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RFC 9043

FFV1 Video Coding Format Versions 0, 1, and 3

Pages: ~51
IETF/art/cellar/draft-ietf-cellar-ffv1-20
Informational

Top   ToC   RFCv3-9043
M. Niedermayer
D. Rice
J. Martinez
August 2021

FFV1 Video Coding Format Versions 0, 1, and 3

Abstract

This document defines FFV1, a lossless, intra-frame video encoding format. FFV1 is designed to efficiently compress video data in a variety of pixel formats. Compared to uncompressed video, FFV1 offers storage compression, frame fixity, and self-description, which makes FFV1 useful as a preservation or intermediate video format.

Status of This Memo

This document is not an Internet Standards Track specification; it is published for informational purposes.
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are candidates for any level of Internet Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9043.

Copyright Notice

Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Top   ToC   RFCv3-9043

Table of Contents

Top   ToC   RFCv3-9043

1.  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 versions 0, 1, and 3 of FFV1. The distinctions of the versions are provided throughout the document, but in summary:
  • Version 0 of FFV1 was the original implementation of FFV1 and was flagged as stable on April 14, 2006 [FFV1_V0].
  • Version 1 of FFV1 adds support of more video bit depths and was flagged as stable on April 24, 2009 [FFV1_V1].
  • Version 2 of FFV1 only existed in experimental form and is not described by this document, but it is available as a LyX file at <https://github.com/FFmpeg/FFV1/blob/8ad772b6d61c3dd8b0171979a2cd9f11924d5532/ffv1.lyx>.
  • Version 3 of FFV1 adds several features such as increased description of the characteristics of the encoding images and embedded Cyclic Redundancy Check (CRC) data to support fixity verification of the encoding. Version 3 was flagged as stable on August 17, 2013 [FFV1_V3].
This document assumes familiarity with mathematical and coding concepts such as Range encoding [Range-Encoding] and YCbCr color spaces [YCbCr].
This specification describes the valid bitstream and how to decode it. Nonconformant bitstreams and the nonconformant handling of bitstreams are outside this specification. A decoder can perform any action that it deems appropriate for an invalid bitstream: reject the bitstream, attempt to perform error concealment, or re-download or use a redundant copy of the invalid part.
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2.  Notation and Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC 2119] [RFC 8174] when, and only when, they appear in all capitals, as shown here.

2.1.  Definitions

FFV1:
The chosen name of this video encoding format, which is the short version of "FF Video 1". The letters "FF" come from "FFmpeg", which is the name of the reference decoder whose first letters originally meant "Fast Forward".
Container:
A format that encapsulates Frames (see Section 4.4) 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 (Y), Blue-difference Chroma (Cb), Red-difference Chroma (Cr), Transparency, Red, Green, and Blue.
Symbol:
A value stored in the bitstream, which is defined and decoded through one of the methods described in Table 4.
Line:
A discrete component of a static image composed of Samples that represent a specific quantification of Samples of that image.
Plane:
A discrete component of a static image composed of Lines that represent a specific quantification of Lines of that image.
Pixel:
The smallest addressable representation of a color in a Frame. It is composed of one or more Samples.
MSB:
Most Significant Bit, the bit that can cause the largest change in magnitude of the symbol.
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 chroma of the pixel (Cb and Cr). The term YCbCr is used for historical reasons and currently references any color space relying on one luma Sample and two chroma Samples, e.g., YCbCr (luma, blue-difference chroma, red-difference chroma), YCgCo, or ICtCp (intensity, blue-yellow, red-green).

2.2.  Conventions

2.2.1.  Pseudocode

The FFV1 bitstream is described in this document using pseudocode. Note that the pseudocode is used to illustrate the structure of FFV1 and is not intended to specify any particular implementation. The pseudocode used is based upon the C programming language [ISO.9899.2018] and uses its if/else, while, and for keywords as well as functions defined within this document.
In some instances, pseudocode is presented in a two-column format such as shown in Figure 1. In this form, the type column provides a symbol as defined in Table 4 that defines the storage of the data referenced in that same line of pseudocode.
pseudocode                                                    | type
--------------------------------------------------------------|-----
ExamplePseudoCode( ) {                                        |
    value                                                     | ur
}                                                             |

2.2.2.  Arithmetic Operators

Note: the operators and the order of precedence are the same as used in the C programming language [ISO.9899.2018], with the exception of >> (removal of implementation-defined behavior) and ^ (power instead of XOR) operators, which are redefined within this section.
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 bitwise "and" of a and b.
a | b means bitwise "or" of a and b.
a >> b means arithmetic right shift of the two's complement integer representation of a by b binary digits. This is equivalent to dividing a by 2, b times, with rounding toward negative infinity.
a << b means arithmetic left shift of the two's complement integer representation of a by b binary digits.

2.2.3.  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.

