1 Introduction

The FFV1 video codec is a simple and efficient lossless intra-frame only codec.

The latest version of this document is available at https://raw.github.com/FFmpeg/FFV1/master/ffv1.md

This document assumes familiarity with mathematical and coding concepts such as Range coding [@?range-coding] and YCbCr colorspaces [@?YCbCr].

2 Notation and Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [@!RFC2119].

2.1 Definitions

2.2 Conventions

Note: the operators and the order of precedence are the same as used in the C programming language [@!ISO.9899.1990].

2.2.1 Arithmetic operators

2.2.2 Assignment operators

2.2.3 Comparison operators

2.2.4 Mathematical functions

2.2.5 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

2.2.6 Range

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

2.2.7 NumBytes

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

2.2.8 Bitstream functions

2.2.8.1 remaining_bits_in_bitstream

remaining_bits_in_bitstream( ) means the count of remaining bits after the current position in that bitstream component. It is computed from the NumBytes value multiplied by 8 minus the count of bits of that component already read by the bitstream parser.

2.2.8.2 byte_aligned

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

3 General Description

Samples within a plane are coded in raster scan order (left->right, top->bottom). Each sample is predicted by the median predictor from samples in the same plane and the difference is stored see Coding of the Sample Difference.

3.1 Border

For the purpose of the predictor and context, samples above the coded slice are assumed to be 0; samples to the right of the coded slice are identical to the closest left sample; samples to the left of the coded slice are identical to the top right sample (if there is one), otherwise 0.

3.2 Median predictor

median(left, top, left + top - diag)

left, top, diag are the left, top and left-top samples

Note, this is also used in [@ISO.14495-1.1999] and [@HuffYUV].

Exception for the media predictor: if colorspace_type == 0 && bits_per_raw_sample == 16 && ( coder_type == 1 || coder_type == 2 ), the following media predictor MUST be used:

median(left16s, top16s, left16s + top16s - diag16s)

with: - left16s = left >= 32768 ? ( left - 65536 ) : left - top16s = top >= 32768 ? ( top - 65536 ) : top - diag16s = diag >= 32768 ? ( diag - 65536 ) : diag

Background: a two's complement signed 16-bit signed integer was used for storing pixel values in all known implementations of FFV1 bitstream. So in some circumstances, the most significant bit was wrongly interpreted (used as a sign bit instead of the 16th bit of an unsigned integer). Note that when the issue is discovered, the only configuration of all known implementations being impacted is 16-bit YCbCr color space with Range Coder coder, as other potentially impacted configurations (e.g. 15/16-bit JPEG2000-RCT color space with Range Coder coder, or 16-bit any color space with Golomb Rice coder) were implemented nowhere. In the meanwhile, 16-bit JPEG2000-RCT color space 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 media predictor in the next version of the bitstream.

3.3 Context

+---+---+---+---+
|   |   | T |   |
+---+---+---+---+
|   |tl | t |tr |
+---+---+---+---+
| L | l | X |   |
+---+---+---+---+

The quantized sample differences L-l, l-tl, tl-t, t-T, t-tr are used as context:

$$context=Q_{0}[l-tl]+\left|Q_{0}\right|(Q_{1}[tl-t]+\left|Q_{1}\right|(Q_{2}[t-tr]+\left|Q_{2}\right|(Q_{3}[L-l]+\left|Q_{3}\right|Q_{4}[T-t])))$$

If the context is smaller than 0 then -context is used and the difference between the sample and its predicted value is encoded with a flipped sign.

3.4 Quantization

There are 5 quantization tables for the 5 sample differences, both the number of quantization steps and their distribution are stored in the bitstream. Each quantization table has exactly 256 entries, and the 8 least significant bits of the sample difference are used as index:

$$Q_{i}[a-b]=Table_{i}[(a-b)\&255]$$

3.5 Colorspace

FFV1 supports two colorspaces: YCbCr and JPEG2000-RCT. Both colorspaces allow an optional Alpha plane that can be used to code transparency data.

