Improving chroma intra prediction for HEVC

YCbCr color space is widely used in previous video coding standards such as H.264 and the latest video coding standard HEVC. Although the conversion from RGB to YCbCr reduces the inter-channel redundancy significantly, there are still some correlations among Y, Cb and Cr. In order to reduce the inter-channel redundancy, LM mode was proposed based on HEVC in which the reconstructed luma component is used to predict the chroma components through a linear model. The model parameters are derived by the reconstructed causal pixels. In this paper, in order to get more accurate parameters, an adaptive template selection method is proposed in which we extend the neighboring pixels and remove unavailable pixels before the calculation of parameters. Besides, adaptive Cr prediction is proposed. Specifically, in the prediction of Cr component, predictions from Y, Cb and both Y and Cb are exploited simultaneously. By combining these two methods, the BD-rate reduction is 0.3%, 2.0% and 3.0% (0.3%, 1.2% and 1.8%) under all-intra high QP (normal QP) setting based on HM12.0 compared to original LM mode for Y, Cb and Cr components respectively.