| cellar | M. Niedermayer |
| Internet-Draft | |
| Intended status: Standards Track | D. Rice |
| Expires: August 10, 2019 | |
| J. Martinez | |
| February 6, 2019 |
FFV1 Video Coding Format Version 4
draft-ietf-cellar-ffv1-v4-04
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.
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Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.
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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].
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 color spaces [YCbCr].
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].
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 1 or more Samples.
ESC: An ESCape symbol to indicate that the symbol to be stored is too large for normal storage and that an alternate storage method is used.
MSB: Most Significant Bit, the bit that can cause the largest change in magnitude of the symbol.
RCT: Reversible Color Transform, a near linear, exactly reversible integer transform that converts between RGB and YCbCr representations of a Pixel.
VLC: Variable Length Code, a code that maps source symbols to a variable number of bits.
RGB: A reference to the method of storing the value of a Pixel by using three numeric values that represent Red, Green, and Blue.
YCbCr: A reference to the method of storing the value of a Pixel by using three numeric values that represent the luma of the Pixel (Y) and the chrominance of the Pixel (Cb and Cr). YCbCr word is used for historical reasons and currently references any color space relying on 1 luma Sample and 2 chrominance Samples, e.g. YCbCr, YCgCo or ICtCp. The exact meaning of the three numeric values is unspecified.
TBA: To Be Announced. Used in reference to the development of future iterations of the FFV1 specification.
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 functions as well as functions defined within this document.
Note: the operators and the order of precedence are the same as used in the C programming language [ISO.9899.1990].
a + b means a plus b.
a - b means a minus b.
-a means negation of a.
a * b means a multiplied by b.
a / b means a divided by b.
a ^ b means a raised to the b-th power.
a & b means bit-wise "and" of a and b.
a | b means bit-wise "or" of a and b.
a >> b means arithmetic right shift of two’s complement integer representation of a by b binary digits.
a << b means arithmetic left shift of two’s complement integer representation of a by b binary digits.
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.
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.
floor(a) the largest integer less than or equal to a
ceil(a) the smallest integer greater than or equal to a
sign(a) extracts the sign of a number, i.e. if a < 0 then -1, else if a > 0 then 1, else 0
abs(a) the absolute value of a, i.e. abs(a) = sign(a)*a
log2(a) the base-two logarithm of a
min(a,b) the smallest of two values a and b
max(a,b) the largest of two values a and b
median(a,b,c) the numerical middle value in a data set of a, b, and c, i.e. a+b+c-min(a,b,c)-max(a,b,c)
a_{b} the b-th value of a sequence of a
a_{b,c} the 'b,c'-th value of a sequence of a
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
a...b means any value starting from a to b, inclusive.
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.
remaining_bits_in_bitstream( ) means the count of remaining bits after the pointer in that Configuration Record or Frame. It is computed from the NumBytes value multiplied by 8 minus the count of bits of that Configuration Record or Frame already read by the bitstream parser.
remaining_symbols_in_syntax( ) is true as long as the RangeCoder has not consumed all the given input bytes.
byte_aligned( ) is true if remaining_bits_in_bitstream( NumBytes ) is a multiple of 8, otherwise false.
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.
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.
A border is assumed for each coded Slice for the purpose of the median predictor and context according to the following rules:
The following table depicts a slice of 9 Samples a,b,c,d,e,f,g,h,i in a 3x3 arrangement along with its assumed border.
+---+---+---+---+---+---+---+---+ | 0 | 0 | | 0 | 0 | 0 | | 0 | +---+---+---+---+---+---+---+---+ | 0 | 0 | | 0 | 0 | 0 | | 0 | +---+---+---+---+---+---+---+---+ | | | | | | | | | +---+---+---+---+---+---+---+---+ | 0 | 0 | | a | b | c | | c | +---+---+---+---+---+---+---+---+ | 0 | a | | d | e | f | | f | +---+---+---+---+---+---+---+---+ | 0 | d | | g | h | i | | i | +---+---+---+---+---+---+---+---+
Relative to any Sample X, six other relatively positioned Samples from the coded Samples and presumed border are identified according to the labels used in the following diagram. The labels for these relatively positioned Samples are used within the median predictor and context.