3.5.1 YCbCr

In YCbCr colorspace, 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 FFV1 frame using YCbCr MUST use one of the following arrangements:

• Y
• Y, Alpha
• Y, Cb, Cr
• Y, Cb, Cr, Alpha

When FFV1 uses the YCbCr colorspace, 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 an Alpha (transparency) plane is used, then it MUST be coded last.

3.5.2 JPEG2000-RCT

JPEG2000-RCT is a Reversible Color Transform that codes RGB (red, green, blue) planes losslessly in a modified YCbCr colorspace. Reversible conversions between YCbCr and RGB use the following formulae.

$$Cb=b-g$$

$$Cr=r-g$$

$$Y=g+(Cb+Cr)>>2$$

$$g=Y-(Cb+Cr)>>2$$

$$r=Cr+g$$

$$b=Cb+g$$

[@!ISO.15444-1.2016]

An FFV1 frame using JPEG2000-RCT MUST use one of the following arrangements:

• Y, Cb, Cr
• Y, Cb, Cr, Alpha

When FFV1 uses the JPEG2000-RCT colorspace, the horizontal lines are interleaved to improve caching efficiency since it is most likely that the RCT will immediately be converted to RGB during decoding. The interleaved coding order is also Y, then Cb, then Cr, and then if used Alpha.

As an example, a frame that is two pixels wide and two pixels high, could be comprised of the following structure:

+------------------------+------------------------+
| Pixel[1,1]             | Pixel[2,1]             |
| Y[1,1] Cb[1,1] Cr[1,1] | Y[2,1] Cb[2,1] Cr[2,1] |
+------------------------+------------------------+
| Pixel[1,2]             | Pixel[2,2]             |
| Y[1,2] Cb[1,2] Cr[1,2] | Y[2,2] Cb[2,2] Cr[2,2] |
+------------------------+------------------------+

In JPEG2000-RCT colorspace, 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]

3.6 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 RCT), only the n (or n+1) 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 RCT or bits_per_raw_sample otherwise:

$$coder\_input=\left[\left(sample\_difference+2^{bits-1}\right)\&\left(2^{bits}-1\right)\right]-2^{bits-1}$$

3.6.1 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].

3.6.1.1 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, which is 128. The length of the bytestream encoding n binary symbols is $j_{n}$ bytes.

$$r_{i}=\left\lfloor \frac{R_{i}S_{i,C_{i}}}{2^{8}}\right\rfloor$$

$$\begin{array}{ccccccccc} S_{i+1,C_{i}}=zero\_state_{S_{i,C_{i}}} & \wedge & l{}_{i}=L_{i} & \wedge & t_{i}=R_{i}-r_{i} & \Longleftarrow & b_{i}=0 & \Longleftrightarrow & L_{i}<R_{i}-r_{i}\\ S_{i+1,C_{i}}=one\_state_{S_{i,C_{i}}} & \wedge & l_{i}=L_{i}-R_{i}+r_{i} & \wedge & t_{i}=r_{i} & \Longleftarrow & b_{i}=1 & \Longleftrightarrow & L_{i}\geq R_{i}-r_{i} \end{array}$$

$$\begin{array}{ccc} S_{i+1,k}=S_{i,k} & \Longleftarrow & C_{i}\neq k \end{array}$$

$$\begin{array}{ccccccc} R_{i+1}=2^{8}t_{i} & \wedge & L_{i+1}=2^{8}l_{i}+B_{j_{i}} & \wedge & j_{i+1}=j_{i}+1 & \Longleftarrow & t_{i}<2^{8}\\ R_{i+1}=t_{i} & \wedge & L_{i+1}=l_{i} & \wedge & j_{i+1}=j_{i} & \Longleftarrow & t_{i}\geq2^{8} \end{array}$$

$$R_{0}=65280$$

$$L_{0}=2^{8}B_{0}+B_{1}$$

$$j_{0}=2$$

3.6.1.2 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 which is not only a waste of memory but also requires more past data to reach a reasonably good estimate of the probabilities. Alternatively assuming a Laplacian distribution and only dealing with its variance and mean (as in Huffman coding) would also be possible, however, for maximum flexibility and simplicity, the chosen method uses a single symbol to encode if a number is 0 and if not encodes the number using its exponent, mantissa and sign. The exact contexts used are best described by the following code, followed by some comments.

