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 decoding a video bitstream that includes at least one coding block, the method comprising:
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 whether a position dependent intra prediction combination (PDPC) mode was combined with the selected at least one of the sixty-seven intra prediction modes to encode the pixel in the at least one coding block, wherein determining if the PDPC mode was combined with the at least one of the sixty-seven intra prediction modes during encoding is determined based on at least one of:
a list of constrained intra prediction modes that are excluded from combination with the PDPC mode; and/or
presence or absence of signaling in the video bitstream identifying whether PDPC mode is applicable;
decoding 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 generated based on the PDPC mode, else decoding the at least one coding block using the generated first level predictor; and
determining a pixel value for a plurality of pixels in the at least one coding block; wherein the step of determining if the PDPC mode was combined with the at least one of the sixty-seven intra prediction modes is determined based on the presence or absence of the signaling in accordance with at least one of the following:
a PDPC flag signaled in the video bitstream indicating if the PDPC mode is enabled for the selected intra prediction mode;
a PDPC flag is signaled in the video bitstream and is set to indicate whether the selected intra prediction mode is combined with the PDPC mode during encoding; and/or
a PDPC flag is absent from the video bitstream, the absence indicating that the selected intra prediction mode for a respective coding unit was not combined with the PDPC mode during encoding.
2. The method of claim 1 , wherein determining if the PDPC mode was combined with the at least one of the sixty-seven intra prediction modes is also determined based on the presence or absence of the PDPC flag in the video bitstream and a value of the PDPC flag if signaled is based on the list of constrained intra prediction modes, including at least one of the following indicators.
3. The method of claim 2 , wherein the PDPC flag is signaled in the video bitstream and indicates either 1) that the PDPC mode is enabled and combined with the selected intra prediction mode or 2) the PDPC mode is disabled and not combined with the selected intra prediction mode during encoding.
4. The method of claim 1 , wherein the PDPC flag is signaled at a coding unit or coding block level.
5. The method of claim 1 , wherein the PDPC flag is signaled but is disabled to indicate the PDPC mode is not applicable.
6. The method of claim 1 , wherein if the PDPC flag is signaled, the selected one of the at least one of the sixty-seven intra prediction modes is signaled before the PDPC flag in the video bitstream.
7. The method of claim 1 , wherein 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.