+---+---+---+---+ | | | T | | +---+---+---+---+ | |tl | t |tr | +---+---+---+---+ | L | l | X | | +---+---+---+---+
The labels for these relative Samples are made of the first letters of the words Top, Left and Right.
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 is discovered, the only configuration of all known implementations being impacted is 16-bit YCbCr with no Pixel transformation with Range Coder coder, as other potentially impacted configurations (e.g. 15/16-bit JPEG2000-RCT with Range Coder coder, or 16-bit content with Golomb Rice coder) were implemented nowhere [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.
Relative to any Sample X, the Quantized Sample Differences L-l, l-tl, tl-t, T-t, and t-tr are used as context:
context = Q_{0}[l − tl] +
Q_{1}[tl − t] +
Q_{2}[t − tr] +
Q_{3}[L − l] +
Q_{4}[T − t]
If context >= 0 then context is used and the difference between the Sample and its predicted value is encoded as is, else -context is used and the difference between the Sample and its predicted value is encoded with a flipped sign.
The FFV1 bitstream contains 1 or more Quantization Table Sets. Each Quantization Table Set contains exactly 5 Quantization Tables with each Quantization Table corresponding to 1 of the 5 Quantized Sample Differences. For each Quantization Table, both the number of quantization steps and their distribution are stored in the FFV1 bitstream; each Quantization Table has exactly 256 entries, and the 8 least significant bits of the Quantized Sample Difference are used as index:
Q_{j}[k] = quant_tables[i][j][k&255]
In this formula, i is the Quantization Table Set index, j is the Quantized Table index, k the Quantized Sample Difference.
For each Plane of each slice, a Quantization Table Set is selected from an index:
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.
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.
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:
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.
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.
Cb=b-g
Cr=r-g
Y=g+(Cb+Cr)>>2
g=Y-(Cb+Cr)>>2
r=Cr+g
b=Cb+g
Exception for the JPEG2000-RCT conversion: if bits_per_raw_sample is between 9 and 15 inclusive and extra_plane is 0, the following formulae for reversible conversions between YCbCr and RGB MUST be used instead of the ones above:
Cb=g-b
Cr=r-b
Y=b+(Cb+Cr)>>2
b=Y-(Cb+Cr)>>2
r=Cr+b
g=Cb+b
Background: At the time of this writing, in all known implementations of 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.
When FFV1 uses the JPEG2000-RCT, the horizontal Lines are interleaved to improve caching efficiency since it is most likely that the JPEG2000-RCT will immediately be converted to RGB during decoding. The interleaved coding order is also Y, then Cb, then Cr, and then if used transparency.
As an example, a Frame that is two Pixels wide and two Pixels high, could be comprised of the following structure:
+------------------------+------------------------+ | Pixel[1,1] | Pixel[2,1] | | Y[1,1] Cb[1,1] Cr[1,1] | Y[2,1] Cb[2,1] Cr[2,1] | +------------------------+------------------------+ | Pixel[1,2] | Pixel[2,2] | | Y[1,2] Cb[1,2] Cr[1,2] | Y[2,2] Cb[2,2] Cr[2,2] | +------------------------+------------------------+
In JPEG2000-RCT, the coding order would be left to right and then top to bottom, with values interleaved by Lines and stored in this order:
Y[1,1] Y[2,1] Cb[1,1] Cb[2,1] Cr[1,1] Cr[2,1] Y[1,2] Y[2,2] Cb[1,2] Cb[2,2] Cr[1,2] Cr[2,2]
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:
coder_input =
[(sample_difference + 2^(bits−1)) & (2^bits − 1)] − 2^(bits−1)
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].