function                                                      | 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|
    }                                                         |
}                                                             |

3.6.1.3 Initial values for the context model

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

3.6.1.4 State transition table

$$one\_state_{i}=default\_state\_transition_{i}+state\_transition\_delta_{i}$$

$$zero\_state_{i}=256-one\_state_{256-i}$$

3.6.1.5 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,

3.6.1.6 alternative state transition table

The alternative state transition table has been build using iterative minimization of frame sizes and generally performs better than the default. To use it, the coder_type MUST be set to 2 and the difference to the default MUST be stored in the parameters. The reference implementation of FFV1 in FFmpeg uses this table by default at the time of this writing when Range coding is used.

  0, 10, 10, 10, 10, 16, 16, 16, 28, 16, 16, 29, 42, 49, 20, 49,

 59, 25, 26, 26, 27, 31, 33, 33, 33, 34, 34, 37, 67, 38, 39, 39,

 40, 40, 41, 79, 43, 44, 45, 45, 48, 48, 64, 50, 51, 52, 88, 52,

 53, 74, 55, 57, 58, 58, 74, 60,101, 61, 62, 84, 66, 66, 68, 69,

 87, 82, 71, 97, 73, 73, 82, 75,111, 77, 94, 78, 87, 81, 83, 97,

 85, 83, 94, 86, 99, 89, 90, 99,111, 92, 93,134, 95, 98,105, 98,

105,110,102,108,102,118,103,106,106,113,109,112,114,112,116,125,

115,116,117,117,126,119,125,121,121,123,145,124,126,131,127,129,

165,130,132,138,133,135,145,136,137,139,146,141,143,142,144,148,

147,155,151,149,151,150,152,157,153,154,156,168,158,162,161,160,

172,163,169,164,166,184,167,170,177,174,171,173,182,176,180,178,

175,189,179,181,186,183,192,185,200,187,191,188,190,197,193,196,

197,194,195,196,198,202,199,201,210,203,207,204,205,206,208,214,

209,211,221,212,213,215,224,216,217,218,219,220,222,228,223,225,

226,224,227,229,240,230,231,232,233,234,235,236,238,239,237,242,

241,243,242,244,245,246,247,248,249,250,251,252,252,253,254,255,

3.6.2 Huffman coding mode

This coding mode uses Golomb Rice codes. The VLC code is split into 2 parts, the prefix stores the most significant bits, the suffix stores the k least significant bits or stores the whole number in the ESC case. The end of the bitstream (of the frame) is filled with 0-bits until that the bitstream contains a multiple of 8 bits.

3.6.2.1 Prefix

3.6.2.2 Suffix

3.6.2.3 Examples

3.6.2.4 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 is coded.

3.6.2.5 Run length coding

The run value is encoded in 2 parts, the prefix part stores the more significant part of the run as well as adjusting the run_index which determines the number of bits in the less significant part of the run. The 2nd part of the value stores the less significant part of the run as it is. The run_index is reset for each plane and slice to 0.

function                                                      | 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_bits1()) {                                        |
        run_count = 1 << log2_run[run_index];                 |
        if (x + run_count <= w)                               |
            run_index++;                                      |
    } else {                                                  |
        if (log2_run[run_index])                              |
            run_count = get_bits(log2_run[run_index]);        |
        else                                                  |
            run_count = 0;                                    |
        if (run_index)                                        |
            run_index--;                                      |
        run_mode = 2;                                         |
    }                                                         |
}                                                             |

The log2_run function is also used within [@ISO.14495-1.1999].

3.6.2.6 Level coding

Level coding is identical to the normal difference coding with the exception that the 0 value is removed as it cannot occur:

    if (diff>0) diff--;
    encode(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.

