Entropy coding is an integral part of all modern hybrid block-based video codecs. The context adaptive binary arithmetic coding (CABAC) is a normative part of ITU-T/ISO/IEC video coding standards (H.264/AVC, HEVC and VVC), while the Alliance for Open Media (AOMedia) video standard (AV1) utilizes a context adaptive multi-symbol version of it for entropy coding. Recent explorations of new coding tools beyond AV1 capabilities have led to the improvement of the context adaptive multi-symbol arithmetic entropy coder. In this study, improvements to the context initialization process of the entropy coder are discussed in detail and results reported. The improvements include a) optimal selection of reference frame pairs and b) context modeling from selected reference frame pairs for tile context initialization. Two variants of the proposed method, Variant1 and Variant2 are implemented on top of the reference codebase (research-v8.0.0). Experimental results with CTCv7 show that for random access (RA) configuration, average BDRATE (PSNR-YUV) gains of -0.13% and -0.18% are achievable for Variant1 and Variant2 respectively. Meanwhile, for low delay (LD) configuration the reported gains are -0.47% and -0.50% respectively for Variant1 and Variant2.

This paper introduces a novel Multi-Hypothesis Cross Component Prediction (MHCCP) method to enhance coding efficiency in image and video compression on top of AOMedia Video Model (AVM). Inspired by prior cross-component coding techniques, this work firstly presents a new cross component prediction method and then introduces a hardware-friendly design for practical deployment. The proposed MHCCP method initially generates multiple hypothesis predictions, and a linear combination of these hypothesis predictions is used to estimate the final chroma intensity. To determine the coefficients of the linear model in MHCCP, both the encoder and decoder use the Gaussian elimination method, which involves significant computational complexity. To address hardware implementation challenges, such as the high complexity for parameter derivation and limited above line buffer at the superblock boundary, the proposed method incorporates several optimizations, including decoupled vertical-horizontal prediction modes to capture diverse texture patterns, a padding mechanism to overcome line buffer constraints, and a sub-sampling strategy to reduce computational complexity. Experimental results on Common Test Condition (CTC) v7 demonstrate consistent coding gains: -0.57% (YUV-PSNR), -0.42% (Y-PSNR), -1.92% (U-PSNR), and -2.09% (V-PSNR) under all-intra settings on anchor research-v8.0.0. Notably, classes A1 and A2 achieve significant gains of -0.88% and -0.61%, respectively, highlighting the efficacy of the proposed approach.

The Alliance for Open Media (AOMedia) has been exploring new coding tools to enhance AV1 capabilities. Semi-Decoupled Partitioning (SDP), originally designed for intra frames in research-v2.0.0, improves coding by decoupling luma and chroma block partitioning. This study extends SDP to inter frames by introducing intra region coding, where the root node is explicitly signaled in the bitstream. Within the intra region, luma components of the intra-coded blocks can be further split, while chroma components remain unsplit. The experiments are implemented on the 8thanchor, research-v8.0.0, of AVM reference software with CTCv7, and experimental results show that the proposed method can achieve 0.12%, 2.39%, 2.58% coding gain for Y, U, and V component separately with random access configuration and 5% encoding time increase and almost no decoding time increase.

The latest initiative of Alliance for Open Media Video (AOM), named as AOM Video Model (AVM), is expected to introduce new coding tools to enhance compression benefits. Semi-Decoupled Partitioning (SDP) in AVM decouples the shared tree to support separate block partitioning for the luma and chroma channels from 64×64. Further, Chroma from Luma (CfL) is a chroma-only coding tool in AVM that applies collocated luma reconstructed samples in predicting chroma samples. The dependency of reconstructed luma samples in CfL can result in a delayed decoding process of chroma blocks in separate tree partitioning and introduce a worst-case latency of 4096 luma samples. In response, this study proposes a CfL constrained strategy to reduce the worst-case latency by selectively disallowing the CfL mode in a given chroma partition tree. Detailed latency analysis is also provided to confirm the reduction of worst-case latency to 2048 luma samples. The experiments are implemented on top research-v10.0.0 under Common Test Conditions (CTC) V7. The experimental results show that when the worst-case decoder latency is minimized to 2048 luma samples, the coding loss can be kept to minimal with an average loss of 0.02% for the YUV components in random access configurations with no change in encoder and decoder timings.