To encode binary digits efficiently a Range coder is used. C_{i} is the i-th Context. B_{i} is the i-th byte of the bytestream. b_{i} is the i-th Range coded binary value, S_{0,i} is the i-th initial state. The length of the bytestream encoding n binary symbols is j_{n} bytes.
r_{i} = floor( ( R_{i} * S_{i,C_{i}} ) / 2^8 )
S_{i+1,C_{i}} = zero_state_{S_{i,C_{i}}} XOR
l_i = L_i XOR
t_i = R_i - r_i <==
b_i = 0 <==>
L_i < R_i - r_i
S_{i+1,C_{i}} = one_state_{S_{i,C_{i}}} XOR
l_i = L_i - R_i + r_i XOR
t_i = r_i <==
b_i = 1 <==>
L_i >= R_i - r_i
S_{i+1,k} = S_{i,k} <== C_i != k
R_{i+1} = 2^8 * t_{i} XOR
L_{i+1} = 2^8 * l_{i} + B_{j_{i}} XOR
j_{i+1} = j_{i} + 1 <==
t_{i} < 2^8
R_{i+1} = t_{i} XOR
L_{i+1} = l_{i} XOR
j_{i+1} = j_{i} <==
t_{i} >= 2^8
R_{0} = 65280
L_{0} = 2^8 * B_{0} + B_{1}
j_{0} = 2
The range coder can be used in 3 modes.
Above describes the range decoding, encoding is defined as any process which produces a decodable bytestream.
There are 3 places where range coder termination is needed in FFV1. First is in the Configuration Record, in this case the size of the range coded bytestream is known and handled as Closed mode. Second is the switch from the Slice Header which is range coded to Golomb coded slices as Sentinel mode. Third is the end of range coded Slices which need to terminate before the CRC at their end. This can be handled as Sentinel mode or as Closed mode if the CRC position has been determined.
To encode scalar integers, it would be possible to encode each bit separately and use the past bits as context. However that would mean 255 contexts per 8-bit symbol that is not only a waste of memory but also requires more past data to reach a reasonably good estimate of the probabilities. Alternatively assuming a Laplacian distribution and only dealing with its variance and mean (as in Huffman coding) would also be possible, however, for maximum flexibility and simplicity, the chosen method uses a single symbol to encode if a number is 0, and if not, encodes the number using its exponent, mantissa and sign. The exact contexts used are best described by the following code, followed by some comments.
pseudo-code | type
--------------------------------------------------------------|-----
void put_symbol(RangeCoder *c, uint8_t *state, int v, int \ |
is_signed) { |
int i; |
put_rac(c, state+0, !v); |
if (v) { |
int a= abs(v); |
int e= log2(a); |
|
for (i=0; i<e; i++) |
put_rac(c, state+1+min(i,9), 1); //1..10 |
|
put_rac(c, state+1+min(i,9), 0); |
for (i=e-1; i>=0; i--) |
put_rac(c, state+22+min(i,9), (a>>i)&1); //22..31 |
|
if (is_signed) |
put_rac(c, state+11 + min(e, 10), v < 0); //11..21|
} |
} |
At keyframes all Range coder state variables are set to their initial state.
one_state_{i} =
default_state_transition_{i} + state_transition_delta_{i}
zero_state_{i} = 256 - one_state_{256-i}
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,
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 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,
The end of the bitstream of the Frame is filled with 0-bits until that the bitstream contains a multiple of 8 bits.