4 Bitstream

The same context which 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 Configuration Record

In the case of a 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 frames. The size of the Configuration Record, NumBytes, is supplied by the underlying container.

function                                                      | type
--------------------------------------------------------------|-----
ConfigurationRecord( NumBytes ) {                             |
    ConfigurationRecordIsPresent = 1                          |
    Parameters( )                                             |
    while( remaining_bits_in_bitstream( NumBytes ) > 32 )     |
        reserved_for_future_use                               | u(1)
    configuration_record_crc_parity                           | u(32)
}                                                             |

4.1.1 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.

4.1.2 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 the standard IEEE CRC polynomial (0x104C11DB7) with initial value 0.

4.1.3 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 container formats:

4.1.3.1 In 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.

4.1.3.2 In ISO/IEC 14496-12 (MP4 File Format)

The Configuration Record extends the sample description box ("moov", "trak", "mdia", "minf", "stbl", "stsd") with a "glbl" box which 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.

4.1.3.3 In 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

4.1.3.4 In 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.

4.2 Frame

A frame consists of the keyframe field, parameters (if version <=1), and a sequence of independent slices.

function                                                      | type
--------------------------------------------------------------|-----
Frame( NumBytes ) {                                           |
    keyframe                                                  | br
    if (keyframe && !ConfigurationRecordIsPresent             |
        Parameters( )                                         |
    while ( remaining_bits_in_bitstream( NumBytes ) )         |
        Slice( )                                              |
}                                                             |

4.3 Slice

function                                                      | type
--------------------------------------------------------------|-----
Slice( ) {                                                    |
    if (version >= 3)                                         |
        SliceHeader( )                                        |
    SliceContent( )                                           |
    if (coder_type == 0)                                      |
        while (!byte_aligned())                               |
            padding                                           | u(1)
    if (version >= 3)                                         |
        SliceFooter( )                                        |
}                                                             |

padding specifies a bit without any significance and used only for byte alignment. MUST be 0.

4.4 Slice Header

function                                                      | type
--------------------------------------------------------------|-----
SliceHeader( ) {                                              |
    slice_x                                                   | ur
    slice_y                                                   | ur
    slice_width - 1                                           | ur
    slice_height - 1                                          | ur
    for( i = 0; i < quant_table_index_count; i++ )            |
        quant_table_index [ i ]                               | ur
    picture_structure                                         | ur
    sar_num                                                   | ur
    sar_den                                                   | ur
    if (version >= 4) {                                       |
        reset_contexts                                        | br
        slice_coding_mode                                     | ur
   }                                                          |
}                                                             |

4.4.1 slice_x

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

4.4.2 slice_y

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

4.4.3 slice_width

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

4.4.4 slice_height

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

4.4.5 quant_table_index_count

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

4.4.6 quant_table_index

quant_table_index indicates the index to select the quantization table set and the initial states for the slice. Inferred to be 0 if not present.

4.4.7 picture_structure

picture_structure specifies the picture structure. Inferred to be 0 if not present.

4.4.8 sar_num

sar_num specifies the sample aspect ratio numerator. Inferred to be 0 if not present. MUST be 0 if sample aspect ratio is unknown.

4.4.9 sar_den

sar_den specifies the sample aspect ratio numerator. Inferred to be 0 if not present. MUST be 0 if sample aspect ratio is unknown.

4.4.10 reset_contexts

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

4.4.11 slice_coding_mode

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

4.5 Slice Content

function                                                      | 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 )                                  |
    }                                                         |
}                                                             |

4.5.1 primary_color_count

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

4.5.2 plane_pixel_height

plane_pixel_height[ p ] is the height in pixels of plane p of the slice. plane_pixel_height[ 0 ] and plane_pixel_height[ 1 + ( chroma_planes ? 2 : 0 ) ] value is slice_pixel_height. If chroma_planes is set to 1, plane_pixel_height[ 1 ] and plane_pixel_height[ 2 ] value is $\lceil slice\_pixel\_height / v\_chroma\_subsample \rceil$.

4.5.3 slice_pixel_height

slice_pixel_height is the height in pixels of the slice. Its value is $\lfloor ( slice\_y + slice\_height ) * slice\_pixel\_height / num\_v\_slices \rfloor - slice\_pixel\_y$.