2.2.4.  Comparison Operators

a > b is true when a is greater than b.
a >= b is true when a is greater than or equal to b.
a < b is true when a is less than b.
a <= b is true when a is less than or equal b.
a == b is true when a is equal to b.
a != b is true when a is not equal to b.
a && b is true when both a is true and b is true.
a || b is true when either a is true or b is true.
!a is true when a is not true.
a ? b : c if a is true, then b, otherwise c.

2.2.5.  Mathematical Functions

floor(a) means the largest integer less than or equal to a.
ceil(a) means 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) means the absolute value of a, i.e., abs(a) = sign(a) * a.
log2(a) means the base-two logarithm of a.
min(a,b) means the smaller of two values a and b.
max(a,b) means the larger of two values a and b.
median(a,b,c) means 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 means a implies b.
a <==> b means a ==> b, b ==> a.
ab means the b-th value of a sequence of a.
ab,c means the 'b,c'-th value of a sequence of a.

2.2.6.  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 : c
a = b, a += b, a -= b, a *= b

2.2.7.  Range

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

2.2.8.  NumBytes

NumBytes is a nonnegative 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 Section 4.3.3.

2.2.9.  Bitstream Functions

2.2.9.1.  remaining_bits_in_bitstream
remaining_bits_in_bitstream( NumBytes ) 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.
2.2.9.2.  remaining_symbols_in_syntax
remaining_symbols_in_syntax( ) is true as long as the range coder has not consumed all the given input bytes.
2.2.9.3.  byte_aligned
byte_aligned( ) is true if remaining_bits_in_bitstream( NumBytes ) is a multiple of 8, otherwise false.
2.2.9.4.  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.
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3.  Sample Coding

For each Slice (as described in Section 4.5) of a Frame, the Planes, Lines, and Samples are coded in an order determined by the color space (see Section 3.7). Each Sample is predicted by the median predictor as described in Section 3.3 from other Samples within the same Plane, and the difference is stored using the method described in Section 3.8.

3.1.  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.
Figure 2 depicts a Slice of nine Samples a,b,c,d,e,f,g,h,i in a three-by-three 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 |
+---+---+---+---+---+---+---+---+

3.2.  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 Figure 3. 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.

3.3.  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 that this prediction template is also used in [ISO.14495-1.1999] and [HuffYUV].

3.3.1.  Exception

If colorspace_type == 0 && bits_per_raw_sample == 16 && ( coder_type == 1 || coder_type == 2 ) (see Sections [4.2.5], [4.2.7], and [4.2.3]), 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 16-bit signed integer was used for storing Sample values in all known implementations of FFV1 bitstream (see Appendix C). 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 was discovered, the only impacted configuration of all known implementations was the 16-bit YCbCr with no pixel transformation and with the range coder coder type, as the other potentially impacted configurations (e.g., the 15/16-bit JPEG 2000 Reversible Color Transform (RCT) [ISO.15444-1.2019] with range coder or the 16-bit content with the Golomb Rice coder type) were not implemented. Meanwhile, the 16-bit JPEG 2000 RCT with range coder was deployed without this issue in one implementation and validated by one conformance checker. It is expected (to be confirmed) that this exception for the median predictor will be removed in the next version of the FFV1 bitstream.

3.4.  Quantization Table Sets

Quantization Tables are used on Sample Differences (see Section 3.8), so Quantized Sample Differences are stored in the bitstream.
The FFV1 bitstream contains one or more Quantization Table Sets. Each Quantization Table Set contains exactly five Quantization Tables with each Quantization Table corresponding to one of the five 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 eight least significant bits of the Quantized Sample Difference are used as an 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, and k is the Quantized Sample Difference (see Section 4.1.1).

3.5.  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.

3.6.  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 the first implementations of the FFV1 bitstream, the index for Cb and Cr Planes was stored even if it was 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.

3.7.  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 JPEG 2000 RCT for each Line from top to bottom, each Plane is coded, and for each Plane, each Sample is encoded from left to right.

3.7.1.  YCbCr

This color space allows one to four Planes.
The Cb and Cr Planes are optional, but if they are used, then they MUST be used together. Omitting the Cb and Cr Planes codes the frames in gray scale 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.

3.7.2.  RGB

This color space allows three or four Planes.
An optional transparency Plane can be used to code transparency data.
JPEG 2000 RCT is a Reversible Color Transform that codes RGB (Red, Green, Blue) Planes losslessly in a modified YCbCr color space [ISO.15444-1.2019]. 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
Cb and Cr are positively offset by 1 << bits_per_raw_sample after the conversion from RGB to the modified YCbCr, and they are negatively offset by the same value before the conversion from the modified YCbCr to RGB in order to have only nonnegative values after the conversion.
When FFV1 uses the JPEG 2000 RCT, the horizontal Lines are interleaved to improve caching efficiency since it is most likely that the JPEG 2000 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 comprise 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 JPEG 2000 RCT, the coding order is 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)
3.7.2.1.  RGB Exception
If bits_per_raw_sample is between 9 and 15 inclusive and extra_plane 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 the FFV1 bitstream, when bits_per_raw_sample was between 9 and 15 inclusive and extra_plane was 0, Green Blue Red (GBR) Planes were used as Blue Green Red (BGR) Planes during both encoding and decoding. Meanwhile, 16-bit JPEG 2000 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.