This coding mode uses Golomb Rice codes. The VLC is split into 2 parts, the prefix stores the most significant bits and the suffix stores the k least significant bits or stores the whole number in the ESC case.
pseudo-code | type
--------------------------------------------------------------|-----
int get_ur_golomb(k) { |
for (prefix = 0; prefix < 12; prefix++) { |
if ( get_bits(1) ) |
return get_bits(k) + (prefix << k) |
} |
return get_bits(bits) + 11 |
} |
|
int get_sr_golomb(k) { |
v = get_ur_golomb(k); |
if (v & 1) return - (v >> 1) - 1; |
else return (v >> 1); |
}
| bits | value |
|---|---|
| 1 | 0 |
| 01 | 1 |
| ... | ... |
| 0000 0000 0001 | 11 |
| 0000 0000 0000 | ESC |
| non ESC | the k least significant bits MSB first |
| ESC | the value - 11, in MSB first order, ESC may only be used if the value cannot be coded as non ESC |
| k | bits | value |
|---|---|---|
| 0 | 1 | 0 |
| 0 | 001 | 2 |
| 2 | 1 00 | 0 |
| 2 | 1 10 | 2 |
| 2 | 01 01 | 5 |
| any | 000000000000 10000000 | 139 |
Run mode 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.
The run value is encoded in 2 parts, the prefix part stores the more significant part of the run as well as adjusting the run_index that determines the number of bits in the less significant part of the run. The 2nd part of the value stores the less significant part of the run as it is. The run_index is reset for each Plane and slice to 0.
pseudo-code | type
--------------------------------------------------------------|-----
log2_run[41]={ |
0, 0, 0, 0, 1, 1, 1, 1, |
2, 2, 2, 2, 3, 3, 3, 3, |
4, 4, 5, 5, 6, 6, 7, 7, |
8, 9,10,11,12,13,14,15, |
16,17,18,19,20,21,22,23, |
24, |
}; |
|
if (run_count == 0 && run_mode == 1) { |
if (get_bits(1)) { |
run_count = 1 << log2_run[run_index]; |
if (x + run_count <= w) |
run_index++; |
} else { |
if (log2_run[run_index]) |
run_count = get_bits(log2_run[run_index]); |
else |
run_count = 0; |
if (run_index) |
run_index--; |
run_mode = 2; |
} |
} |
The log2_run function is also used within [ISO.14495-1.1999].
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.
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;
}
At keyframes all coder state variables are set to their initial state.
drift = 0;
error_sum = 4;
bias = 0;
count = 1;
An FFV1 bitstream is composed of a series of 1 or more Frames and (when required) a Configuration Record.
Within the following sub-sections, pseudo-code is used to explain the structure of each FFV1 bitstream component, as described in Section 2.2.1. The following table lists symbols used to annotate that pseudo-code in order to define the storage of the data referenced in that line of pseudo-code.
| Symbol | Definition |
|---|---|
| u(n) | unsigned big endian integer using n bits |
| sg | Golomb Rice coded signed scalar symbol coded with the method described in 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 |
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.
The Parameters section contains significant characteristics about the decoding configuration used for all instances of Frame (in FFV1 version 0 and 1) or the whole FFV1 bitstream (other versions), including the stream version, color configuration, and quantization tables. The pseudo-code below describes the contents of the bitstream.
pseudo-code | type
--------------------------------------------------------------|-----
Parameters( ) { |
version | ur
if (version >= 3) |
micro_version | ur
coder_type | ur
if (coder_type > 1) |
for (i = 1; i < 256; i++) |
state_transition_delta[ i ] | sr
colorspace_type | ur
if (version >= 1) |
bits_per_raw_sample | ur
chroma_planes | br
log2_h_chroma_subsample | ur
log2_v_chroma_subsample | ur
extra_plane | br
if (version >= 3) { |
num_h_slices - 1 | ur
num_v_slices - 1 | ur
quant_table_set_count | ur
} |
for( i = 0; i < quant_table_set_count; i++ ) |
QuantizationTableSet( i ) |
if (version >= 3) { |
for( i = 0; i < quant_table_set_count; i++ ) { |
states_coded | br
if (states_coded) |
for( j = 0; j < context_count[ i ]; j++ ) |
for( k = 0; k < CONTEXT_SIZE; k++ ) |
initial_state_delta[ i ][ j ][ k ] | sr
} |
ec | ur
intra | ur
} |
} |
version specifies the version of the FFV1 bitstream.
