Research On Efficient Chroma Coding In H.266/VVC

In recent years,with the demands of video services growing rapidly,the requirement of video coding is increasingly higher.Facing to this challenge,experts and scholars in video coding have bent themselves to the research and development of the next generation video coding standard,H.266/VVC.Compared with H.265/HEVC,H.266/VVC has improved the main coding modules of the hybrid video coding framework by adding a lot of new technology.The performance of coding is improved significantly.To improve the efficiency of coding video content,H.266/VVC has turned on the tools of separate chroma partition structures for I Slice.The technology makes the chroma coding more flexible and shows a huge performance boost.However,with the separate partition,the correspondence of coding information of luma and chroma is no longer unique.Under this framework,utilizing the cross-component redundancy to improve the video coding efficiency has become into a new research direction.In this paper,it is proposed improved chroma coding methods in two aspect,intra prediction and transform coding.The concrete work is as follows:1.Optimization of chroma intra candidatesDirect Mode(DM)is an effective mode which directly uses the intra prediction information of the co-located luma block.DM is based on hypothesis that the textures of luma and chroma have the same direction.Under the separate chroma partition structures,there might exist multiple luma blocks corresponding to the current chroma block.In this way,the intra prediction mode from DM maybe a local optimal mode.To solve the above problem,the chroma texture distribution characteristics when DM is an intra directional mode and nondirectional mode are analyzed and the optimization methods are proposed for the two case.(1)Chroma intra candidates modification based on directional DMIn the proposed method,Vertical and Horizontal candidates are replaced by two derived modes from DM.The simulation results show-0.03%,-0.31%,and-0.39% BD-rates on Y,Cb,and Cr components,respectively,compared to VTM3.0 in All Intra configuration.(2)Chroma intra candidates modification based on non-directional DMThe direction of local texture tends to tiny direction.Based on the analysis,it is proposed to modify chroma intra mode candidates when DM is non-directional.Specifically,three tiny directionanl modes(mode 6,40,61)are used instead of Vertical,Horizontal and mode 66.The simulation results show-0.02%,-0.02%,and-0.02% BD-rates on Y,Cb,and Cr components,respectively,compared to VTM3.0 in All Intra configuration.(3)Uniform chroma intra candidates modification based on DMWhen the two methods are combined,it is found that the derived modes of non-angle DM conforms to the tiny direction.A uniform chroma intra candidates modification based on DM is proposed.Specifically,Vertical and Horizontal are replaced by two derived modes of DM,no matter DM is directional mode or not.The simulation results show-0.04%,-0.25%,and-0.34% BD-rates on Y,Cb,and Cr components,respectively,compared to VTM3.0 in All Intra configuration.2.Chroma multiple transform codingIn this paper,a chroma multiple transform method under the separate chroma partition structures is designed.There are differences between luma and chroma in residual distribution.Chroma DM is a mode which directly uses the intra prediction mode of the colocated luma block.In this case,chroma blocks will use the same prediction mode as luma does.And the residual distribution of chroma will be similar to luma.Based on the above analysis,a chroma multiple transform method is proposed.The chroma blocks using DM mode will borrow the luma prediction mode and transform core from the co-located luma PU.In this method,the chroma block chooses the transform core by inheriting with no increase in complexity and additional signal transmission.The simulation results show 0.11%,0.09% BD-rate reduction on Y and overall,respectively,compared to VTM3.0 in All Intra configuration.