3.8.  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 JPEG 2000 RCT), only the n (or n+1, in the case of JPEG 2000 RCT) least significant bits are used, since this is sufficient to recover the original Sample. In Figure 10, the term bits represents bits_per_raw_sample + 1 for JPEG 2000 RCT or bits_per_raw_sample otherwise:
coder_input = ((sample_difference + 2 ^ (bits - 1)) &
              (2 ^ bits - 1)) - 2 ^ (bits - 1)

3.8.1.  Range Coding Mode

Early experimental versions of FFV1 used the Context-Adaptive Binary Arithmetic Coding (CABAC) coder from H.264 as defined in [ISO.14496-10.2020], 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 N. Martin in 1979 [Range-Encoding].
3.8.1.1.  Range Binary Values
To encode binary digits efficiently, a range coder is used. A range coder encodes a series of binary symbols by using a probability estimation within each context. The sizes of each of the two subranges are proportional to their estimated probability. The Quantization Table is used to choose the context used from the surrounding image sample values for the case of coding the Sample Differences. The coding of integers is done by coding multiple binary values. The range decoder will read bytes until it can determine into which subrange the input falls to return the next binary symbol.
To describe Range coding for FFV1, the following values are used:
Ci
the i-th context.
Bi
the i-th byte of the bytestream.
Ri
the Range at the i-th symbol.
ri
the boundary between two subranges of Ri: a subrange of ri values and a subrange Ri - ri values.
Li
the Low value of the Range at the i-th symbol.
li
a temporary variable to carry over or adjust the Low value of the Range between range coding operations.
ti
a temporary variable to transmit subranges between range coding operations.
bi
the i-th range-coded binary value.
S0, i
the i-th initial state.
jn
the length of the bytestream encoding n binary symbols.
The following range coder state variables are initialized to the following values. The Range is initialized to a value of 65,280 (expressed in base 16 as 0xFF00) as depicted in Figure 11. The Low is initialized according to the value of the first two bytes as depicted in Figure 12. ji tracks the length of the bytestream encoding while incrementing from an initial value of j0 to a final value of jn. j0 is initialized to 2 as depicted in Figure 13.
R_0 = 65280
L_0 = 2 ^ 8 * B_0 + B_1
j_0 = 2
The following equations define how the range coder variables evolve as it reads or writes symbols.
r_i = floor( ( R_i * S_(i, C_i) ) / 2 ^ 8 )
           b_i =  0                        <==>
           L_i <  R_i - r_i                ==>
   S_(i+1,C_i) =  zero_state_(S_(i, C_i))  AND
           l_i =  L_i                      AND
           t_i =  R_i - r_i

           b_i =  1                        <==>
           L_i >= R_i - r_i                ==>
   S_(i+1,C_i) =  one_state_(S_(i, C_i))   AND
           l_i =  L_i - R_i + r_i          AND
           t_i =  r_i
C_i != k ==> S_(i + 1, k) = S_(i, k)
t_i       <  2 ^ 8                         ==>
R_(i + 1) =  2 ^ 8 * t_i                   AND
L_(i + 1) =  2 ^ 8 * l_i + B_(j_i)         AND
j_(i + 1) =  j_i + 1

t_i       >= 2 ^ 8                         ==>
R_(i + 1) =  t_i                           AND
L_(i + 1) =  l_i                           AND
j_(i + 1) =  j_i
    range = 0xFF00;
    end   = 0;
    low   = get_bits(16);
    if (low >= range) {
        low = range;
        end = 1;
    }
refill() {
    if (range < 256) {
        range = range * 256;
        low   = low * 256;
        if (!end) {
            c.low += get_bits(8);
            if (remaining_bits_in_bitstream( NumBytes ) == 0) {
                end = 1;
            }
        }
    }
}
get_rac(state) {
    rangeoff  = (range * state) / 256;
    range    -= rangeoff;
    if (low < range) {
        state = zero_state[state];
        refill();
        return 0;
    } else {
        low   -= range;
        state  = one_state[state];
        range  = rangeoff;
        refill();
        return 1;
    }
}
3.8.1.1.1.  Termination
The range coder can be used in three 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 one 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 uncorrupted and be of value 0.
The above describes the range decoding. Encoding is defined as any process that produces a decodable bytestream.
There are three places where range coder termination is needed in FFV1. The first is in the Configuration Record, which in this case the size of the range-coded bytestream is known and handled as Closed mode. The second is the switch from the Slice Header, which is range coded to Golomb-coded Slices as Sentinel mode. The 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.
3.8.1.2.  Range Nonbinary 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, which is not only a waste of memory but also requires more past data to reach a reasonably good estimate of the probabilities. Alternatively, it would also be possible to assume a Laplacian distribution and only deal with its variance and mean (as in Huffman coding). However, for maximum flexibility and simplicity, the chosen method uses a single symbol to encode if a number is 0, and if the number is nonzero, it encodes the number using its exponent, mantissa, and sign. The exact contexts used are best described by Figure 21.
int get_symbol(RangeCoder *c, uint8_t *state, int is_signed) {
    if (get_rac(c, state + 0) {
        return 0;
    }

