Luma-based chroma intra-prediction for video coding

An encoder for video encoding using luma-based chroma intra-prediction, the encoder comprising:

• at least one processor;
• and
• a non-transitory computer readable storage medium storing a program for execution by the at least one processor, the program including instructions to:
• receive, from a video capture device, a picture;
• divide the picture into a plurality of coding units (CUs) that includes:
• a first CU that includes a first row;
• and
• a second CU that includes a second row immediately adjacent the first row and a third row immediately adjacent the second row, wherein the second row includes a first portion and a second portion immediately adjacent the first CU;
• perform sample padding to obtain padded luma samples for the second portion of the second row without performing sample padding on the third row;
• down sample a first reconstructed luma block of the first CU, to generate a down sampled first reconstructed luma block;
• compute parameters α and β of a linear model by linear regression using a top neighboring reconstructed luma sample and left neighboring reconstructed luma samples of the first reconstructed luma block and reconstructed neighboring chroma samples of a chroma block corresponding to the first reconstructed luma block, and wherein the instructions to compute the parameters α and β are configured to use reconstructed luma samples in the first portion of the second row, to use the padded luma samples in the second portion of the second row, and to omit reconstructed luma samples in the third row;
• and
• compute samples of a first predicted chroma block from corresponding samples of the down sampled first reconstructed luma block using the linear model and the parameters.

A non-transitory computer readable storage medium storing a program for execution by at least one processor, for video encoding using luma-based chroma intra-prediction in a video encoder, the program including instructions to:

• receive, from a video capture device, a picture;
• divide the picture into a plurality of coding units (CUs) that includes:
• a first CU that includes a first row;
• and
• a second CU that includes a second row immediately adjacent the first row and a third row immediately adjacent the second row;
• perform sample padding to obtain padded luma samples for a first portion of the second row, wherein the performing of the sample padding is not performed on the third row;
• down sample a first reconstructed luma block of the first CU to generate a down sampled first reconstructed luma block;
• compute parameters α and β of a linear model by linear regression using a top neighboring reconstructed luma sample and left neighboring reconstructed luma samples of the first reconstructed luma block and reconstructed neighboring chroma samples of a chroma block corresponding to the first reconstructed luma block, and wherein the instructions to compute the parameters α and β use the padded luma samples in the second row and omit reconstructed luma samples in the third row;
• and
• compute samples of a first predicted chroma block from corresponding samples of the down sampled first reconstructed luma block using the linear model and the parameters.