Each version is incompatible with other versions: decoders SHOULD reject a file due to an unknown version.
Decoders SHOULD reject a file with version <= 1 && ConfigurationRecordIsPresent == 1.
Decoders SHOULD reject a file with version >= 3 && ConfigurationRecordIsPresent == 0.
| value | version |
|---|---|
| 0 | FFV1 version 0 |
| 1 | FFV1 version 1 |
| 2 | reserved* |
| 3 | FFV1 version 3 |
| 4 | FFV1 version 4 |
| Other | reserved for future use |
* Version 2 was never enabled in the encoder thus version 2 files SHOULD NOT exist, and this document does not describe them to keep the text simpler.
micro_version specifies the micro-version of the FFV1 bitstream.
After a version is considered stable (a micro-version value is assigned to be the first stable variant of a specific version), each new micro-version after this first stable variant is compatible with the previous micro-version: decoders SHOULD NOT reject a file due to an unknown micro-version equal or above the micro-version considered as stable.
Meaning of micro_version for version 3:
| value | micro_version |
|---|---|
| 0...3 | reserved* |
| 4 | first stable variant |
| Other | reserved for future use |
* development versions may be incompatible with the stable variants.
Meaning of micro_version for version 4 (note: at the time of writing of this specification, version 4 is not considered stable so the first stable version value is to be announced in the future):
| value | micro_version |
|---|---|
| 0...TBA | reserved* |
| TBA | first stable variant |
| Other | reserved for future use |
* development versions which may be incompatible with the stable variants.
coder_type specifies the coder used.
| value | coder used |
|---|---|
| 0 | Golomb Rice |
| 1 | Range Coder with default state transition table |
| 2 | Range Coder with custom state transition table |
| Other | reserved for future use |
state_transition_delta 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.
colorspace_type specifies the color space encoded, the pixel transformation used by the encoder, the extra plane content, as well as interleave method.
| value | color space encoded | pixel transformation | extra plane content | interleave method |
|---|---|---|---|---|
| 0 | YCbCr | None | Transparency | Plane then Line |
| 1 | RGB | JPEG2000-RCT | Transparency | Line then Plane |
| Other | reserved for future use | reserved for future use | reserved for future use | reserved for future use |
Restrictions:
If colorspace_type is 1, then chroma_planes MUST be 1, log2_h_chroma_subsample MUST be 0, and log2_v_chroma_subsample MUST be 0.
chroma_planes indicates if chroma (color) Planes are present.
| value | presence |
|---|---|
| 0 | chroma Planes are not present |
| 1 | chroma Planes are present |
bits_per_raw_sample indicates the number of bits for each Sample. Inferred to be 8 if not present.
| value | bits for each sample |
|---|---|
| 0 | reserved* |
| Other | the actual bits for each Sample |
* Encoders MUST NOT store bits_per_raw_sample = 0 Decoders SHOULD accept and interpret bits_per_raw_sample = 0 as 8.
log2_h_chroma_subsample indicates the subsample factor, stored in powers to which the number 2 must be raised, between luma and chroma width (chroma_width = 2^(-log2_h_chroma_subsample) * luma_width).
log2_v_chroma_subsample indicates the subsample factor, stored in powers to which the number 2 must be raised, between luma and chroma height (chroma_height=2^(-log2_v_chroma_subsample) * luma_height).
extra_plane indicates if an extra Plane is present.
| value | presence |
|---|---|
| 0 | extra Plane is not present |
| 1 | extra Plane is present |
num_h_slices indicates the number of horizontal elements of the slice raster.
Inferred to be 1 if not present.
num_v_slices indicates the number of vertical elements of the slice raster.
Inferred to be 1 if not present.
quant_table_set_count indicates the number of Quantization Table Sets.