    int e = 0;
    while (get_rac(c, state + 1 + min(e, 9)) { //1..10
        e++;
    }

    int a = 1;
    for (int i = e - 1; i >= 0; i--) {
        a = a * 2 + get_rac(c, state + 22 + min(i, 9));  // 22..31
    }

    if (!is_signed) {
        return a;
    }

    if (get_rac(c, state + 11 + min(e, 10))) { //11..21
        return -a;
    } else {
        return a;
    }
}
get_symbol is used for the read out of sample_difference indicated in Figure 10.
get_rac returns a boolean computed from the bytestream as described by the formula found in Figure 14 and by the pseudocode found in Figure 20.
3.8.1.3.  Initial Values for the Context Model
When the keyframe value (see Section 4.4) is 1, all range coder state variables are set to their initial state.
3.8.1.4.  State Transition Table
In Range Coding Mode, a state transition table is used, indicating to which state the decoder will move based on the current state and the value extracted from Figure 20.
one_state_i =
       default_state_transition_i + state_transition_delta_i
zero_state_i = 256 - one_state_(256-i)
3.8.1.5.  default_state_transition
By default, the following state transition table is used:
  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,
3.8.1.6.  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 Section 4.2.3) MUST be set to 2, and the difference to the default MUST be stored in the Parameters, see Section 4.2. At the time of this writing, the reference implementation of FFV1 in FFmpeg uses Figure 25 by default 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,

3.8.2.  Golomb Rice Mode

The end of the bitstream of the Frame is padded with zeroes until the bitstream contains a multiple of eight bits.
3.8.2.1.  Signed Golomb Rice Codes
This coding mode uses Golomb Rice codes. The VLC is split into two parts: the prefix and suffix. The prefix stores the most significant bits or indicates if the symbol is too large to be stored (this is known as the ESC case, see Section 3.8.2.1.1). The suffix either stores the k least significant bits or stores the whole number in the ESC case.
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);
}
3.8.2.1.1.  Prefix
bits value
1 0
01 1
... ...
0000 0000 01 9
0000 0000 001 10
0000 0000 0001 11
0000 0000 0000 ESC
Table 1: Description of the coding of the prefix of signed Golomb Rice codes.
ESC is 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.
3.8.2.1.2.  Suffix
non-ESC the k least significant bits MSB first
ESC the value - 11, in MSB first order
Table 2: Description of the coding of the suffix of signed Golomb Rice codes.
ESC MUST NOT be used if the value can be coded as non-ESC.
3.8.2.1.3.  Examples
Table 3 shows practical examples of how signed Golomb Rice codes are decoded based on the series of bits extracted from the bitstream as described by the method above:
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
Table 3: Examples of decoded, signed Golomb Rice codes.
3.8.2.2.  Run Mode
Run mode is entered when the context is 0 and left as soon as a nonzero difference is found. The Sample Difference is identical to the predicted one. The run and the first different Sample Difference are coded as defined in Section 3.8.2.4.1.
3.8.2.2.1.  Run Length Coding
The run value is encoded in two 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 second part of the value stores the less significant part of the run as it is. The run_index is reset to zero for each Plane and Slice.
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 array is also used within [ISO.14495-1.1999].
3.8.2.3.  Sign Extension
sign_extend is the function of increasing the number of bits of an input binary number in two's complement signed number representation while preserving the input number's sign (positive/negative) and value, in order to fit in the output bit width. It MAY be computed with the following:
sign_extend(input_number, input_bits) {
    negative_bias = 1 << (input_bits - 1);
    bits_mask = negative_bias - 1;
    output_number = input_number & bits_mask; // Remove negative bit
    is_negative = input_number & negative_bias; // Test negative bit
    if (is_negative)
        output_number -= negative_bias;
    return output_number
}
3.8.2.4.  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;
}
3.8.2.4.1.  Golomb Rice Sample Difference 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 that this is different from JPEG-LS (lossless JPEG), 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.
3.8.2.5.  Initial Values for the VLC Context State
When keyframe (see Section 4.4) value is 1, all VLC coder state variables are set to their initial state.
    drift     = 0;
    error_sum = 4;
    bias      = 0;
    count     = 1;
Top   ToC   RFCv3-9043

4.  Bitstream

An FFV1 bitstream is composed of a series of one or more Frames and (when required) a Configuration Record.
Within the following subsections, pseudocode as described in Section 2.2.1 is used to explain the structure of each FFV1 bitstream component. Table 4 lists symbols used to annotate that pseudocode in order to define the storage of the data referenced in that line of pseudocode.
symbol definition
u(n) Unsigned, big-endian integer symbol using n bits
br Boolean (1-bit) symbol that is range coded with the method described in Section 3.8.1.1
ur Unsigned scalar symbol that is range coded with the method described in Section 3.8.1.2
sr Signed scalar symbol that is range coded with the method described in Section 3.8.1.2
sd Sample Difference symbol that is coded with the method described in Section 3.8
Table 4: Definition of pseudocode symbols for this document.
The following MUST be provided by external means during the 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.

