Modern Video Encoders Research Articles

Heuristic-Based Algorithms for Low-Complexity AV1 Intraprediction

Modern video encoders, such as the state-of-the-art AOMedia Video 1 (AV1) and Versatile Video Coding (VVC), offer significant compression gains at the cost of an increase in computational effort. This is partially because of the novel intra prediction techniques, including an increased number of prediction modes and block sizes supported by the codecs. To reduce this computational effort, this work proposes two heuristic-based algorithms for AV1 intra prediction: a fast mode decision that reduces the number of intra candidates with a low-cost direction detection method, and a mode-adaptive subsampling algorithm that significantly reduces the number of Sum of Squared Errors (SSE) operations after prediction. On average, a notable encoding time reduction of 29.8% was achieved, at the cost of a compression efficiency loss of 1.28%.

Perceptual Video Quality Prediction Emphasizing Chroma Distortions

Measuring the quality of digital videos viewed by human observers has become a common practice in numerous multimedia applications, such as adaptive video streaming, quality monitoring, and other digital TV applications. Here we explore a significant, yet relatively unexplored problem: measuring perceptual quality on videos arising from both luma and chroma distortions from compression. Toward investigating this problem, it is important to understand the kinds of chroma distortions that arise, how they relate to luma compression distortions, and how they can affect perceived quality. We designed and carried out a subjective experiment to measure subjective video quality on both luma and chroma distortions, introduced both in isolation as well as together. Specifically, the new subjective dataset comprises a total of 210 videos afflicted by distortions caused by varying levels of luma quantization commingled with different amounts of chroma quantization. The subjective scores were evaluated by 34 subjects in a controlled environmental setting. Using the newly collected subjective data, we were able to demonstrate important shortcomings of existing video quality models, especially in regards to chroma distortions. Further, we designed an objective video quality model which builds on existing video quality algorithms, by considering the fidelity of chroma channels in a principled way. We also found that this quality analysis implies that there is room for reducing bitrate consumption in modern video codecs by creatively increasing the compression factor on chroma channels. We believe that this work will both encourage further research in this direction, as well as advance progress on the ultimate goal of jointly optimizing luma and chroma compression in modern video encoders.

Higher precision range estimation for context‐based adaptive binary arithmetic coding

The Lagrangian rate distortion optimisation is widely employed in modern video encoders, such as high-efficiency video coding (H.265/HEVC). In this work, the authors propose a more accurate context-based adaptive binary arithmetic coding look-up table that can enhance compression quality and provide substantially better accuracy of range estimation, by employing one-more bit with 64 probability states. For the hardware implementation, they propose a higher precision look-up table instead of the HEVC Test Model (HM) standard table. The authors also define a new finite-state machine to handle the probability changing in real-time. The significant BD-RATE gain of the proposed context modelling is up to 6.0% for all-intra mode and 13.0% for inter mode. This finite state machine offers no divergence from the H.265/HEVC standards and can be used in the current systems.

Improved rate-distortion optimized video coding using non-integer bit estimation and multiple Lambda search

Many modern video encoders use the Lagrangian rate-distortion optimization (RDO) algorithm for mode decisions during the compression procedure. For each encoding stage, this approach involves minimizing a cost, which is a function of rate, distortion and a multiplier called Lambda. This paper proposes to improve the RDO process by applying two modifications. The first modification is to increase the accuracy of rate estimation, which is achieved by computing a non-integer number of bits for arithmetic coding of the syntax elements. This leads to a more accurate cost computation and therefore a better mode decision. The second modification is to search and adjust the value of Lambda based on the characteristics of each coding stage. For the encoder used, this paper proposes to search multiple values of Lambda for the intra-4×4mode decision. Moreover, a simple shift in Lambda value is proposed for motion estimation. Each of these modifications offers a certain gain in RDO performance, and, when all are combined, an average bit-rate saving of up to 7.0% can be achieved for the H.264/AVC codec while the same concept is applicable to the H.265/HEVC codec as well. The extra added complexity is contained to a certain level, and is also adjustable according to the processing resources available.