Pi Sheng Chang

Hybrid Variable Length Coding for Image and Video Compression

The conventional run-level variable length coding (RL-VLC), commonly adopted in block-based image and video compression to code quantized transform coefficients, is not efficient in coding consecutive nonzero coefficients. To overcome the deficiency, hybrid variable length coding (HVLC) is proposed in this paper. HVLC takes advantage of the clustered nature of nonzero transform coefficients in the low-frequency (LF) region and the scattered nature of nonzero transform coefficients in the high-frequency (HF) region by employing two types of VLC schemes. A novel two-dimensional position and one-dimensional amplitude coding scheme is proposed to code the LF coefficients while RL-VLC or an equivalent VLC scheme is retained to code the HF coefficients. Experimental results show that HVLC greatly favors the coding of high-resolution, high-complexity scenes, while it preserves low computational complexity.

Hybrid Variable Length Coding in Video Compression using Variable Breakpoint

Hybrid variable length coding (HVLC) was recently proposed as a novel entropy coding scheme for block-based image and video compression, in observation of the inefficiency of the conventional run-level variable length coding scheme in coding consecutive nonzero transform coefficients. In HVLC, each transform block is partitioned into low-frequency and high-frequency regions, and the coefficients in the two regions are coded separately by different schemes. The partition of the transform block was performed based on a predefined, constant breakpoint. In this paper, we propose to partition the transform block using a variable breakpoint that is adaptive to the local context. We present a method to find one optimal breakpoint per transform block or per multi-block partition efficiently, and we show that by using variable breakpoint, the efficiency of HVLC can be improved considerably compared to the constant breakpoint case.

Three-dimensional position and amplitude VLC coding in H.264/AVC

Hybrid variable length coding (HVLC) was recently proposed as a novel entropy coding scheme for block-based image and video compression, which divides each transform block into low frequency (LF) region and high frequency (HF) region and codes them differently. To take advantage of the clustered nature of the nonzero coefficients in the LF region, a two-dimensional position and one- dimensional amplitude coding scheme (2DP1DA) was also proposed, which codes the 2D position information, i.e., run of consecutive zero-valued coefficients and run of consecutive nonzero coefficients, jointly by a 2D code table. To further improve the coding efficiency, joint position and amplitude coding is desirable. In this paper, we propose a three-dimensional position and amplitude coding scheme (3DPA) to jointly code the position and amplitude information for each nonzero cluster by a 3D code table. The experimental results show that HVLC with 3DPA for coding LF region achieves about 3.2%~3.7% bit rate reduction for a wide range of quantization parameters (QP) compared with CAVLC in H.264. Moreover, the 3DPA scheme is a compact way to realize the joint position and amplitude coding and it is very suitable for context-adaptive HVLC design.

Joint position and amplitude coding in hybrid variable length coding for video compression

Hybrid variable length coding (HVLC) was recently proposed as a novel entropy coding scheme for block-based image and video compression, which divides each transform block into low frequency (LF) region and high frequency (HF) region and codes them differently. To efficiently code LF region, a two-dimensional position and one-dimensional amplitude coding scheme (2DP1DA) was also proposed, which jointly codes the 2D position information, i.e., run of consecutive zero-valued coefficients and run of consecutive nonzero coefficients. To further explore the potential of HVLC concept, we propose a new scheme for coding LF region, which codes the 2D position and amplitude information of each nonzero cluster jointly with manageable complexity. The experimental results show that compared with CAVLC in H.264, about 3.5% bit rate reduction is achieved by the proposed method for a wide range of quantization parameters (QP).

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.