4.1.  Quantization Table Set

The Quantization Table Sets store a sequence of values that are equal to one less than the count of equal concurrent entries for each set of equal concurrent entries within the first half of the table (represented as len - 1 in the pseudocode below) using the method described in Section 3.8.1.2. 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 pseudocode.
Example:
Table: 0 0 1 1 1 1 2 2 -2 -2 -2 -1 -1 -1 -1 0
Stored values: 1, 3, 1
QuantizationTableSet has its own initial states, all set to 128.
pseudocode                                                    | 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.
pseudocode                                                    | type
--------------------------------------------------------------|-----
QuantizationTable(i, j, scale) {                              |
    v = 0                                                     |
    for (k = 0; k < 128;) {                                   |
        len - 1                                               | ur
        for (n = 0; n < len; n++) {                           |
            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                                   |
}                                                             |

4.1.1.  quant_tables

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

4.1.2.  context_count

context_count[ i ] indicates the count of contexts for Quantization Table Set i. context_count[ i ] MUST be less than or equal to 32768.

4.2.  Parameters

The Parameters section, which could be in a global header of a container file that may or may not be considered to be part of the bitstream, contains significant characteristics about the decoding configuration used for all instances of Frame (in FFV1 versions 0 and 1) or the whole FFV1 bitstream (other versions), including the stream version, color configuration, and Quantization Tables. Figure 28 describes the contents of the bitstream.
Parameters has its own initial states, all set to 128.
pseudocode                                                    | 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
    }                                                         |
}                                                             |
CONTEXT_SIZE is 32.

4.2.1.  version

version specifies the version of the FFV1 bitstream.
Each version is incompatible with other versions: decoders SHOULD reject FFV1 bitstreams due to an unknown version.
Decoders SHOULD reject FFV1 bitstreams with version <= 1 && ConfigurationRecordIsPresent == 1.
Decoders SHOULD reject FFV1 bitstreams with version >= 3 && ConfigurationRecordIsPresent == 0.
value version
0 FFV1 version 0
1 FFV1 version 1
2 reserved*
3 FFV1 version 3
Other reserved for future use
Table 5: The definitions for version values.
* Version 2 was experimental and this document does not describe it.

4.2.2.  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 FFV1 bitstreams 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
Table 6: The definitions for micro_version values for FFV1 version 3.
* Development versions may be incompatible with the stable variants.

4.2.3.  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
Table 7: The definitions for coder_type values.
Restrictions:
If coder_type is 0, then bits_per_raw_sample SHOULD NOT be > 8.
Background: At the time of this writing, there is no known implementation of FFV1 bitstream supporting the Golomb Rice algorithm with bits_per_raw_sample greater than eight, and range coder is preferred.

4.2.4.  state_transition_delta

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

4.2.5.  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 JPEG 2000 RCT Transparency Line then Plane
Other reserved for future use reserved for future use reserved for future use reserved for future use
Table 8: The definitions for colorspace_type values.
FFV1 bitstreams with colorspace_type == 1 && (chroma_planes != 1 || log2_h_chroma_subsample != 0 || log2_v_chroma_subsample != 0) are not part of this specification.

4.2.6.  chroma_planes

chroma_planes indicates if chroma (color) Planes are present.
value presence
0 chroma Planes are not present
1 chroma Planes are present
Table 9: The definitions for chroma_planes values.

4.2.7.  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
Table 10: The definitions for bits_per_raw_sample values.
* Encoders MUST NOT store bits_per_raw_sample = 0. Decoders SHOULD accept and interpret bits_per_raw_sample = 0 as 8.

4.2.8.  log2_h_chroma_subsample

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

4.2.9.  log2_v_chroma_subsample

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

4.2.10.  extra_plane

extra_plane indicates if an extra Plane is present.
value presence
0 extra Plane is not present
1 extra Plane is present
Table 11: The definitions for extra_plane values.

4.2.11.  num_h_slices

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

4.2.12.  num_v_slices

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

4.2.13.  quant_table_set_count

quant_table_set_count indicates the number of Quantization Table Sets. quant_table_set_count MUST be less than or equal to 8.
Inferred to be 1 if not present.
MUST NOT be 0.

4.2.14.  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
Table 12: The definitions for states_coded values.

