Implicit coding of reference line index used in intra prediction

A device for decoding video data, the device comprising:

• one or more memory devices;
• and
• one or more processors configured to:
• determine that a current block of video data is coded using a linear model prediction mode, wherein the linear model prediction mode is used to predict a chroma component of the current block, wherein the current block comprises a luma component and at least one chroma component including the chroma component predicted using the linear model prediction mode;
• determine that the luma component of the current block is coded using an intra prediction mode;
• receive a syntax element set to a value;
• in response to the value of the syntax element being equal to 2, determine that a reference line index for the luma component of the current block is equal to 3, wherein the reference line index identifies a starting line in the luma component of the one or more already decoded neighboring blocks from which to intra predict luma samples for the luma component of the current block;
• for the luma component of the current block, determine reconstructed luma samples from the intra predicted luma samples;
• based on the reference line index being equal to 3, determine the starting line in the luma component of the one or more already decoded neighboring blocks to be used for determining the values for the linear parameters;
• based on luma samples in a luma component of one or more already decoded neighboring blocks and chroma samples in a chroma component of the one or more already decoded neighboring blocks, determine values for linear parameters, wherein the luma samples in the luma component of the one or more already decoded neighboring blocks comprise luma samples from the starting line in the luma component of the one or more already decoded neighboring blocks, wherein the starting line in the luma component of the one or more already decoded neighboring blocks is at least one line removed from a border line of the luma component of the current block;
• based on the linear parameters and the reconstructed luma samples, generate prediction chroma samples for the chroma component of the current block;
• and
• reconstruct the chroma component of the current block based on the generated prediction chroma samples and received residual data.

A non-transitory computer-readable storage medium storing instructions that when executed cause one or more processors of a device for coding video data to:

• determine that a current block of video data is coded using a linear model prediction mode, wherein the linear model prediction mode is used to predict a chroma component of the current block, wherein the current block comprises a luma component and at least one chroma component including the chroma component predicted using the linear model prediction mode;
• determine that the luma component of the current block is coded using an intra prediction mode;
• receive a syntax element set to a value;
• in response to the value of the syntax element being equal to 2, determine that a reference line index for the luma component of the current block is equal to 3, wherein the reference line index identifies a starting line in the luma component of the one or more already decoded neighboring blocks from which to intra predict luma samples for the luma component of the current block;
• for the luma component of the current block, determine reconstructed luma samples from the intra predicted luma samples;
• based on the reference line index being equal to 3, determine the starting line in the luma component of the one or more already decoded neighboring blocks to be used for determining the values for the linear parameters;
• based on luma samples in a luma component of one or more already decoded neighboring blocks and chroma samples in a chroma component of the one or more already decoded neighboring blocks, determine values for linear parameters, wherein the luma samples in the luma component of the one or more already decoded neighboring blocks comprise luma samples from the starting line in the luma component of the one or more already decoded neighboring blocks, wherein the starting line in the luma component of the one or more already decoded neighboring blocks is at least one line removed from a border line of the luma component of the current block;
• based on the linear parameters and the reconstructed luma samples, generate prediction chroma samples for the chroma component of the current block;
• and
• reconstruct the chroma component of the current block based on the generated prediction chroma samples and received residual data.

A device for decoding video data, the device comprising:

• means for determining that a current block of video data is coded using a linear model prediction mode, wherein the linear model prediction mode is used to predict a chroma component of the current block, wherein the current block comprises a luma component and at least one chroma component including the chroma component predicted using the linear model prediction mode;
• means for determining that the luma component of the current block is coded using an intra prediction mode;
• means for receiving a syntax element set to a value;
• means for determining that a reference line index for the luma component of the current block is equal to 3 in response to the value of the syntax element being equal to 2, wherein the reference line index identifies a starting line in the luma component of the one or more already decoded neighboring blocks from which to intra predict luma samples for the luma component of the current block;
• means for determing reconstructed luma samples for the luma component of the current block;
• means for determining values for linear parameters based on luma samples in a luma component of one or more already decoded neighboring blocks and chroma samples in a chroma component of the one or more already decoded neighboring blocks, wherein the luma samples in the luma component of the one or more already decoded neighboring blocks comprise luma samples from the starting line in the luma component of the one or more already decoded neighboring blocks, wherein the starting line in the luma component of the one or more already decoded neighboring blocks is at least one line removed from a border line of the luma component of the current block;
• means for generating prediction chroma samples for the chroma component of the current block based on the linear parameters and the reconstructed luma samples;
• and
• means for reconstructing the chroma component of the current block based on the generated prediction chroma samples and received residual data.