Method and apparatus for intra prediction using linear model

An encoding device, comprising:

• a video data memory;
• and
• a video encoder configured to:
• perform intra prediction using a linear model by:
• determining subsampling ratio information in horizontal and vertical directions based on chroma format information, wherein the chroma format information indicates a chroma format of a picture that a current block belongs to, wherein the subsampling ratio information in horizontal and vertical directions are represented by values of variables SubWidthC and SubHeightC, respectively;
• determining a down-sampling filter used for the values of the variables SubWidthC and SubHeightC;
• obtaining down-sampled luma samples of reconstructed luma samples in a luma block of the current block and down-sampled luma reference samples of selected neighboring luma reference samples of the luma block using the down-sampling filter;
• determining one or more linear model coefficients based on the obtained down-sampled luma reference samples and neighboring chroma reference samples that correspond to the obtained down-sampled luma reference samples;
• and
• obtaining prediction samples of a chroma block that corresponds to the luma block based on the determined one or more linear model coefficients and the obtained down-sampled luma samples of the reconstructed luma samples in the luma block;
• and
• generate a bitstream including a plurality of syntax elements, wherein the plurality of syntax elements includes a SPS flag indicating whether a subsampled chroma sample is co-located with the corresponding luma sample.

A non-transitory computer-readable medium storing computer instructions that, when executed by one or more processors, cause the one or more processors to perform encoding operations comprising:

• performing intra prediction using a linear model by:
• determining subsampling ratio information in horizontal and vertical directions based on chroma format information, wherein the chroma format information indicates a chroma format of a picture that a current block belongs to, wherein the subsampling ratio information in horizontal and vertical directions are represented by values of variables SubWidthC and SubHeightC, respectively;
• determining a down-sampling filter used for the values of the variables SubWidthC and SubHeightC;
• obtaining down-sampled luma samples of reconstructed luma samples in a luma block of the current block and down-sampled luma reference samples of selected neighboring luma reference samples of the luma block using the down-sampling filter;
• determining one or more linear model coefficients based on the obtained down-sampled luma reference samples and neighboring chroma reference samples that correspond to the obtained down-sampled luma reference samples;
• and
• obtaining prediction samples of a chroma block that corresponds to the luma block based on the determined one or more linear model coefficients and the obtained down-sampled luma samples of the reconstructed luma samples in the luma block;
• and
• generating a bitstream including a plurality of syntax elements, wherein the plurality of syntax elements includes a SPS flag indicating whether a subsampled chroma sample is co-located with the corresponding luma sample.

A non-transitory computer-readable medium storing an encoded bitstream for video signals, the encoded bitstream comprises a plurality of syntax elements, wherein the plurality of syntax elements comprises a SPS flag indicating whether a subsampled chroma sample is co-located with the corresponding luma sample:

• wherein a value of the SPS flag corresponds to an intra prediction process comprising:
• determining subsampling ratio information in horizontal and vertical directions based on chroma format information, wherein the chroma format information indicates a chroma format of a picture that a current block belongs to, wherein the subsampling ratio information in horizontal and vertical directions are represented by values of variables SubWidthC and SubHeightC, respectively;
• determining a down-sampling filter corresponding to the values of the variables SubWidthC and SubHeightC based on whether the subsampled chroma sample is co-located with the corresponding luma sample;
• obtaining down-sampled luma samples of reconstructed luma samples in a luma block of the current block and down-sampled luma reference samples of selected neighboring luma reference samples of the luma block using the down-sampling filter;
• determining one or more linear model coefficients based on the obtained down-sampled luma reference samples and neighboring chroma reference samples that correspond to the obtained down-sampled luma reference samples;
• and
• obtaining prediction samples of a chroma block that corresponds to the luma block based on the determined one or more linear model coefficients and the obtained down-sampled luma samples of the reconstructed luma samples in the luma block.