4.2.15.  initial_state_delta

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

4.2.16.  ec

ec indicates the error detection/correction type.
value error detection/correction type
0 32-bit CRC in ConfigurationRecord
1 32-bit CRC in Slice and ConfigurationRecord
Other reserved for future use
Table 13: The definitions for ec values.

4.2.17.  intra

intra indicates the constraint on keyframe in each instance of Frame.
Inferred to be 0 if not present.
value relationship
0 keyframe can be 0 or 1 (non keyframes or keyframes)
1 keyframe MUST be 1 (keyframes only)
Other reserved for future use
Table 14: The definitions for intra values.

4.3.  Configuration Record

In the case of a FFV1 bitstream with version >= 3, a Configuration Record is stored in the underlying container as described in Section 4.3.3. It contains the Parameters used for all instances of Frame. The size of the Configuration Record, NumBytes, is supplied by the underlying container.
pseudocode                                                 | 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)
}                                                          |

4.3.1.  reserved_for_future_use

reserved_for_future_use is a placeholder for future updates of this specification.
Encoders conforming to this version of this specification SHALL NOT write reserved_for_future_use.
Decoders conforming to this version of this specification SHALL ignore reserved_for_future_use.

4.3.2.  configuration_record_crc_parity

configuration_record_crc_parity is 32 bits that are chosen so that the Configuration Record as a whole has a CRC remainder of zero.
This is equivalent to storing the CRC remainder in the 32-bit parity.
The CRC generator polynomial used is described in Section 4.9.3.

4.3.3.  Mapping FFV1 into Containers

This Configuration Record can be placed in any file format that supports Configuration Records, fitting as much as possible with how the file format stores Configuration Records. The Configuration Record storage place and NumBytes are currently defined and supported for the following formats:
4.3.3.1.  Audio Video Interleave (AVI) File Format
The Configuration Record extends the stream format chunk ("AVI ", "hdlr", "strl", "strf") with the ConfigurationRecord bitstream.
See [AVI] 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.
4.3.3.2.  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 [ISO.14496-12.2020] 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.
4.3.3.3.  NUT File Format
The codec_specific_data element (in stream_header packet) contains the ConfigurationRecord bitstream. See [NUT] 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.
4.3.3.4.  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 [Matroska] for more information about elements.
NumBytes is defined as the Element Data Size of the CodecPrivate Element.

4.4.  Frame

A Frame is an encoded representation of a complete static image. The whole Frame is provided by the underlying container.
A Frame consists of the keyframe field, Parameters (if version <= 1), and a sequence of independent Slices. The pseudocode below describes the contents of a Frame.
The keyframe field has its own initial state, set to 128.
pseudocode                                                    | type
--------------------------------------------------------------|-----
Frame( NumBytes ) {                                           |
    keyframe                                                  | br
    if (keyframe && !ConfigurationRecordIsPresent {           |
        Parameters( )                                         |
    }                                                         |
    while (remaining_bits_in_bitstream( NumBytes )) {         |
        Slice( )                                              |
    }                                                         |
}                                                             |
The following is an 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                                               |
+-----------------------------------------------------------------+
  

4.5.  Slice

A Slice is an independent, spatial subsection of a Frame that is encoded separately from another region of the same Frame. The use of more than one Slice per Frame provides opportunities for taking advantage of multithreaded encoding and decoding.
A Slice consists of a Slice Header (when relevant), a Slice Content, and a Slice Footer (when relevant). The pseudocode below describes the contents of a Slice.
pseudocode                                                    | 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. padding MUST be 0.
reserved specifies a bit without any significance in this specification but may have a significance in a later revision of this specification.
Encoders SHOULD NOT fill reserved.
Decoders SHOULD ignore reserved.

4.6.  Slice Header

A Slice Header provides information about the decoding configuration of the Slice, such as its spatial position, size, and aspect ratio. The pseudocode below describes the contents of the Slice Header.
Slice Header has its own initial states, all set to 128.
pseudocode                                                    | 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
}                                                             |

4.6.1.  slice_x

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

4.6.2.  slice_y

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

4.6.3.  slice_width

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

4.6.4.  slice_height

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

4.6.5.  quant_table_set_index_count

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

4.6.6.  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 Content.
Inferred to be 0 if not present.

4.6.7.  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
Table 15: The definitions for picture_structure values.

4.6.8.  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 the Sample aspect ratio is unknown.
If sar_den is 0, decoders SHOULD ignore the encoded value and consider that sar_num is 0.

4.6.9.  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 the Sample aspect ratio is unknown.
If sar_num is 0, decoders SHOULD ignore the encoded value and consider that sar_den is 0.