4.5.4 slice_pixel_y

slice_pixel_y is the slice vertical position in pixels. Its value is $\lfloor slice\_y * frame\_pixel\_height / num\_v\_slices \rfloor$.

4.6 Line

function                                                      | type
--------------------------------------------------------------|-----
Line( p, y ) {                                                |
    if (colorspace_type == 0) {                               |
        for( x = 0; x < plane_pixel_width[ p ]; x++ )         |
            Pixel( p, y, x )                                  |
    } else if (colorspace_type == 1) {                        |
        for( x = 0; x < slice_pixel_width; x++ )              |
            Pixel( p, y, x )                                  |
    }                                                         |
}                                                             |

4.6.1 plane_pixel_width

plane_pixel_width[ p ] is the width in pixels of plane p of the slice. plane_pixel_width[ 0 ] and plane_pixel_width[ 1 + ( chroma_planes ? 2 : 0 ) ] value is slice_pixel_width. If chroma_planes is set to 1, plane_pixel_width[ 1 ] and plane_pixel_width[ 2 ] value is $\lceil slice\_pixel\_width / v\_chroma\_subsample \rceil$.

4.6.2 slice_pixel_width

slice_pixel_width is the width in pixels of the slice. Its value is $\lfloor ( slice\_x + slice\_width ) * slice\_pixel\_width / num\_h\_slices \rfloor - slice\_pixel\_x$.

4.6.3 slice_pixel_x

slice_pixel_x is the slice horizontal position in pixels. Its value is $\lfloor slice\_x * frame\_pixel\_width / num\_h\_slices \rfloor$.

4.7 Slice Footer

Note: slice footer is always byte aligned.

function                                                      | type
--------------------------------------------------------------|-----
SliceFooter( ) {                                              |
    slice_size                                                | u(24)
    if (ec) {                                                 |
        error_status                                          | u(8)
        slice_crc_parity                                      | u(32)
    }                                                         |
}                                                             |

4.7.1 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 this way parallel decoding as well as error resilience.

4.7.2 error_status

error_status specifies the error status.

4.7.3 slice_crc_parity

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

4.8 Parameters

function                                                      | 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
    alpha_plane                                               | br
    if (version >= 3) {                                       |
        num_h_slices - 1                                      | ur
        num_v_slices - 1                                      | ur
        quant_table_count                                     | ur
    }                                                         |
    for( i = 0; i < quant_table_count; i++ )                  |
        QuantizationTable( i )                                |
    if (version >= 3) {                                       |
        for( i = 0; i < quant_table_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
    }                                                         |
}                                                             |

4.8.1 version

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

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

4.8.2 micro_version

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

Meaning of micro_version for version 3:

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

Meaning of micro_version for version 4 (note: at the time of writing of this specification, version 4 is not considered stable so the first stable version value is to be announced in the future):

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

4.8.3 coder_type

coder_type specifies the coder used

4.8.4 state_transition_delta

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

4.8.5 colorspace_type

colorspace_type specifies the color space.

4.8.6 chroma_planes

chroma_planes indicates if chroma (color) planes are present.

4.8.7 bits_per_raw_sample

bits_per_raw_sample indicates the number of bits for each luma and chroma sample. Inferred to be 8 if not present.

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

4.8.8 h_chroma_subsample

h_chroma_subsample indicates the subsample factor between luma and chroma width ($chroma\_width=2^{-log2\_h\_chroma\_subsample}luma\_width$).

4.8.9 v_chroma_subsample

v_chroma_subsample indicates the subsample factor between luma and chroma height ($chroma\_height=2^{-log2\_v\_chroma\_subsample}luma\_height$).

4.8.10 alpha_plane

alpha_plane

indicates if a transparency plane is present.

4.8.11 num_h_slices

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

4.8.12 num_v_slices

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

4.8.13 quant_table_count

quant_table_count indicates the number of quantization table sets. Inferred to be 1 if not present.

4.8.14 states_coded

states_coded indicates if the respective quantization table set has the initial states coded. Inferred to be 0 if not present.

4.8.15 initial_state_delta

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

4.8.16 ec

ec indicates the error detection/correction type.

