2.2.1. Pseudocode

The FFV1 bitstream is described in this document using pseudocode. Note that the pseudocode is used to illustrate the structure of FFV1 and is not intended to specify any particular implementation. The pseudocode used is based upon the C programming language [ISO.9899.2018] and uses its if/else, while, and for keywords as well as functions defined within this document.¶

In some instances, pseudocode is presented in a two-column format such as shown in Figure 1. In this form, the type column provides a symbol as defined in Table 4 that defines the storage of the data referenced in that same line of pseudocode.¶

pseudocode                                                    | type
--------------------------------------------------------------|-----
ExamplePseudoCode( ) {                                        |
    value                                                     | ur
}                                                             |

2.2.2. Arithmetic Operators

Note: the operators and the order of precedence are the same as used in the C programming language [ISO.9899.2018], with the exception of >> (removal of implementation-defined behavior) and ^ (power instead of XOR) operators, which are redefined within this section.¶

a + b means a plus b.¶

a - b means a minus b.¶

-a means negation of a.¶

a * b means a multiplied by b.¶

a / b means a divided by b.¶

a ^ b means a raised to the b-th power.¶

a & b means bitwise "and" of a and b.¶

a | b means bitwise "or" of a and b.¶

a >> b means arithmetic right shift of the two's complement integer representation of a by b binary digits. This is equivalent to dividing a by 2, b times, with rounding toward negative infinity.¶

a << b means arithmetic left shift of the two's complement integer representation of a by b binary digits.¶

2.2.3. Assignment Operators

a = b means a is assigned b.¶

a++ is equivalent to a is assigned a + 1.¶

a-- is equivalent to a is assigned a - 1.¶

a += b is equivalent to a is assigned a + b.¶

a -= b is equivalent to a is assigned a - b.¶

a *= b is equivalent to a is assigned a * b.¶

2.2.4. Comparison Operators

a > b is true when a is greater than b.¶

a >= b is true when a is greater than or equal to b.¶

a < b is true when a is less than b.¶

a <= b is true when a is less than or equal b.¶

a == b is true when a is equal to b.¶

a != b is true when a is not equal to b.¶

a && b is true when both a is true and b is true.¶

a || b is true when either a is true or b is true.¶

!a is true when a is not true.¶

a ? b : c if a is true, then b, otherwise c.¶

2.2.5. Mathematical Functions

floor(a) means the largest integer less than or equal to a.¶

ceil(a) means the smallest integer greater than or equal to a.¶

sign(a) extracts the sign of a number, i.e., if a < 0 then -1, else if a > 0 then 1, else 0.¶

abs(a) means the absolute value of a, i.e., abs(a) = sign(a) * a.¶

log2(a) means the base-two logarithm of a.¶

min(a,b) means the smaller of two values a and b.¶

max(a,b) means the larger of two values a and b.¶

median(a,b,c) means the numerical middle value in a data set of a, b, and c, i.e., a+b+c-min(a,b,c)-max(a,b,c).¶

a ==> b means a implies b.¶

a <==> b means a ==> b, b ==> a.¶

ab means the b-th value of a sequence of a.¶

ab,c means the 'b,c'-th value of a sequence of a.¶

2.2.6. Order of Operation Precedence

When order of precedence is not indicated explicitly by use of parentheses, operations are evaluated in the following order (from top to bottom, operations of same precedence being evaluated from left to right). This order of operations is based on the order of operations used in Standard C.¶

a++, a--
!a, -a
a ^ b
a * b, a / b
a + b, a - b
a << b, a >> b
a < b, a <= b, a > b, a >= b
a == b, a != b
a & b
a | b
a && b
a || b
a ? b : c
a = b, a += b, a -= b, a *= b

2.2.7. Range

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

2.2.8. NumBytes

NumBytes is a nonnegative integer that expresses the size in 8-bit octets of a particular FFV1 Configuration Record or Frame. FFV1 relies on its container to store the NumBytes values; see Section 4.3.3.¶