4.7.  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).
pseudocode                                                    | 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 )                                  |
            }                                                 |
        }                                                     |
    }                                                         |
}                                                             |

4.7.1.  primary_color_count

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

4.7.2.  plane_pixel_height

plane_pixel_height[ p ] is the height in pixels of Plane p of the Slice. It is defined as the following:
chroma_planes == 1 && (p == 1 || p == 2)
    ? ceil(slice_pixel_height / (1 << log2_v_chroma_subsample))
    : slice_pixel_height

4.7.3.  slice_pixel_height

slice_pixel_height is the height in pixels of the Slice. It is defined as the following:
floor(
        ( slice_y + slice_height )
        * slice_pixel_height
        / num_v_slices
    ) - slice_pixel_y.

4.7.4.  slice_pixel_y

slice_pixel_y is the Slice vertical position in pixels. It is defined as the following:
floor( slice_y * frame_pixel_height / num_v_slices )

4.8.  Line

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

4.8.1.  plane_pixel_width

plane_pixel_width[ p ] is the width in pixels of Plane p of the Slice. It is defined as the following:
chroma_planes == 1 && (p == 1 || p == 2)
    ? ceil( slice_pixel_width / (1 << log2_h_chroma_subsample) )
    : slice_pixel_width.

4.8.2.  slice_pixel_width

slice_pixel_width is the width in pixels of the Slice. It is defined as the following:
floor(
        ( slice_x + slice_width )
        * slice_pixel_width
        / num_h_slices
    ) - slice_pixel_x

4.8.3.  slice_pixel_x

slice_pixel_x is the Slice horizontal position in pixels. It is defined as the following:
floor( slice_x * frame_pixel_width / num_h_slices )

4.8.4.  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 Section 3.2.

4.9.  Slice Footer

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

4.9.1.  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.

4.9.2.  error_status

error_status specifies the error status.
value error status
0 no error
1 Slice contains a correctable error
2 Slice contains an uncorrectable error
Other reserved for future use
Table 16: The definitions for error_status values.

4.9.3.  slice_crc_parity

slice_crc_parity is 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, without pre-inversion, and without post-inversion.
Top   ToC   RFCv3-9043

5.  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 and no Slice overlapping).
For each Frame with a keyframe value of 0, each Slice MUST have the same value of slice_x, slice_y, slice_width, and slice_height as a Slice in the previous Frame.
Top   ToC   RFCv3-9043

6.  Security Considerations

Like any other codec (such as [RFC 6716]), 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. Those related to denial of service are outlined in Section 2.1 of RFC 4732. 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. An overrun in allocated memory could lead to arbitrary code execution by an attacker. 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 failure to check for integer overflows. An example is allocating frame_pixel_width * frame_pixel_height in pixel count computations without considering that the multiplication result may have overflowed the range of the arithmetic type. 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.
None of the content carried in FFV1 is intended to be executable.
Top   ToC   RFCv3-9043

7.  IANA Considerations

IANA has registered the following values.

7.1.  Media Type Definition

This registration is done using the template defined in [RFC 6838] and following [RFC 4855].
Type name:
video
Subtype name:
FFV1
Required parameters:
None.
Optional parameters:
These parameters are used to signal the capabilities of a receiver implementation. These parameters MUST NOT be used for any other purpose.
version:
The version of the FFV1 encoding as defined by Section 4.2.1.
micro_version:
The micro_version of the FFV1 encoding as defined by Section 4.2.2.
coder_type:
The coder_type of the FFV1 encoding as defined by Section 4.2.3.
colorspace_type:
The colorspace_type of the FFV1 encoding as defined by Section 4.2.5.
bits_per_raw_sample:
The bits_per_raw_sample of the FFV1 encoding as defined by Section 4.2.7.
max_slices:
The value of max_slices is an integer indicating the maximum count of Slices within a Frame of the FFV1 encoding.
Encoding considerations:
This media type is defined for encapsulation in several audiovisual container formats and contains binary data; see Section 4.3.3. This media type is framed binary data; see Section 4.8 of RFC 6838.
Security considerations:
See Section 6 of this document.
Interoperability considerations:
None.
Published specification:
RFC 9043.
Applications that 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 (mailto:michael@niedermayer.cc)
Intended usage:
COMMON
Restrictions on usage:
None.
Author:
Dave Rice (mailto:dave@dericed.com)
Change controller:
IETF CELLAR Working Group delegated from the IESG.
Top   ToC   RFCv3-9043

8.  References

8.1.  Normative References

[ISO.9899.2018]
International Organization for Standardization, "Information technology - Programming languages - C", June 2018.
[RFC2119]
S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4732]
IAB, M. Handley, and E. Rescorla, "Internet Denial-of-Service Considerations", RFC 4732, DOI 10.17487/RFC4732, December 2006,
<https://www.rfc-editor.org/info/rfc4732>.
[RFC4855]
S. Casner, "Media Type Registration of RTP Payload Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
<https://www.rfc-editor.org/info/rfc4855>.
[RFC6838]
N. Freed, J. Klensin, and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC8174]
B. Leiba, "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017,
<https://www.rfc-editor.org/info/rfc8174>.

