XXXX. [RFC Editor: Upon publication as an RFC, please replace "XXXX" with the number assigned to this document and remove this note.] Applications which use this media type: Any application that requires the transport of lossless video can use this media type. Some examples are, but not limited to screen recording, scientific imaging, and digital video preservation. Fragment identifier considerations: N/A. Additional information: None. Person & email address to contact for further information: Michael Niedermayer michael@niedermayer.cc Intended usage: COMMON Restrictions on usage: None. Author: Dave Rice dave@dericed.com Change controller: IETF cellar working group delegated from the IESG.

IANA Considerations The IANA is requested to register the following values:

Media type registration as described in Media Type Definition.

Appendixes

Decoder implementation suggestions

Multi-threading Support and Independence of Slices The FFV1 bitstream is parsable in two ways: in sequential order as described in this document or with the pre-analysis of the footer of each slice. Each slice footer contains a slice_size field so the boundary of each slice is computable without having to parse the slice content. That allows multi-threading as well as independence of slice content (a bitstream error in a slice header or slice content has no impact on the decoding of the other slices). After having checked keyframe field, a decoder SHOULD parse slice_size fields, from slice_size of the last slice at the end of the Frame up to slice_size of the first slice at the beginning of the Frame, before parsing slices, in order to have slices boundaries. A decoder MAY fallback on sequential order e.g. in case of a corrupted Frame (frame size unknown, slice_size of slices not coherent...) or if there is no possibility of seeking into the stream.

Changelog See https://github.com/FFmpeg/FFV1/commits/master