.H264 File Format – CAVLC tables

Decoding a block

1. Read coeff_token from the column of Table 9-5 chosen by nC. It gives TotalCoeff (0 to 16) and TrailingOnes (0 to 3). With 0 coefficients the block is all zeros.
2. Read one trailing_ones_sign_flag per trailing one: 0 means +1, 1 means −1.
3. Read the remaining TotalCoeff − TrailingOnes levels, each as level_prefix and level_suffix (below).
4. If TotalCoeff is less than the block size maxNumCoeff, read total_zeros from Tables 9-7 to 9-9, using the column tzVlcIndex = TotalCoeff.
5. For each coefficient but the last, while zeros are left, read run_before from Table 9-10 using the column for the zeros left; the last coefficient takes all remaining zeros.
6. Place the levels from the highest frequency down: starting at position −1, each level advances by its run + 1 (clause 9.2.4). Levels are in reverse scan order: the first level read is the last non-zero coefficient.

Choosing nC

For chroma DC, nC is −1 for 4:2:0 (ChromaArrayType 1) and −2 for 4:2:2. Otherwise nC comes from the 4 × 4 blocks to the left (A) and above (B), of the same colour component (clause 9.2.1):

• A neighbour is unavailable if it is outside the picture or slice, or if the current macroblock is intra, constrained_intra_pred_flag is 1, the neighbour is inter and data partitioning is used (NAL types 2 to 4).
• An available neighbour counts 0 if it is P_Skip or B_Skip or its coded block pattern bit is 0, 16 if it is I_PCM, and otherwise its own TotalCoeff (AC coefficients only, for Intra 16 × 16 and chroma blocks).
• nC = (nA + nB + 1) >> 1 with both, nA or nB with one, 0 with none. For Intra 16 × 16 DC, the block index is taken as 0.

With a 15-coefficient block (AC only), TotalCoeff may not be 16.

coeff_token (Table 9-5)

T1 is TrailingOnes, TC TotalCoeff. A dash means the combination cannot occur in that column. For 8 ≤ nC the code is a fixed 6 bits: the top four are TotalCoeff − 1 and the bottom two TrailingOnes, except that 0000 11 means no coefficients.

Levels

level_prefix is the count of zero bits before a 1 (Table 9-6): 1 is 0, 01 is 1, and so on, up to 15 zeros then a 1 for 15. Baseline, Constrained Baseline, Main and Extended streams never exceed 15; other profiles may go higher (it has been reported to stay at or below 11 + bit depth).

suffixLength starts at 1 if TotalCoeff > 10 and TrailingOnes < 3, otherwise 0. For each level (clause 9.2.2):

1. levelSuffixSize is 4 if level_prefix is 14 and suffixLength is 0; level_prefix − 3 if the prefix is 15 or more; otherwise suffixLength.
2. level_suffix is read as an unsigned number of that many bits (0 when the size is 0).
3. levelCode = (Min(15, level_prefix) << suffixLength) + level_suffix; add 15 if the prefix is 15 or more and suffixLength is 0; add (1 << (level_prefix − 3)) − 4096 if the prefix is 16 or more; add 2 to the first level after the trailing ones when there are fewer than 3 of them.
4. Level = (levelCode + 2) >> 1 if levelCode is even, (−levelCode − 1) >> 1 if odd.
5. Then suffixLength becomes 1 if it was 0, and grows by 1 (up to 6) when the level’s magnitude exceeds 3 << (suffixLength − 1).

total_zeros

The number of zero coefficients before the last non-zero one in scan order. Columns are tzVlcIndex (= TotalCoeff). 4 × 4 blocks (and 8 × 8 blocks, which are sent as four 4 × 4 blocks) use Tables 9-7 and 9-8:

Chroma DC uses Table 9-9: (a) for the 2 × 2 block of 4:2:0, (b) for the 2 × 4 block of 4:2:2.

run_before (Table 9-10)

The number of zeros directly before each coefficient, read from the last coefficient backwards while zeros remain. Columns are zerosLeft.