HEVC Encoder Research Articles

Square-type-first inter-CU tree search algorithm for acceleration of HEVC encoder

In this paper, a fast inter-coding algorithm is proposed to reduce the computational load of HEVC encoders. The HEVC reference model (HM) employs the recursive depth-first-search (DFS) of the quad-tree search in terms of rate-distortion optimization in selecting the best coding modes for the best CU, PU, TU partitions, and many associated coding modes. The proposed algorithm evaluates the RD costs of the current CU only for its square-type PUs in the top-down search of the DFS. When the CU partition with the square-type PU is better than its sub-level CU partitions in terms of RD cost in bottom-up search of the DFS, the square type of current CU partition, along with its coding mode, is selected as the best partition. Otherwise, non-square-type PUs for the current CU level are evaluated. If the sub-partition is better than the CU with the non-square PUs, the sub-partition is finally selected as the optimum PU. Otherwise, the best non-square PU is selected as the best PU for the current level. Experimental results demonstrate that the proposed square-type-first inter-PU search can reduce the computational load in average encoding time by 66.7 % with 1---2 % BD loss over HM reference software. In addition, the proposed algorithm can yield an additional average time saving of 26.8---35.5 % against the three fast encoding algorithms adopted in HM.

Fast TU size decision algorithm for HEVC encoders using Bayesian theorem detection

The emerging internationalhigh efficiency video coding (HEVC) uses variable block-size transform coding of the prediction error residual. The residual block is partitioned into multiple square transform units. The supported transform unit (TU) sizes are 4×4, 8×8, 16×16, and 32×32. The optimal TU size is chosen based on rate distortion optimization techniques. Variable TU sizes have a coding performance superior to the traditional fixed-block-size transform. Meanwhile, it causes high computational complexity at the encoder. In this paper, we propose an early termination algorithm of TU size decision for HEVC encoders. The proposed method uses the Bayesian decision theory and the correlation between the variance of residual coefficients and the transform size to reduce the number of candidate transform sizes for a given block. Simulation results show that the proposed algorithm can save 30–46% of transform processing complexity with negligible loss of coding efficiency.

Fast Intra Mode Decision in High Efficiency Video Coding

In this paper a coding unit early termination algorithm resulting in a fast intra prediction is proposed that terminates complete full search prediction for the coding unit. This is followed by a prediction unit mode decision to find the optimal modes HEVC encoder 35 prediction modes. This includes a two-step process: firstly calculating the Sum of Absolute Differences (SAD) of all the modes by down sampling method and secondly applying a three-step search algorithm to remove unnecessary modes. This is followed by early RDOQ (Rate Distortion Optimization Quantization) termination algorithm to further reduce the encoding time. Experimental results based on several video test sequences for 30 frames from each test sequence show for HEVC a decrease of about 35%-48% in encoding, with negligible degradation in peak signal to noise ratio (PSNR). Metrics such as BD-bitrate (Bjontegaard Delta bitrate), BD-PSNR (Bjontegaard Delta Peak Signal to Noise Ratio) and RD plots (Rate Distortion) are also used.

Parallel strategies analysis over the HEVC encoder

Recently, a new video coding standard called HEVC has been developed to deal with the nowadays media market challenges, being able to reduce to the half, on average, the bit stream size produced by the former video coding standard H.264/AVC at the same video quality. However, the computing requirements to encode video improving compression efficiency have significantly been increased. In this paper, we focus on applying parallel processing techniques to HEVC encoder to significantly reduce the computational power requirements without disturbing the coding efficiency. So, we propose several parallelization approaches to the HEVC encoder which are well suited to multicore architectures. Our proposals use OpenMP programming paradigm working at a coarse grain level parallelization which we call GOP-based level. GOP-based approaches encode simultaneously several groups of consecutive frames. Depending on how these GOPs are conformed and distributed, it is critical to obtain good parallel performance, taking also into account the level of coding efficiency degradation. The results show that near ideal efficiencies are obtained using up to 12 cores.

Skipping Prediction Directions Based on the Cost Relationship between Multi-Directional Predictions for an HEVC Encoder

Skipping Prediction Directions Based on the Cost Relationship between Multi-Directional Predictions for an HEVC Encoder

An Effective CU Size Decision Method for HEVC Encoders

The emerging high efficiency video coding standard (HEVC) adopts the quadtree-structured coding unit (CU). Each CU allows recursive splitting into four equal sub-CUs. At each depth level (CU size), the test model of HEVC (HM) performs motion estimation (ME) with different sizes including 2N × 2N, 2N × N, N × 2N and N × N. ME process in HM is performed using all the possible depth levels and prediction modes to find the one with the least rate distortion (RD) cost using Lagrange multiplier. This achieves the highest coding efficiency but requires a very high computational complexity. In this paper, we propose a fast CU size decision algorithm for HM. Since the optimal depth level is highly content-dependent, it is not efficient to use all levels. We can determine CU depth range (including the minimum depth level and the maximum depth level) and skip some specific depth levels rarely used in the previous frame and neighboring CUs. Besides, the proposed algorithm also introduces early termination methods based on motion homogeneity checking, RD cost checking and SKIP mode checking to skip ME on unnecessary CU sizes. Experimental results demonstrate that the proposed algorithm can significantly reduce computational complexity while maintaining almost the same RD performance as the original HEVC encoder.

