Constrained position dependent intra prediction combination (PDPC)
A second level intra prediction mode can be combined with one or more of sixty-seven JVET intra prediction modes during encoding of a coding unit in a video bitstream. Embodiments include making a position dependent intra prediction combination (PDPC) mode available as the second level intra prediction mode. In embodiments, when a PDPC (position dependent intra prediction combination) mode is enabled, the second level intra prediction is combined with one of the 67 selected intra predictor modes. In embodiments, the PDPC mode is only enabled or available for a predetermined subset of intra prediction modes (out of 67 possible modes), in order to reduce encoder complexity and potentially improve coding efficiency. The PDPC mode may be identifies as enabled or available by a list of modes or signaling in the video bitstream.
1. A method for coding a video bitstream, the method comprising:
receiving the video bitstream including at least one coding block;
generating a first level predictor for a pixel within said at least one coding block based on neighboring pixels and an intra prediction mode selected from sixty-seven intra prediction modes during encoding of said pixel;
determining if a second level prediction mode was combined with the at least one of the sixty-seven intra prediction modes selected during encoding of the pixel in the at least one coding block;
if the second level prediction mode was combined with the at least one of the sixty-seven intra prediction modes selected during encoding, deriving the pixel in the at least one coding block using the first level predictor generated based on a selected at least one intra prediction mode and a second level prediction process based on the second level prediction mode, else deriving the pixel using the generated first level predictor;
decoding the at least one coding block based on the derived pixel values within the coding block wherein the second level prediction mode is a position dependent intra prediction combination (PDPC) mode that combines the second level prediction process generated using the PDPC mode with the first level predictor;
wherein determining if the second level intra prediction mode was combined with the at least one of the intra prediction modes is determined based on a list of first level prediction modes for which the second level intra prediction mode is enabled and/or disabled;
wherein the list identifies constrained first level prediction modes, wherein constrained first level prediction modes are excluded from combination with the second level intra prediction mode;
wherein planar mode and DC mode are first level intra prediction modes included in the list of constrained modes and a position dependent intra prediction combination (PDPC) mode is the second level prediction mode, the list thereby excluding combination of either the planar mode or the DC mode with the PDPC mode;
wherein the position dependent intra prediction combination (PDPC) mode is further constrained for decoding I slices and excluded for P and/or B slices;
and wherein when the list indicates that a position dependent intra prediction combination (PDPC) mode is disabled for the selected at least one of the intra prediction modes used to generate the first level predictor, only the first level predictor is used for intra prediction of the pixel.
2. The method of claim 1 , wherein the list identifies first level prediction modes for which a position dependent intra prediction combination (PDPC) mode is enabled and/or disabled.
3. The method of claim 1 , wherein the list identifies first level prediction modes that are excluded from combination with a position dependent intra prediction combination (PDPC) mode.
4. The method of claim 1 , wherein a decoder prevents or prohibits the second level intra prediction mode with first level prediction modes listed in the constrained first level prediction modes list.
5. The method of claim 1 , wherein when the list indicates that a PDPC mode is enabled, a final predictor for decoding the at least one coding block is created from the at least one of the sixty-seven intra prediction modes combined with the second level prediction mode.
6. The method of claim 1 , wherein the list indicates whether a PDPC mode is disabled or enabled by identifying which of the sixty-seven intra prediction modes for the first level predictor apply and/or do not apply to the PDPC mode.
7. The method of claim 1 , wherein the list is predetermined and accessible by a decoder.