Internet-Draft FFV1 May 2020
Niedermayer, et al. Expires 27 November 2020 [Page]
Workgroup:
cellar
Internet-Draft:
draft-ietf-cellar-ffv1-v4-11
Published:
Intended Status:
Standards Track
Expires:
Authors:
M. Niedermayer
D. Rice
J. Martinez

FFV1 Video Coding Format Version 4

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 Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on 27 November 2020.

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Table of Contents

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 a version 4 of FFV1. Prior versions of FFV1 are defined within [I-D.ietf-cellar-ffv1].

This document assumes familiarity with mathematical and coding concepts such as Range coding [range-coding] and YCbCr color spaces [YCbCr].

This specification describes the valid bitstream and how to decode such valid bitstream. Bitstreams not conforming to this specification or how they are handled is outside this specification. A decoder could reject every invalid bitstream or attempt to perform error concealment or re-download or use a redundant copy of the invalid part or any other action it deems appropriate.

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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

2.1. Definitions

Container: Format that encapsulates Frames (see Section 4.3) 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 one 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.

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.

2.2. Conventions

2.2.1. 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 [ISO.9899.1990] and uses its if/else, while and for keywords as well as functions defined within this document.

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 re-defined 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 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. This is equivalent to dividing a by 2, b times, with rounding toward negative infinity.

a << b means arithmetic left shift of 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 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.

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 smallest of two values a and b.

max(a,b) means the largest 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 B implies A.

A <==> B means A <== B , B <== 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
a ? b : c
a = b, a += b, a -= b, a *= b

2.2.7. Range

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

2.2.8. 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 Section 4.2.3.

2.2.9. Bitstream Functions

2.2.9.1. 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.

2.2.9.2. remaining_symbols_in_syntax

remaining_symbols_in_syntax( ) is true as long as the RangeCoder 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.

3. Sample Coding

For each Slice (as described in Section 4.4) 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 1 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 |
+---+---+---+---+---+---+---+---+
Figure 1: A depiction of FFV1's assumed border for a set example Samples.

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 2. The labels for these relatively positioned Samples are used within the median predictor and context.

+---+---+---+---+
|   |   | T |   |
+---+---+---+---+
|   |tl | t |tr |
+---+---+---+---+
| L | l | X |   |
+---+---+---+---+
Figure 2: A depiction of how relatively positions Samples are references within this document.

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

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 was 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 [ISO.15444-1.2016]. 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.

3.4. 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:

Figure 3

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.

3.5. Quantization Table Sets

The FFV1 bitstream contains one or more Quantization Table Sets. Each Quantization Table Set contains exactly 5 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 8 least significant bits of the Quantized Sample Difference are used as index:

Figure 4

In this formula, i is the Quantization Table Set index, j is the Quantized Table index, k the Quantized Sample Difference.

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 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.

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 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.

3.7.1. 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.

3.7.2. 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 [ISO.15444-1.2016]. Reversible Pixel transformations between YCbCr and RGB use the following formulae.

Figure 5

Exception for the JPEG2000-RCT conversion: 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:

Figure 6

Background: At the time of this writing, in all known implementations of FFV1 bitstream, when bits_per_raw_sample was between 9 and 15 inclusive and extra_plane 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.

Cb and Cr are positively offset by 1 << bits_per_raw_sample after the conversion from RGB to the modified YCbCr and are negatively offseted by the same value before the conversion from the modified YCbCr to RGB, in order to have only non-negative values after the conversion.

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 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 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)

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 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 bits_per_raw_sample + 1 for JPEG2000-RCT or bits_per_raw_sample otherwise:

Figure 7

3.8.1. Range Coding Mode

Early experimental versions of FFV1 used the CABAC Arithmetic coder from H.264 as defined in [ISO.14496-10.2014] 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-coding].

3.8.1.1. 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.

Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
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 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 three 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.

3.8.1.2. 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 Figure 15, 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|
        }                                                     |
    }                                                         |
}                                                             |
Figure 15: A pseudo-code description of the contexts of Range Non Binary Values.
3.8.1.3. Initial Values for the Context Model

At keyframes all Range coder state variables are set to their initial state.