Inferred to be 1 if not present.
MUST NOT be 0.
states_coded indicates if the respective Quantization Table Set has the initial states coded.
Inferred to be 0 if not present.
| value | initial states |
|---|---|
| 0 | initial states are not present and are assumed to be all 128 |
| 1 | initial states are present |
initial_state_delta[ i ][ j ][ k ] indicates the initial Range coder state, it is encoded using k as context index and
pred = j ? initial_states[ i ][j - 1][ k ] : 128
initial_state[ i ][ j ][ k ] =
( pred + initial_state_delta[ i ][ j ][ k ] ) & 255
ec indicates the error detection/correction type.
| value | error detection/correction type |
|---|---|
| 0 | 32-bit CRC on the global header |
| 1 | 32-bit CRC per slice and the global header |
| Other | reserved for future use |
intra indicates the relationship between the instances of Frame.
Inferred to be 0 if not present.
| value | relationship |
|---|---|
| 0 | Frames are independent or dependent (keyframes and non keyframes) |
| 1 | Frames are independent (keyframes only) |
| Other | reserved for future use |
In the case of a FFV1 bitstream with version >= 3, a Configuration Record is stored in the underlying Container, at the track header level. It contains the Parameters used for all instances of Frame. The size of the Configuration Record, NumBytes, is supplied by the underlying Container.
pseudo-code | type
--------------------------------------------------------------|-----
ConfigurationRecord( NumBytes ) { |
ConfigurationRecordIsPresent = 1 |
Parameters( ) |
while( remaining_symbols_in_syntax( NumBytes - 4 ) ) |
reserved_for_future_use | br/ur/sr
configuration_record_crc_parity | u(32)
} |
reserved_for_future_use has semantics that are reserved for future use.
Encoders conforming to this version of this specification SHALL NOT write this value.
Decoders conforming to this version of this specification SHALL ignore its value.
configuration_record_crc_parity 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.
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:
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.
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.
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
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.
A Frame is an encoded representation of a complete static image. The whole Frame is provided by the underlaying container.
A Frame consists of the keyframe field, Parameters (if version <=1), and a sequence of independent slices. The pseudo-code below describes the contents of a Frame.
pseudo-code | type
--------------------------------------------------------------|-----
Frame( NumBytes ) { |
keyframe | br
if (keyframe && !ConfigurationRecordIsPresent |
Parameters( ) |
while ( remaining_bits_in_bitstream( NumBytes ) ) |
Slice( ) |
} |
Architecture overview of slices in a Frame:
| first slice header |
| first slice content |
| first slice footer |
| --------------------------------------------------------------- |
| second slice header |
| second slice content |
| second slice footer |
| --------------------------------------------------------------- |
| ... |
| --------------------------------------------------------------- |
| last slice header |
| last slice content |
| last slice footer |
A Slice is an independent spatial sub-section of a Frame that is encoded separately from an other region of the same Frame. The use of more than one Slice per Frame can be useful for taking advantage of the opportunities of multithreaded encoding and decoding.
A Slice consists of a Slice Header (when relevant), a Slice Content, and a Slice Footer (when relevant). The pseudo-code below describes the contents of a Slice.
pseudo-code | type
--------------------------------------------------------------|-----
Slice( ) { |
if (version >= 3) |
SliceHeader( ) |
SliceContent( ) |
if (coder_type == 0) |
while (!byte_aligned()) |
padding | u(1)
if (version <= 1) { |
while (remaining_bits_in_bitstream( NumBytes ) != 0 ) |
reserved | u(1)
} |
if (version >= 3) |
SliceFooter( ) |
} |
padding specifies a bit without any significance and used only for byte alignment. MUST be 0.
reserved specifies a bit without any significance in this revision of the specification and may have a significance in a later revision of this specification.
Encoders SHOULD NOT fill these bits.
Decoders SHOULD ignore these bits.