2.2.9. Bitstream Functions

3.3.1. Exception

If colorspace_type == 0 && bits_per_raw_sample == 16 && ( coder_type == 1 || coder_type == 2 ) (see Sections 4.2.5, 4.2.7, and 4.2.3), the following median predictor MUST be used:¶

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

where:¶

left16s = l  >= 32768 ? ( l  - 65536 ) : l
top16s  = t  >= 32768 ? ( t  - 65536 ) : t
diag16s = tl >= 32768 ? ( tl - 65536 ) : tl

Background: a two's complement 16-bit signed integer was used for storing Sample values in all known implementations of FFV1 bitstream (see Appendix C). So in some circumstances, the most significant bit was wrongly interpreted (used as a sign bit instead of the 16th bit of an unsigned integer). Note that when the issue was discovered, the only impacted configuration of all known implementations was the 16-bit YCbCr with no pixel transformation and with the range coder coder type, as the other potentially impacted configurations (e.g., the 15/16-bit JPEG 2000 Reversible Color Transform (RCT) [ISO.15444-1.2019] with range coder or the 16-bit content with the Golomb Rice coder type) were not implemented. Meanwhile, the 16-bit JPEG 2000 RCT with range coder was deployed without this issue in one implementation and validated by one conformance checker. It is expected (to be confirmed) that this exception for the median predictor will be removed in the next version of the FFV1 bitstream.¶

3.7.1. YCbCr

This color space allows one to four Planes.¶

The Cb and Cr Planes are optional, but if they are used, then they MUST be used together. Omitting the Cb and Cr Planes codes the frames in gray scale without color data.¶

An optional transparency Plane can be used to code transparency data.¶

An FFV1 Frame using YCbCr MUST use one of the following arrangements:¶

• Y¶
• Y, Transparency¶
• Y, Cb, Cr¶
• Y, Cb, Cr, Transparency¶

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

3.7.2. RGB

This color space allows three or four Planes.¶

An optional transparency Plane can be used to code transparency data.¶

JPEG 2000 RCT is a Reversible Color Transform that codes RGB (Red, Green, Blue) Planes losslessly in a modified YCbCr color space [ISO.15444-1.2019]. Reversible pixel transformations between YCbCr and RGB use the following formulae:¶

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

When FFV1 uses the JPEG 2000 RCT, the horizontal Lines are interleaved to improve caching efficiency since it is most likely that the JPEG 2000 RCT will immediately be converted to RGB during decoding. The interleaved coding order is also Y, then Cb, then Cr, and then, if used, transparency.¶

As an example, a Frame that is two pixels wide and two pixels high could comprise the following structure:¶

+------------------------+------------------------+
| Pixel(1,1)             | Pixel(2,1)             |
| Y(1,1) Cb(1,1) Cr(1,1) | Y(2,1) Cb(2,1) Cr(2,1) |
+------------------------+------------------------+
| Pixel(1,2)             | Pixel(2,2)             |
| Y(1,2) Cb(1,2) Cr(1,2) | Y(2,2) Cb(2,2) Cr(2,2) |
+------------------------+------------------------+

In JPEG 2000 RCT, the coding order is left to right and then top to bottom, with values interleaved by Lines and stored in this order:¶

Y(1,1) Y(2,1) Cb(1,1) Cb(2,1) Cr(1,1) Cr(2,1) Y(1,2) Y(2,2) Cb(1,2) Cb(2,2) Cr(1,2) Cr(2,2)¶

3.8.1. Range Coding Mode

Early experimental versions of FFV1 used the Context-Adaptive Binary Arithmetic Coding (CABAC) coder from H.264 as defined in [ISO.14496-10.2020], but due to the uncertain patent/royalty situation, as well as its slightly worse performance, CABAC was replaced by a range coder based on an algorithm defined by G. Nigel N. Martin in 1979 [Range-Encoding].¶

    range = 0xFF00;
    end   = 0;
    low   = get_bits(16);
    if (low >= range) {
        low = range;
        end = 1;
    }

refill() {
    if (range < 256) {
        range = range * 256;
        low   = low * 256;
        if (!end) {
            c.low += get_bits(8);
            if (remaining_bits_in_bitstream( NumBytes ) == 0) {
                end = 1;
            }
        }
    }
}

get_rac(state) {
    rangeoff  = (range * state) / 256;
    range    -= rangeoff;
    if (low < range) {
        state = zero_state[state];
        refill();
        return 0;
    } else {
        low   -= range;
        state  = one_state[state];
        range  = rangeoff;
        refill();
        return 1;
    }
}

int get_symbol(RangeCoder *c, uint8_t *state, int is_signed) {
    if (get_rac(c, state + 0) {
        return 0;
    }

