Mode list generation for multi-line intra prediction

1. A method of signaling an intra prediction mode used to encode a current block in an encoded video bitstream using at least one processor, the method comprising:

generating a first most probable mode (MPM) list corresponding to a zero reference line of the current block, wherein the first MPM list comprises a first plurality of intra prediction modes;

generating a second MPM list corresponding to one or more non-zero reference lines of the current block, wherein the second MPM list comprises a second plurality of intra prediction modes, the second plurality of intra prediction modes comprising a subset of the first plurality of intra prediction modes;

signalling a reference line index indicating a reference line used to encode the current block from among the zero reference line and the one or more non-zero reference lines; and

signalling an intra mode index indicating the intra prediction mode,

wherein, based on the reference line index indicating that the reference line is the zero reference line, the intra mode index indicates the intra prediction mode within the first MPM list, or based on the reference line index indicating that the reference line is one from among the one or more non-zero reference lines, the intra mode index indicates the intra prediction mode within the second MPM list,

wherein based on the reference line index indicating that the reference line is the zero reference line, the intra mode index is signaled using 5 bins.

2. The method of claim 1 , wherein the zero reference line comprises a nearest reference line from among a plurality of reference lines of the current block.

3. The method of claim 1 , wherein the second MPM list is smaller than the first MPM list.

4. The method of claim 3 , wherein a number of the modes of the first MPM list is six, and a number of modes of the second MPM list is four.

5. The method of claim 1 , wherein the reference line index is signaled before the intra mode index.

6. A device for signaling an intra prediction mode used to encode a current block in an encoded video bitstream, the device comprising:

at least one memory configured to store program code; and

at least one processor configured to read the program code and operate as instructed by the program code, the program code including:

first generating code configured to cause the processor to generate a first most probable mode (MPM) list corresponding to a zero reference line of the current block, wherein the first MPM list comprises a first plurality of intra prediction modes;

second generating code configured to cause the processor to generate a second MPM list corresponding to one or more non-zero reference lines of the current block, wherein the second MPM list comprises a second plurality of intra prediction modes, the second plurality of intra prediction modes comprising a subset of the first plurality of intra prediction modes;

first signaling code configured to cause the processor to signal a reference line index indicating a reference line used to encode the current block from among the zero reference line and the one or more non-zero reference lines; and

second signalling code configured to cause the processor to signal an intra mode index indicating the intra prediction mode,

wherein, based on the reference line index indicating that the reference line is the zero reference line, the intra mode index indicates the intra prediction mode within the first MPM list, or based on the reference line index indicating that the reference line is one from among the one or more non-zero reference lines, the intra mode index indicates the intra prediction mode within the second MPM list,

wherein based on the reference line index indicating that the reference line is the zero reference line, the intra mode index is signaled using 5 bins.

7. The device of claim 6 , wherein the second MPM list is smaller than the first MPM list.

8. The device of claim 7 , wherein a number of the modes of the first MPM list is six, and a number of modes of the second MPM list is four.

9. The device of claim 6 , wherein the reference line index is signaled before the intra mode index.

10. A non-transitory computer-readable medium storing instructions, the instructions comprising: one or more instructions that, when executed by one or more processors of a device for signaling an intra prediction mode used to encode a current block in an encoded video bitstream, cause the one or more processors to:

generate a first most probable mode (MPM) list corresponding to a zero reference line of the current block, wherein the first MPM list comprises a first plurality of intra prediction modes;

generate a second MPM list corresponding to one or more non-zero reference lines of the current block, wherein the second MPM list comprises a second plurality of intra prediction modes, the second plurality of intra prediction modes comprising a subset of the first plurality of intra prediction modes;

signal a reference line index indicating a reference line used to encode the current block from among the zero reference line and the one or more non-zero reference lines; and

signal an intra mode index indicating the intra prediction mode,

wherein, based on the reference line index indicating that the reference line is the zero reference line, the intra mode index indicates the intra prediction mode within the first MPM list, or based on the reference line index indicating that the reference line is one from among the one or more non-zero reference lines, the intra mode index indicates the intra prediction mode within the second MPM list,

wherein based on the reference line index indicating that the reference line is the zero reference line, the intra mode index is signaled using 5 bins.

11. The method of claim 1 , wherein the first MPM list includes at least one of a DC mode and a planar mode, and

wherein the second MPM list includes all modes of the first MPM list except for the at least one of the DC mode and the planar mode.

12. The method of claim 1 , wherein based on the reference line index indicating that the reference line is the one from among the one or more non-zero reference lines, the intra mode index is signaled using 4 bins.