3.8.1.5. 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,
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 4.1.3) MUST be set to 2 and the difference to the default MUST be stored in the Parameters, see Section 4.1. The reference implementation of FFV1 in FFmpeg uses Figure 18 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,
Figure 18: Alternative state transition table for Range coding.

3.8.2. 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.

3.8.2.1. Signed Golomb Rice Codes

This coding mode uses Golomb Rice codes. The VLC is split into two 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);                             |
}
3.8.2.1.1. Prefix
Table 1
bits value
1 0
01 1
... ...
0000 0000 01 9
0000 0000 001 10
0000 0000 0001 11
0000 0000 0000 ESC
3.8.2.1.2. Suffix
Table 2
non ESC the k least significant bits MSB first
ESC the value - 11, in MSB first order

ESC MUST NOT be used if the value can be coded as non ESC.

3.8.2.1.3. Examples
Table 3
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
3.8.2.2. 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.

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 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 array is also used within [ISO.14495-1.1999].

3.8.2.3. 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;
}
3.8.2.3.1. 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.

3.8.2.4. 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;

4. Bitstream

An FFV1 bitstream is composed of a series of one 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 Section 2.2.1. Table 4 lists symbols used to annotate that pseudo-code in order to define the storage of the data referenced in that line of pseudo-code.

Table 4: Definition of pseudo-code symbols for this document.
Symbol Definition
u(n) unsigned big endian integer using n bits
sg Golomb Rice coded signed scalar symbol coded with the method described in Section 3.8.2
br Range coded Boolean (1-bit) symbol with the method described in Section 3.8.1.1
ur Range coded unsigned scalar symbol coded with the method described in Section 3.8.1.2
sr Range coded signed scalar symbol coded with the method described in Section 3.8.1.2
sd Sample difference coded with the method described in Section 3.8

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.

4.1. 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. Figure 19 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
    }                                                         |
}                                                             |
Figure 19: A pseudo-code description of the bitstream contents.

CONTEXT_SIZE is 32.

4.1.1. 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.

Table 5
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 experimental and this document does not describe it.

4.1.2. 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:

Table 6: The definitions for micro_version values for FFV1 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 micro_version value is to be announced in the future):

Table 7: The definitions for micro_version values for FFV1 version 4.
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.

4.1.3. coder_type

coder_type specifies the coder used.

Table 8
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

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 Golomb Rice algorithm with bits_per_raw_sample greater than 8, and Range Coder is prefered.

4.1.4. 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.

4.1.5. 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.

Table 9
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

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.

4.1.6. chroma_planes

chroma_planes indicates if chroma (color) Planes are present.

Table 10
value presence
0 chroma Planes are not present
1 chroma Planes are present

4.1.7. bits_per_raw_sample

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

Table 11
value bits for each sample
0 reserved*
Other the actual bits for each Sample

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

4.1.8. 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).

4.1.9. 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).

4.1.10. extra_plane

extra_plane indicates if an extra Plane is present.

Table 12
value presence
0 extra Plane is not present
1 extra Plane is present

4.1.11. num_h_slices

num_h_slices indicates the number of horizontal elements of the slice raster.

Inferred to be 1 if not present.

4.1.12. num_v_slices

num_v_slices indicates the number of vertical elements of the slice raster.

Inferred to be 1 if not present.

4.1.13. 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.

4.1.14. states_coded

states_coded indicates if the respective Quantization Table Set has the initial states coded.

Inferred to be 0 if not present.

Table 13
value initial states
0 initial states are not present and are assumed to be all 128
1 initial states are present

4.1.15. initial_state_delta

initial_state_delta[ i ][ j ][ k ] indicates the initial Range coder state, it is encoded using k as context index and

Figure 20
Figure 21

4.1.16. ec

ec indicates the error detection/correction type.

Table 14
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

4.1.17. intra

intra indicates the constraint on keyframe in each instance of Frame.

Inferred to be 0 if not present.

Table 15
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

4.2. 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)
}                                                          |

4.2.1. 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.

4.2.2. 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 described in Section 4.8.3.

4.2.3. 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:

4.2.3.1. 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.

4.2.3.2. 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.14496-12.2015] 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.

4.2.3.3. 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.

4.2.3.4. 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.

4.3. 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:

Table 16
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

4.4. Slice

A Slice is an independent spatial sub-section of a Frame that is encoded separately from another 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.

