Context-adaptive hybrid variable length coding in H.264/AVC

It is well observed that the nonzero transform coefficients for block-based hybrid video coding is more clustered in the low frequency region while more scattered in the high frequency region. Based on this observation, hybrid variable length coding (HVLC) was recently proposed, which divides each transform block into low frequency (LF) region and high frequency (HF) region by a pre-defined breakpoint and encodes the clustered low frequency coefficients with new coding schemes, such as two dimensional position and one-dimensional amplitude coding (2DP1DA) or three-dimensional position and amplitude coding (3DPA). However, the transition between clustered and scattered nonzero coefficients varies with different blocks and sequences. In this paper, we propose a context-adaptive hybrid variable length coding (CAHVLC) scheme, where multiple code tables are used and the code table is adaptively chosen based on the derivable context information from coded neighboring blocks or from the coded portion of the currently being coded block. The experimental results show that CAHVLC achieves about 6.5% ~ 8% bit rate reduction for a wide range of quantization parameters (QP) compared with CAVLC in H.264.