8.2.  Informative References

[AddressSanitizer]
Clang Project, "AddressSanitizer",
<https://clang.llvm.org/docs/AddressSanitizer.html>.
[AVI]
Microsoft, "AVI RIFF File Reference",
<https://docs.microsoft.com/en-us/windows/win32/directshow/avi-riff-file-reference>.
[FFV1GO]
D. Buitenhuis, "FFV1 Decoder in Go", 2019,
<https://github.com/dwbuiten/go-ffv1>.
[FFV1_V0]
M. Niedermayer, "Commit to mark FFV1 version 0 as non-experimental", April 2006,
<https://git.videolan.org/?p=ffmpeg.git;a=commit;h=b548f2b91b701e1235608ac882ea6df915167c7e>.
[FFV1_V1]
M. Niedermayer, "Commit to release FFV1 version 1", April 2009,
<https://git.videolan.org/?p=ffmpeg.git;a=commit;h=68f8d33becbd73b4d0aa277f472a6e8e72ea6849>.
[FFV1_V3]
M. Niedermayer, "Commit to mark FFV1 version 3 as non-experimental", August 2013,
<https://git.videolan.org/?p=ffmpeg.git;a=commit;h=abe76b851c05eea8743f6c899cbe5f7409b0f301>.
[HuffYUV]
B. Rudiak-Gould, "HuffYUV revisited", December 2003,
<https://web.archive.org/web/20040402121343/http://cultact-server.novi.dk/kpo/huffyuv/huffyuv.html>.
[ISO.14495-1.1999]
International Organization for Standardization, "Information technology -- Lossless and near-lossless compression of continuous-tone still images: Baseline", December 1999.
[ISO.14496-10.2020]
International Organization for Standardization, "Information technology -- Coding of audio-visual objects -- Part 10: Advanced Video Coding", December 2020.
[ISO.14496-12.2020]
International Organization for Standardization, "Information technology -- Coding of audio-visual objects -- Part 12: ISO base media file format", December 2020.
[ISO.15444-1.2019]
International Organization for Standardization, "Information technology -- JPEG 2000 image coding system: Core coding system", October 2019.
[Matroska]
, , and , "Matroska Media Container Format Specifications", Internet-Draft draft-ietf-cellar-matroska-07, April 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-cellar-matroska-07>.
[MediaConch]
MediaArea.net, "MediaConch", 2018,
<https://mediaarea.net/MediaConch>.
[NUT]
M. Niedermayer, "NUT Open Container Format", December 2013,
<https://ffmpeg.org/~michael/nut.txt>.
[Range-Encoding]
G. N. N. Martin, "Range encoding: an algorithm for removing redundancy from a digitised message", July 1979.
[REFIMPL]
M. Niedermayer, "The reference FFV1 implementation / the FFV1 codec in FFmpeg",
<https://ffmpeg.org/doxygen/trunk/ffv1_8h.html>.
[RFC6716]
JM. Valin, K. Vos, and T. Terriberry, "Definition of the Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716, September 2012,
<https://www.rfc-editor.org/info/rfc6716>.
[Valgrind]
Valgrind Developers, "Valgrind website",
<https://valgrind.org/>.
[YCbCr]
Wikipedia, "YCbCr", May 2021,
<https://en.wikipedia.org/w/index.php?title=YCbCr&oldid=1025097882>.
Top   ToC   RFCv3-9043

Appendix A.  Multithreaded Decoder Implementation Suggestions

This appendix is informative.
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 multithreading 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 the 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 Slice boundaries. A decoder may fall back on sequential order e.g., in case of a corrupted Frame (e.g., frame size unknown or slice_size of Slices not coherent) or if there is no possibility of seeking into the stream.
Top   ToC   RFCv3-9043

Appendix B.  Future Handling of Some Streams Created by Nonconforming Encoders

This appendix is informative.
Some bitstreams were found with 40 extra bits corresponding to error_status and slice_crc_parity in the reserved bits of Slice. Any revision of this specification should avoid adding 40 bits of content after SliceContent if version == 0 or version == 1, otherwise a decoder conforming to the revised specification could not distinguish between a revised bitstream and such buggy bitstream in the wild.
Top   ToC   RFCv3-9043

Appendix C.  FFV1 Implementations

This appendix provides references to a few notable implementations of FFV1.

C.1.  FFmpeg FFV1 Codec

This reference implementation [REFIMPL] 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 [REFIMPL] 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 Valgrind memory debugger [Valgrind] as well as the Clang AddressSanitizer [AddressSanitizer], which tracks 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.

C.2.  FFV1 Decoder in Go

An FFV1 decoder [FFV1GO] was written in Go by Derek Buitenhuis during the work to develop this document.

C.3.  MediaConch

The developers of the MediaConch project [MediaConch] created an independent FFV1 decoder as part of that project to validate FFV1 bitstreams. This work led to the discovery of three conflicts between existing FFV1 implementations and draft versions of this document. These issues are addressed by Section 3.3.1, Section 3.7.2.1, and Appendix B.