In Versatile Video Coding (VVC), Cross-component Linear Model (CCLM) predicts chroma samples by assuming a linear relationship between luma and chroma components. In performing CCLM for video in YUV 4:2:0 chroma format, collocated luma samples are firstly downsampled by a low-pass filter to match luma resolution with chroma, and one linear model of luma-chroma sample pairs is applied on the reconstructed luma samples to generate the predicted chroma samples. However, the low-pass downsampling procedure ignores relative spatial variations among luma samples in proximity, such as edge and gradient information. To solve this issue, a new coding technique, namely gradient linear model (GLM), is proposed for further compression efficiency exploration beyond VVC. Instead of using a low-pass filter in CCLM, the GLM utilizes high-pass gradient filters to generate the downsampled luma values. In this paper, two GLM schemes are provided with different trade-offs between coding gain and complexity, including: 1) a 2-parameter scheme that shares the CCLM module framework but replaces the downsampling filter with high-pass gradient filters; 2) a 3-parameter scheme that further combines the luma gradients with the low-pass downsampled luma values. Based on the enhanced compression model (ECM-5.0) software from the joint video experts team (JVET), simulation results show that the 2-parameter GLM achieves average Bjontegaard delta-rate (BD-rate) savings of {1.01%, 1.66%, 1.81%} and {0.69%, 0.95%, 1.12%} for {Y, U, V} components under the All Intra and Random Access configurations, respectively, and the 3-parameter GLM provides {1.28%, 3.23%, 3.28%} and {0.92%, 2.19%, 2.26%} BD-rate savings for {Y, U, V} components under the All Intra and Random Access configurations, respectively. Both of the proposed GLM schemes have been adopted to the ECM software platform.

This paper proposes one new In-loop filtering technique cross-component sample adaptive offset (CCSAO) for further coding efficiency improvement beyond Versatile Video Coding (VVC). The CCSAO reduces the sample distortion by 1) utilizing the strong correlation between luma and chroma components to classify the reconstructed samples into different categories and 2) deriving one offset for each category and adding the offset to the samples in the category. The offset of each category is properly derived at encoder and signaled to decoder. To keep the design at low complexity, only band information of reconstructed samples is considered for the sample classification of the CCSAO. To verify the performance, the proposed CCSAO is implemented on top of the enhanced compression model (ECM) for the joint video exploration team (JVET)'s exploratory work of future video coding technologies beyond VVC. Simulation results show that the CCSAO achieves average {0.20%, 2.83%, 2.98%} and {0.41%, 7.36%, 7.36%} Bjentegaard delta (BD)-rate savings for {Y, U, V} components under the Random Access and Low Delay B configuration, with negligible complexity impacts on encoding and decoding complexity. The proposed CCSAO scheme has been adopted to the ECM-2.0 software platform.

Intra-picture prediction as in HEVC exploits the nearest reference line adjacent to the current coding unit (CU) for prediction of samples. If this reference line represents a discontinuity, the reference samples in this reference line can differ to a large extent from the original samples and may lead to a large prediction error. We propose a multiple reference lines (MRLs) coding to allow not only the nearest reference line 0 but also reference lines 1 and 3 to be candidates for angular intra prediction as shown in Fig. 1. To reduce the complexity arising from additional lines to be checked at encoder side, we further propose to restrict the MRL to angular most probable modes (MPMs) only. The MRL coding signals the reference line index before the intra prediction mode. This allows to not signal the MPM flag of the current CU and implicitly derive it as true when a non-zero reference line index is signaled. Experimental results are provided to evaluate the performance of the proposed MRL coding on top of the VVC test model VTM-2.0.1. 26 test sequences in different categories, including 4k, 1080p, 720p, WVGA, WQVGA resolutions and screen contents are tested. Two coding structures are evaluated, all intra (AI) and random access (RA). The objective coding efficiency is measured in terms of Bjøntegaard Delta (BD) rate (%) computed using four rate/PSNR points that were generated by using quantization parameters 22, 27, 32 and 37. Lower (negative) BD-rate implies better compression rate. Table 1 shows that the presented MRL provides 0.46% bitrate savings for an all-intra and 0.2% for a random-access configuration on average. Furthermore, it provides 1.45% bitrate reduction for screen content test sequences, which are representing an increasingly important video application. Because of a fairly good trade-off between coding efficiency and complexity, the proposed MRL coding mode with MPM restriction was adopted into the current VVC draft standard.