4.5. 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
    }                                                         |
}                                                             |

4.5.1. slice_x

slice_x indicates the x position on the slice raster formed by numhslices.

Inferred to be 0 if not present.

4.5.2. slice_y

slice_y indicates the y position on the slice raster formed by numvslices.

Inferred to be 0 if not present.

4.5.3. slice_width

slice_width indicates the width on the slice raster formed by numhslices.

Inferred to be 1 if not present.

4.5.4. slice_height

slice_height indicates the height on the slice raster formed by numvslices.

Inferred to be 1 if not present.

4.5.5. quant_table_set_index_count

quant_table_set_index_count is defined as:

1 + ( ( chroma_planes || version <= 3 ) ? 1 : 0 ) + ( extra_plane ? 1 : 0 )

4.5.6. 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.

Inferred to be 0 if not present.

4.5.7. picture_structure

picture_structure specifies the temporal and spatial relationship of each Line of the Frame.

Inferred to be 0 if not present.

Table 17
value picture structure used
0 unknown
1 top field first
2 bottom field first
3 progressive
Other reserved for future use

4.5.8. 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 Sample aspect ratio is unknown.

If sar_den is 0, decoders SHOULD ignore the encoded value and consider that sar_num is 0.

4.5.9. 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 Sample aspect ratio is unknown.

If sar_num is 0, decoders SHOULD ignore the encoded value and consider that sar_den is 0.

4.5.10. reset_contexts

reset_contexts indicates if slice contexts must be reset.

Inferred to be 0 if not present.

4.5.11. slice_coding_mode

slice_coding_mode indicates the slice coding mode.

Inferred to be 0 if not present.

Table 18
value slice coding mode
0 Range Coding or Golomb Rice
1 raw PCM
Other reserved for future use

4.6. 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 )                                  |
            }                                                 |
        }                                                     |
    }                                                         |
}                                                             |

4.6.1. primary_color_count

primary_color_count is defined as:

1 + ( chroma_planes ? 2 : 0 ) + ( extra_plane ? 1 : 0 )

4.6.2. plane_pixel_height

plane_pixel_height[ p ] is the height in Pixels of Plane p of the Slice. It is defined as:

(chroma_planes == 1 && (p == 1 || p == 2)) ? ceil(slice_pixel_height / (1 << log2_v_chroma_subsample)) : slice_pixel_height

4.6.3. slice_pixel_height

slice_pixel_height is the height in pixels of the slice. It is defined as:

floor( ( slice_y + slice_height ) * slice_pixel_height / num_v_slices ) - slice_pixel_y.

4.6.4. slice_pixel_y

slice_pixel_y is the slice vertical position in pixels. It is defined as:

floor( slice_y * frame_pixel_height / num_v_slices )

4.7. 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 ]                  | sd
        }                                                     |
    } else if (colorspace_type == 1) {                        |
        for (x = 0; x < slice_pixel_width; x++) {             |
            sample_difference[ p ][ y ][ x ]                  | sd
        }                                                     |
    }                                                         |
}                                                             |

4.7.1. plane_pixel_width

plane_pixel_width[ p ] is the width in Pixels of Plane p of the Slice. It is defined as:

(chroma_planes == 1 && (p == 1 || p == 2)) ? ceil( slice_pixel_width / (1 << log2_h_chroma_subsample) ) : slice_pixel_width.

4.7.2. slice_pixel_width

slice_pixel_width is the width in Pixels of the slice. It is defined as:

floor( ( slice_x + slice_width ) * slice_pixel_width / num_h_slices ) - slice_pixel_x

4.7.3. slice_pixel_x

slice_pixel_x is the slice horizontal position in Pixels. It is defined as:

floor( slice_x * frame_pixel_width / num_h_slices )

4.7.4. 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. 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 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 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                                   |
}                                                             |

4.9.1. 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.

4.9.2. 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.

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, 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.

6. Security Considerations

Like any other codec, (such as [RFC6716]), 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 [RFC4732]. 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 [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:

In all of the conditions above, the decoder and encoder was run inside the [VALGRIND] memory debugger as well as clangs address sanitizer [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.

7. Media Type Definition

This section completes the media type registration template defined in [RFC6838] and following [RFC4855].