4.8.17 intra

intra indicates the relationship between frames. Inferred to be 0 if not present.

4.9 Quantization Tables

The quantization tables are stored by storing the number of equal entries -1 of the first half of the table using the method described in Range Non Binary Values. The second half doesn’t need to be stored as it is identical to the first with flipped sign.

example:

Table: 0 0 1 1 1 1 2 2-2-2-2-1-1-1-1 0

Stored values: 1, 3, 1

function                                                      | type
--------------------------------------------------------------|-----
QuantizationTable( i ) {                                      |
    scale = 1                                                 |
    for( j = 0; j < MAX_CONTEXT_INPUTS; j++ ) {               |
        QuantizationTablePerContext( i, j, scale )            |
        scale *= 2 * len_count[ i ][ j ] - 1                  |
    }                                                         |
    context_count[ i ] = ( scale + 1 ) / 2                    |
}                                                             |

MAX_CONTEXT_INPUTS is 5.

function                                                      | type
--------------------------------------------------------------|-----
QuantizationTablePerContext(i, j, scale) {                    |
    v = 0                                                     |
    for( k = 0; k < 128; ) {                                  |
        len - 1                                               | sr
        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 indicates the quantification table values.

4.9.2 context_count

context_count indicates the count of contexts.

4.9.3 Restrictions

To ensure that fast multithreaded decoding is possible, starting 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.

5 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. Although problems in encoders are typically rarer, the same applies to the encoder. 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 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 encoders 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 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.

6 Appendixes

6.1 Decoder implementation suggestions

6.1.1 Multi-threading support and independence of slices

The 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 corrupted frame (frame size unknown, slice_size of slices not coherent...) or if there is no possibility of seek into the stream.

Architecture overview of slices in a frame:

7 Changelog

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

8 ToDo

• mean,k estimation for the Golomb Rice codes

9 Copyright

Copyright 2003-2013 Michael Niedermayer <michaelni@gmx.at> This text can be used under the GNU Free Documentation License or GNU General Public License. See http://www.gnu.org/licenses/fdl.txt.

10 Bibliography

10.1 References

RFC 2119 - Key words for use in RFCs to Indicate Requirement Levels https://www.ietf.org/rfc/rfc2119.txt

ISO/IEC 9899 - Programming languages - C http://www.open-std.org/JTC1/SC22/WG14/www/standards

JPEG-LS FCD 14495 https://www.jpeg.org/public/fcd14495p.pdf

H.264 Draft http://bs.hhi.de/~wiegand/JVT-G050.pdf

HuffYuv http://web.archive.org/web/20040402121343/http://cultact-server.novi.dk/kpo/huffyuv/huffyuv.html

FFmpeg https://ffmpeg.org

JPEG2000 https://www.jpeg.org/jpeg2000/

Range encoding: an algorithm for removing redundancy from a digitised message. Presented by G. Nigel N. Martin at the Video & Data Recording Conference, IBM UK Scientific Center held in Southampton July 24-27 1979.

AVI RIFF File Format https://msdn.microsoft.com/en-us/library/windows/desktop/dd318189%28v=vs.85%29.aspx

Information technology Coding of audio-visual objects Part 12: ISO base media file format https://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnumber=61988

NUT Open Container Format https://www.ffmpeg.org/~michael/nut.txt

DOS Handley, M., Rescorla, E., and IAB, "Internet Denial-of-Service Considerations", RFC 4732, December 2006.

VALGRIND "Valgrind website", http://valgrind.org/.

ASAN Addresss Sanitizer, http://clang.llvm.org/docs/AddressSanitizer.html.

REFIMPL, The reference FFV1 implementation / the FFV1 codec in FFmpeg, https://ffmpeg.org/.

OPUS, Definition of the Opus Audio Codec, https://www.ietf.org/rfc/rfc6716.txt

YCbCr, Wikipedia, "YCbCr", https://en.wikipedia.org/w/index.php?title=YCbCr.

Matroska, IETF, "Matroska", https://datatracker.ietf.org/doc/draft-lhomme-cellar-matroska/, 2016.