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

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

    if (!is_signed) {
        return a;
    }

    if (get_rac(c, state + 11 + min(e, 10))) { //11..21
        return -a;
    } else {
        return a;
    }
}

  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,

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

3.8.2. Golomb Rice Mode

The end of the bitstream of the Frame is padded with zeroes until the bitstream contains a multiple of eight bits.¶

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);
}

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;
    }
}

sign_extend(input_number, input_bits) {
    negative_bias = 1 << (input_bits - 1);
    bits_mask = negative_bias - 1;
    output_number = input_number & bits_mask; // Remove negative bit
    is_negative = input_number & negative_bias; // Test negative bit
    if (is_negative)
        output_number -= negative_bias;
    return output_number
}

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;
}

    diff = get_vlc_symbol(context_state);
    if (diff >= 0) {
        diff++;
    }

    drift     = 0;
    error_sum = 4;
    bias      = 0;
    count     = 1;

4.1.1. quant_tables

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

4.1.2. context_count

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

4.2.1. version

version specifies the version of the FFV1 bitstream.¶

Each version is incompatible with other versions: decoders SHOULD reject FFV1 bitstreams due to an unknown version.¶

Decoders SHOULD reject FFV1 bitstreams with version <= 1 && ConfigurationRecordIsPresent == 1.¶

Decoders SHOULD reject FFV1 bitstreams with version >= 3 && ConfigurationRecordIsPresent == 0.¶

* Version 2 was experimental and this document does not describe it.¶

4.2.2. micro_version

micro_version specifies the micro-version of the FFV1 bitstream.¶

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

Meaning of micro_version for version 3:¶

* Development versions may be incompatible with the stable variants.¶

4.2.3. coder_type

coder_type specifies the coder used.¶

Restrictions:¶

If coder_type is 0, then bits_per_raw_sample SHOULD NOT be > 8.¶

Background: At the time of this writing, there is no known implementation of FFV1 bitstream supporting the Golomb Rice algorithm with bits_per_raw_sample greater than eight, and range coder is preferred.¶

4.2.4. state_transition_delta

state_transition_delta specifies the range coder custom state transition table.¶

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

4.2.5. colorspace_type

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

FFV1 bitstreams with colorspace_type == 1 && (chroma_planes != 1 || log2_h_chroma_subsample != 0 || log2_v_chroma_subsample != 0) are not part of this specification.¶

4.2.6. chroma_planes

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

4.2.7. bits_per_raw_sample

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

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

4.2.8. log2_h_chroma_subsample

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

4.2.9. log2_v_chroma_subsample

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

4.2.10. extra_plane

extra_plane indicates if an extra Plane is present.¶

4.2.11. num_h_slices

num_h_slices indicates the number of horizontal elements of the Slice raster.¶

Inferred to be 1 if not present.¶

4.2.12. num_v_slices

num_v_slices indicates the number of vertical elements of the Slice raster.¶

Inferred to be 1 if not present.¶

4.2.13. quant_table_set_count

quant_table_set_count indicates the number of Quantization Table Sets. quant_table_set_count MUST be less than or equal to 8.¶

Inferred to be 1 if not present.¶

MUST NOT be 0.¶

4.2.14. states_coded

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

Inferred to be 0 if not present.¶

4.2.15. initial_state_delta

initial_state_delta[ i ][ j ][ k ] indicates the initial range coder state, and it is encoded using k as context index for the range coder and the following pseudocode:¶