Note in case these bits are used in a later revision of this specification: any revision of this specification SHOULD care about avoiding to add 40 bits of content after SliceContent for version 0 and 1 of the bitstream. Background: due to some non conforming encoders, some bitstreams where found with 40 extra bits corresponding to error_status and slice_crc_parity, a decoder conforming to the revised specification could not do the difference between a revised bitstream and a buggy bitstream.
A Slice Header provides information about the decoding configuration of the Slice, such as its spatial position, size, and aspect ratio. The pseudo-code below describes the contents of the Slice Header.
pseudo-code | type
--------------------------------------------------------------|-----
SliceHeader( ) { |
slice_x | ur
slice_y | ur
slice_width - 1 | ur
slice_height - 1 | ur
for( i = 0; i < quant_table_set_index_count; i++ ) |
quant_table_set_index [ i ] | ur
picture_structure | ur
sar_num | ur
sar_den | ur
if (version >= 4) { |
reset_contexts | br
slice_coding_mode | ur
} |
} |
slice_x indicates the x position on the slice raster formed by num_h_slices.
Inferred to be 0 if not present.
slice_y indicates the y position on the slice raster formed by num_v_slices.
Inferred to be 0 if not present.
slice_width indicates the width on the slice raster formed by num_h_slices.
Inferred to be 1 if not present.
slice_height indicates the height on the slice raster formed by num_v_slices.
Inferred to be 1 if not present.
quant_table_set_index_count is defined as 1 + ( ( chroma_planes || version \<= 3 ) ? 1 : 0 ) + ( extra_plane ? 1 : 0 ).
quant_table_set_index indicates the Quantization Table Set index to select the Quantization Table Set and the initial states for the slice.
Inferred to be 0 if not present.
picture_structure specifies the temporal and spatial relationship of each Line of the Frame.
Inferred to be 0 if not present.
| value | picture structure used |
|---|---|
| 0 | unknown |
| 1 | top field first |
| 2 | bottom field first |
| 3 | progressive |
| Other | reserved for future use |
sar_num specifies the Sample aspect ratio numerator.
Inferred to be 0 if not present.
A value of 0 means that aspect ratio is unknown.
Encoders MUST write 0 if Sample aspect ratio is unknown.
If sar_den is 0, decoders SHOULD ignore the encoded value and consider that sar_num is 0.
sar_den specifies the Sample aspect ratio denominator.
Inferred to be 0 if not present.
A value of 0 means that aspect ratio is unknown.
Encoders MUST write 0 if Sample aspect ratio is unknown.
If sar_num is 0, decoders SHOULD ignore the encoded value and consider that sar_den is 0.
reset_contexts indicates if slice contexts must be reset.
Inferred to be 0 if not present.
slice_coding_mode indicates the slice coding mode.
Inferred to be 0 if not present.
| value | slice coding mode |
|---|---|
| 0 | Range Coding or Golomb Rice |
| 1 | raw PCM |
| Other | reserved for future use |
A Slice Content contains all Line elements part of the Slice.
Depending on the configuration, Line elements are ordered by Plane then by row (YCbCr) or by row then by Plane (RGB).
pseudo-code | type
--------------------------------------------------------------|-----
SliceContent( ) { |
if (colorspace_type == 0) { |
for( p = 0; p < primary_color_count; p++ ) |
for( y = 0; y < plane_pixel_height[ p ]; y++ ) |
Line( p, y ) |
} else if (colorspace_type == 1) { |
for( y = 0; y < slice_pixel_height; y++ ) |
for( p = 0; p < primary_color_count; p++ ) |
Line( p, y ) |
} |
} |
primary_color_count is defined as 1 + ( chroma_planes ? 2 : 0 ) + ( extra_plane ? 1 : 0 ).
plane_pixel_height[ p ] is the height in pixels of plane p of the slice.
plane_pixel_height[ 0 ] and plane_pixel_height[ 1 + ( chroma_planes ? 2 : 0 ) ] value is slice_pixel_height.
