New chroma intra prediction modes based on linear model for HEVC

RGB-to-YUV conversion reduces the inter-channel redundancy in video compression. However, inter-channel correlation is still observed locally. In HEVC HM4.0, LM mode uses a linear model to predict chroma from luma as a chroma intra prediction mode. The linearity is derived from the reconstructed causal pixels. Specifically, for a NxN chroma block, the above N neighbors and the left N neighbors are used for derivation. Considering the variety of the content inside a block, the linearity derived in LM mode does not always match the linearity of the current block. In this paper, we propose two new modes LML and LMA for chroma intra prediction. Those two modes are quite similar to LM mode, except that they use different neighbors for linearity derivation. By adding the proposed modes, about an average of 0.2%, 5.9%, 6.7% BD-rate reduction is achieved under all intra configuration for Y, Cb and Cr components respectively.