Lossless compression plays an important role in the storage and transmission of data with stringent quality requirements. There is a substantial demand for enhancing the lossless compression performance of the current AV1 codec. In this paper, two novel techniques, named Residual Block Refinement (RBR) mode and Multi-Residual Blocks (MRB) mode, are introduced to improve the lossless coding performance beyond AV1. For the RBR mode, the main idea is to perform a lossless block refinement within the residual block to further reduce redundancy. For the MRB mode, the first partial residual block utilizes a traditional transform and quantization process to generate a lossy representation of the original residual samples with efficient energy compaction. The second partial residual block is further coded to achieve a perfect representation of the difference between the original residual block and the reconstructed first residual block. The experimental results reveal that an average coding performance -2.76 %, -1.04 %, and -1.18 % are achieved on top of the AOMedia Video Model (AVM) v6.0.0 in terms of Bitrate savings for allIntra (AI), Random Access (RA), and Low Delay (LD) configurations, respectively.

Intra Block Copy (IntraBC) is a key coding tool for screen content video, enabling blocks to reference previously reconstructed regions within the same frame. While IntraBC was introduced in AV1 and further enhanced in VVC, the AVM development framework offers new opportunities to improve its coding efficiency and hardware compatibility. This paper proposes three enhancements adopted into the AVM reference software: (1) a unified and extended local reference buffer design supporting multiple superblock sizes with fixed memory constraints; (2) a decoupled global-local search strategy that improves block vector prediction efficiency; and (3) the integration of intra Block-Adaptive Weighted Prediction (BAWP) into local IntraBC. Experimental results show significant screen content coding gains: -3.53%, -2.25%, -1.43% (YUV-PSNR) for all intra, random access and low delay, respectively, with minimal runtime overhead. These contributions have been adopted into the AVM standard and reference software.

Context-adaptive arithmetic coding is an essential entropy coding scheme used in all modern video codecs. An arithmetic coder with binary symbol size is used by video codecs like H.264/AVC, HEVC and VVC while AV1 utilizes an arithmetic coder with syntax adaptive M-ary symbols. Recently, the Alliance for Open Media (AOMedia) has initiated exploration activities towards next-generation video coding tools beyond AV1. In this regard, improvements on probability estimation techniques for the context-adaptive M-ary arithmetic coder in AV1, are explored in this paper. The proposed improvements are applied and tested on top of the reference implementation of the exploratory codec beyond AV1, known as AVM (AOMedia Video Model). Experimental results show that, compared to AVM, the proposed method achieves an average 0.27%, 0.34% and 0.32% overall BD-rate coding gains for All Intra (AI), Random Access (RA) and Low Delay (LD) coding configurations for a wide range of video content.

It is well-known that non-separable transforms can efficiently decorrelate arbitrarily directed textures that are often present in image and video content. Due to the computational complexity involved, it is usually applied as a secondary transform operating on low frequency primary transform coefficients. In order to represent a variety of arbitrary directional textures in natural images /videos, it is ideal to have sufficient coverage of secondary transform kernels for the codec to choose from. However, this may lead to increased signaling cost and encoder complexity. This paper proposes a context-adaptive secondary transform (CAST) kernel selection approach to enable the usage of more secondary transform kernels with no signaling cost increase and minimal encoder and decoder complexity increase. The proposed approach uses the variance of the top row and left column of reconstructed pixels adjacent to the transform block, if available, as a context for selecting the set of transform kernels. Experimental results show that, compared to libaom, the proposed algorithm achieves a luma BD-rate reduction of 2.17% and 3.11% for All Intra coding using PSNR and SSIM quality metrics, respectively.

The Alliance for Open Media has recently initiated coding tool exploration activities towards the next-generation video coding beyond AV1. With this regard, this paper presents a package of coding tools that have been investigated, implemented and tested on top of the codebase, known as libaom, which is used for the exploration of next-generation video compression tools. The proposed tools cover several technical areas based on a traditional hybrid video coding structure, including block partitioning, prediction, transform and loop filtering. The proposed coding tools are integrated as a package, and a combined coding gain over AV1 is demonstrated in this paper. Furthermore, to better understand the behavior of each tool, besides the combined coding gain, the tool-on and tool-off tests are also simulated and reported for each individual coding tool. Experimental results show that, compared to libaom, the proposed methods achieve an average 8.0% (up to 22.0%) overall BD-rate reduction for All Intra coding configuration a wide range of image and video content.

The Alliance for Open Media has recently initiated coding tool exploration activities towards the next-generation video coding beyond AV1. In this regard, a frequency-domain coding tool, which is designed to leverage the cross-component correlation existing between collocated chroma blocks, is explored in this paper. The tool, henceforth known as multi-component secondary transform (MCST), is implemented as a low complexity secondary transform with primary transform coefficients of multiple color components as input. The proposed tool is implemented and tested on top of libaom. Experimental results show that, compared to libaom, the proposed method achieves an average 0.34% to 0.44% overall coding efficiency for All Intra (AI) coding configuration for a wide range of video content.