HEVC 부호화기 소프트웨어의 통계적 특성 및 복잡도 분석

In this paper, we analyzed statistical characteristics and complexity of HEVC encoder as a leading research of acceleration, optimization and parallelization. Computational complexity of the HEVC encoder is approximately twice the compression performance compared to H.264/AVC. But, the increase of encoder complexity remains a problem to be solved in the future. Before performing the research on acceleration, optimization and parallelization to reduce high complexity of HEVC encoder, we measure the complexity each module for HEVC encoder using it`s reference software HM 7.1. We also measured the predicted complexity of fast HEVC encoder software, used in real applications, using HM 7.1 applying fast encoding method. The complexity is measured in terms of the operating cycle of the encoder software under the common test sequences and conditions in the Windows PC environment. In addition, we analyze statistical characteristics of HEVC encoder software according to encoding structures and limitation using coded bitstreams.

HEVC에서 인코더 계산 복잡도 개선 및 분할 정보 부호화 방법

본 논문에서는 참조 프레임 혹은 시간적으로 이전에 부호화한 프레임을 통해 현재 프레임의 LCU 분할구조를 예측하여 부호화하는 방법을 제안한다. HEVC에서는 CU로 부호화 및 복호화를 수행하는데, CU의 기본이 되는 LCU 단위로 영상의 특성에 따라 분할구조를 결정하여 영상을 적응적으로 부호화한다. 이 때, 현재 부호화하려는 LCU의 분할구조와 참조 프레임 및 시간적으로 이전에 부호화한 프레임 내의 동일한 위치에 대응되는 LCU(Co-located LCU)의 분할구조는 매우 유사한 특성이 있다. 따라서 본 논문에서는 인코더의 복잡도를 낮추기 위하여 현재 LCU의 분할구조를 결정할 때, Co-located LCU의 복잡성을 통해 현재 부호화하는 LCU의 분할구조 정보를 예측하고 분할구조에 포함될 확률이 높은 CU만 부호화하는 방법을 제안한다. 제안 방법의 시뮬레이션 결과로서, 인코더만을 변경하여 인코더 복잡도를 낮추는 방법이 기존 대비 인코더 복잡도가 평균 21.3% 감소하였고, 디코더 복잡도는 거의 비슷했으며, BD-Bitrate는 최대 0.6% 증가하였다. 또한 인코더에서 분할구조를 결정할 때 LCU의 분할 정보를 예측하여 부호화하고, CU 분할 정보를 부호화 및 복호화하는 과정을 변경하는 방법을 통해 BD-Bitrate를 감소시키는 방법을 제안하였다. 제안 방법의 시뮬레이션 결과는 인코더 복잡도가 평균 22% 감소하였고, 디코더 복잡도는 거의 비슷했으며, BD-Bitrate는 최대 0.3% 정도만 증가하여 제안하는 방법의 우수함을 확인할 수 있었다. This paper proposes the coding method to predict the split structure of LCU in the current frame on the basis of the reference frame or temporally-previous frame. HEVC encoder determines split structure according to image characteristics in LCU which is an basic element of CU. The split structure of the current LCU is very similar to the split structure of collocated LCU in the reference frame or temporally-previous frame. Thus, this paper proposes the method to reduce the encoder computational complexity by predicting split structure of the current LCU on the basis of that of collocated LCU in the reference frame or temporally-previous frame. And it also proposes the method to reduce the BD-Bitrate by coding after the prediction of the CU split information. The simulation results of changing only encoder showed that the mean of encoder computational complexity was lower by 21.3%, the decoder computational complexity was negligible change and the BD-Bitrate increase by the maximum of 0.6%. Also, the method changing encoder, bitstream, and decoder improves the mean of encoder computational complexity was lower by 22%, the decoder computational complexity was negligible change and the BD-Bitrate is improved to the maximum of 0.3%. When compared with the conventional method, indicating that the proposed method is superior.

Reference frame selection in a hardware-based HEVC encoder

The present invention relates to a method for selecting a reference frame in a hardware-based HEVC encoder, which performs integer motion estimation (IME) on multiple reference frames, and performs fractional motion estimation (FME) only on the optimum reference frame detected by IME. Therefore, the method can significantly reduce encoding time and can prevent compression efficiency from being largely reduced, by effectively reducing computational complexity through the removal of searches on the optimum reference frame and reference frames with no large change when performing FME.