4.2.16. ec

ec indicates the error detection/correction type.¶

4.2.17. intra

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

Inferred to be 0 if not present.¶

4.3.1. reserved_for_future_use

reserved_for_future_use is a placeholder for future updates of this specification.¶

Encoders conforming to this version of this specification SHALL NOT write reserved_for_future_use.¶

Decoders conforming to this version of this specification SHALL ignore reserved_for_future_use.¶

4.3.2. configuration_record_crc_parity

configuration_record_crc_parity is 32 bits that are chosen so that the Configuration Record as a whole has a CRC remainder of zero.¶

This is equivalent to storing the CRC remainder in the 32-bit parity.¶

The CRC generator polynomial used is described in Section 4.9.3.¶

4.3.3. Mapping FFV1 into Containers

This Configuration Record can be placed in any file format that supports Configuration Records, fitting as much as possible with how the file format stores Configuration Records. The Configuration Record storage place and NumBytes are currently defined and supported for the following formats:¶

4.6.1. slice_x

slice_x indicates the x position on the Slice raster formed by num_h_slices.¶

Inferred to be 0 if not present.¶

4.6.2. slice_y

slice_y indicates the y position on the Slice raster formed by num_v_slices.¶

Inferred to be 0 if not present.¶

4.6.3. slice_width

slice_width indicates the width on the Slice raster formed by num_h_slices.¶

Inferred to be 1 if not present.¶

4.6.4. slice_height

slice_height indicates the height on the Slice raster formed by num_v_slices.¶

Inferred to be 1 if not present.¶

4.6.5. quant_table_set_index_count

quant_table_set_index_count is defined as the following:¶

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

4.6.6. quant_table_set_index

quant_table_set_index indicates the Quantization Table Set index to select the Quantization Table Set and the initial states for the Slice Content.¶

Inferred to be 0 if not present.¶

4.6.7. picture_structure

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

Inferred to be 0 if not present.¶

4.6.8. sar_num

sar_num specifies the Sample aspect ratio numerator.¶

Inferred to be 0 if not present.¶

A value of 0 means that aspect ratio is unknown.¶

Encoders MUST write 0 if the Sample aspect ratio is unknown.¶

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

4.6.9. sar_den

sar_den specifies the Sample aspect ratio denominator.¶

Inferred to be 0 if not present.¶

A value of 0 means that aspect ratio is unknown.¶

Encoders MUST write 0 if the Sample aspect ratio is unknown.¶

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

4.7.1. primary_color_count

primary_color_count is defined as the following:¶

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

4.7.2. plane_pixel_height

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

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

4.7.3. slice_pixel_height

slice_pixel_height is the height in pixels of the Slice. It is defined as the following:¶

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

4.7.4. slice_pixel_y

slice_pixel_y is the Slice vertical position in pixels. It is defined as the following:¶

floor( slice_y * frame_pixel_height / num_v_slices )

4.8.1. plane_pixel_width

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

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

4.8.2. slice_pixel_width

slice_pixel_width is the width in pixels of the Slice. It is defined as the following:¶

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

4.8.3. slice_pixel_x

slice_pixel_x is the Slice horizontal position in pixels. It is defined as the following:¶

floor( slice_x * frame_pixel_width / num_h_slices )

4.8.4. sample_difference

sample_difference[ p ][ y ][ x ] is the Sample Difference for Sample at Plane p, y position y, and x position x. The Sample value is computed based on median predictor and context described in Section 3.2.¶

4.9.1. slice_size

slice_size indicates the size of the Slice in bytes.¶

Note: this allows finding the start of Slices before previous Slices have been fully decoded and allows parallel decoding as well as error resilience.¶

4.9.2. error_status

error_status specifies the error status.¶

4.9.3. slice_crc_parity

slice_crc_parity is 32 bits that are chosen so that the Slice as a whole has a CRC remainder of 0.¶

This is equivalent to storing the CRC remainder in the 32-bit parity.¶

The CRC generator polynomial used is the standard IEEE CRC polynomial (0x104C11DB7) with initial value 0, without pre-inversion, and without post-inversion.¶