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 Section 4.1.1.

micro_version: The micro_version of the FFV1 encoding as defined by Section 4.1.2.

coder_type: The coder_type of the FFV1 encoding as defined by Section 4.1.3.

colorspace_type: The colorspace_type of the FFV1 encoding as defined by Section 4.1.5.

bits_per_raw_sample: The bits_per_raw_sample of the FFV1 encoding as defined by Section 4.1.7.

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 Section 4.2.3. This media type is framed binary data; see Section 4.8 of [RFC6838].

Security considerations:

See Section 6 of this document.

Interoperability considerations: None.

Published specification:

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.

8. IANA Considerations

The IANA is requested to register the following values:

9. Changelog

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

[RFC Editor: Please remove this Changelog section prior to publication.]

10. Normative References

[Matroska]
IETF, "Matroska", , <https://datatracker.ietf.org/doc/draft-ietf-cellar-matroska/>.
[RFC4732]
Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet Denial-of-Service Considerations", RFC 4732, DOI 10.17487/RFC4732, , <https://www.rfc-editor.org/info/rfc4732>.
[ISO.9899.2018]
International Organization for Standardization, "Programming languages - C", .
[ISO.9899.1990]
International Organization for Standardization, "Programming languages - C", .
[RFC4855]
Casner, S., "Media Type Registration of RTP Payload Formats", RFC 4855, DOI 10.17487/RFC4855, , <https://www.rfc-editor.org/info/rfc4855>.
[ISO.15444-1.2016]
International Organization for Standardization, "Information technology -- JPEG 2000 image coding system: Core coding system", .
[RFC6716]
Valin, JM., Vos, K., and T. Terriberry, "Definition of the Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716, , <https://www.rfc-editor.org/info/rfc6716>.
[RFC6838]
Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, DOI 10.17487/RFC6838, , <https://www.rfc-editor.org/info/rfc6838>.
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.

11. Informative References

[ISO.14496-12.2015]
International Organization for Standardization, "Information technology -- Coding of audio-visual objects -- Part 12: ISO base media file format", .
[VALGRIND]
Valgrind Developers, "Valgrind website", , <https://valgrind.org/>.
[I-D.ietf-cellar-ffv1]
Niedermayer, M., Rice, D., and J. Martinez, "FFV1 Video Coding Format Version 0, 1, and 3", Work in Progress, Internet-Draft, draft-ietf-cellar-ffv1-13, , <https://tools.ietf.org/html/draft-ietf-cellar-ffv1-13>.
[range-coding]
Nigel, G. and N. Martin, "Range encoding: an algorithm for removing redundancy from a digitised message.", .
[ISO.14495-1.1999]
International Organization for Standardization, "Information technology -- Lossless and near-lossless compression of continuous-tone still images: Baseline", .
[ISO.14496-10.2014]
International Organization for Standardization, "Information technology -- Coding of audio-visual objects -- Part 10: Advanced Video Coding", .
[YCbCr]
Wikipedia, "YCbCr", , <https://en.wikipedia.org/w/index.php?title=YCbCr>.
[AVI]
Microsoft, "AVI RIFF File Reference", , <https://msdn.microsoft.com/en-us/library/windows/desktop/dd318189%28v=vs.85%29.aspx>.
[Address-Sanitizer]
The Clang Team, "ASAN AddressSanitizer website", , <https://clang.llvm.org/docs/AddressSanitizer.html>.
[NUT]
Niedermayer, M., "NUT Open Container Format", , <https://ffmpeg.org/~michael/nut.txt>.
[REFIMPL]
Niedermayer, M., "The reference FFV1 implementation / the FFV1 codec in FFmpeg", , <https://ffmpeg.org>.
[HuffYUV]
Rudiak-Gould, B., "HuffYUV", , <https://web.archive.org/web/20040402121343/http://cultact-server.novi.dk/kpo/huffyuv/huffyuv.html>.

Appendix A. Multi-theaded 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 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.

Appendix B. Future handling of some streams created by non conforming 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 care about avoiding to add 40 bits of content after SliceContent if version == 0 or version == 1. Else a decoder conforming to the revised specification could not distinguish between a revised bitstream and such buggy bitstream in the wild.

Authors' Addresses

Michael Niedermayer
Dave Rice
Jerome Martinez