Context-adaptive variable-length coding

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Context-adaptive variable-length coding (CAVLC) is a form of entropy coding used in H.264/MPEG-4 AVC video encoding. It is an inherently lossless compression technique, like almost all entropy-coders. In H.264/MPEG-4 AVC, it is used to encode residual, zig-zag order, blocks of transform coefficients. It is an alternative to context-based adaptive binary arithmetic coding (CABAC). CAVLC requires considerably less processing to decode than CABAC, although it does not compress the data quite as effectively. CAVLC is supported in all H.264 profiles, unlike CABAC which is not supported in Baseline and Extended profiles.

CAVLC is used to encode residual, zig-zag ordered 4×4 (and 2×2) blocks of transform coefficients. CAVLC is designed to take advantage of several characteristics of quantized 4×4 blocks:

• After prediction, transformation and quantization, blocks are typically sparse (containing mostly zeros).
• The highest non-zero coefficients after zig-zag scan are often sequences of +/− 1. CAVLC signals the number of high-frequency +/−1 coefficients in a compact way.
• The number of non-zero coefficients in neighbouring blocks is correlated. The number of coefficients is encoded using a look-up table; the choice of look-up table depends on the number of non-zero coefficients in neighbouring blocks.
• The level (magnitude) of non-zero coefficients tends to be higher at the start of the reordered array (near the DC coefficient) and lower towards the higher frequencies. CAVLC takes advantage of this by adapting the choice of VLC look-up table for the “level” parameter depending on recently coded level magnitudes.