If chroma_planes is set to 1, plane_pixel_height[ 1 ] and plane_pixel_height[ 2 ] value is ceil(slice_pixel_height / log2_v_chroma_subsample).
slice_pixel_height is the height in pixels of the slice.
Its value is floor(( slice_y + slice_height ) * slice_pixel_height / num_v_slices) - slice_pixel_y.
slice_pixel_y is the slice vertical position in pixels.
Its value is floor(slice_y * frame_pixel_height / num_v_slices).
A Line is a list of the sample differences (relative to the predictor) of primary color components. The pseudo-code below describes the contents of the Line.
pseudo-code | type
--------------------------------------------------------------|-----
Line( p, y ) { |
if (colorspace_type == 0) { |
for( x = 0; x < plane_pixel_width[ p ]; x++ ) |
sample_difference[ p ][ y ][ x ] |
} else if (colorspace_type == 1) { |
for( x = 0; x < slice_pixel_width; x++ ) |
sample_difference[ p ][ y ][ x ] |
} |
} |
plane_pixel_width[ p ] is the width in Pixels of Plane p of the slice.
plane_pixel_width[ 0 ] and plane_pixel_width[ 1 + ( chroma_planes ? 2 : 0 ) ] value is slice_pixel_width.
If chroma_planes is set to 1, plane_pixel_width[ 1 ] and plane_pixel_width[ 2 ] value is ceil(slice_pixel_width / (1 << log2_h_chroma_subsample)).
slice_pixel_width is the width in Pixels of the slice.
Its value is floor(( slice_x + slice_width ) * slice_pixel_width / num_h_slices) - slice_pixel_x.
slice_pixel_x is the slice horizontal position in Pixels.
Its value is floor(slice_x * frame_pixel_width / num_h_slices).
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.
A Slice Footer provides information about slice size and (optionally) parity. The pseudo-code below describes the contents of the Slice Header.
Note: Slice Footer is always byte aligned.
pseudo-code | type
--------------------------------------------------------------|-----
SliceFooter( ) { |
slice_size | u(24)
if (ec) { |
error_status | u(8)
slice_crc_parity | u(32)
} |
} |
slice_size indicates the size of the slice in bytes.
Note: this allows finding the start of slices before previous slices have been fully decoded, and allows parallel decoding as well as error resilience.
error_status specifies the error status.
| value | error status |
|---|---|
| 0 | no error |
| 1 | slice contains a correctable error |
| 2 | slice contains a uncorrectable error |
| Other | reserved for future use |
slice_crc_parity 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.
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 |
} |
quant_tables[ i ][ j ][ k ] indicates the quantification table value of the Quantized Sample Difference k of the Quantization Table j of the Set Quantization Table Set i.
context_count[ i ] indicates the count of contexts for Quantization Table Set i.
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.
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 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.
This registration is done using the 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 version 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 Section 4.8 of [RFC6838].
Security considerations:
See Section 6 of this document.
Interoperability considerations: None.
Published specification:
[I-D.ietf-cellar-ffv1] and RFC XXXX.
[RFC Editor: Upon publication as an RFC, please replace "XXXX" with the number assigned to this document and remove this note.]
Applications which use this media type:
Any application that requires the transport of lossless video can use this media type. Some examples are, but not limited to screen recording, scientific imaging, and digital video preservation.
Fragment identifier considerations: N/A.
Additional information: None.
Person & email address to contact for further information: Michael Niedermayer <mailto:michael@niedermayer.cc>
Intended usage: COMMON
Restrictions on usage: None.
Author: Dave Rice <mailto:dave@dericed.com>
Change controller: IETF cellar working group delegated from the IESG.
The IANA is requested to register the following values:
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 seek into the stream.
See <https://github.com/FFmpeg/FFV1/commits/master>