JCTVC-M0380 Crosscheck of JCTVC-M0106 proposed by Samsung [J. Kim (LG)] [late]
JCTVC-M0107 AHG5: Chroma intra prediction for extended color formats [J. Min, S. Jeong, S. Lee, C. Kim (Samsung)]
(Discussion chaired by D. Flynn.)
In extended chroma formats, the effect of prediction accuracy of chroma components is large for both coding efficiency and subjective visual quality. In this proposal, new designs of chroma intra prediction optimized for extended color formats are proposed. The current range extension design supports, only five intra prediction modes that are used for chroma intra prediction. In the proposed methods, the effect of increasing this to 7 and 11 modes was tested with appropriate changes in mode coding methods. In all-intra configurations, use of 7 intra prediction modes yields gains of 0.0%, 0.1%, 0.2%, and 11 modes yields gains of 0.0%, 0.4%, 0.6%. Additional tests were performed with chroma reference sample filtering method of JCTVC-M0106. The proposed methods with combination of chroma reference sample filtering approach provide higher gains which are 0.1%, 0.4%, 0.5% and 0.1%, 0.7%, 0.9% for 7 and 11 prediction modes.
Adding more angles doesn't seem to do much in terms of coding gain, especially when considering the cost associated. Increasing the chroma angles from 5 to 7, results in a reported encoder runtime increases of 10%, whereas increasing the chroma angles from 5 to 11, results in with reported encoder runtime increases of 40%.
It was asked as to what the performance would be for an encoder that doesn't search all modes, restricting the search to the current five. Such results are not available.
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