Intra interpolation filter for multi-line intra prediction

1. A method performed by at least one processor for selecting an intra interpolation filter for multi-line intra prediction based on a reference line index for decoding of a video sequence, the method comprising:

identifying a set of reference lines associated with a coding unit;

applying both an edge smoothing filter with only positive or zero coefficients and an edge preserving filter to reference samples included in a first reference line, of the set of reference lines, that is adjacent to the coding unit to generate a first set of prediction samples based on the first reference line being associated with a first reference line index; and

applying only the edge preserving filter but not the edge smoothing filter to reference samples included in a second reference line, of the set of reference lines, that is non-adjacent to the coding unit to generate a second set of prediction samples based on the second reference line being associated with a second reference line index.

2. The method of claim 1 , wherein the edge smoothing filter comprises a linear interpolation filter having only positive or zero filter coefficients, and

wherein the edge preserving filter comprises at least one of a linear interpolation filter having at least one or two negative filter coefficients, a non-linear filter, or a bilateral filter.

3. The method of claim 1 , wherein the first reference line has a first reference line index having a value of zero.

4. The method of claim 1 , wherein the edge preserving filter comprises at least one of a Cubic filter, a discrete cosine transform (DCT)-based filter, a polynomial-based interpolation filter, a bilateral filter, and a Hermite interpolation filter.

5. The method of claim 1 , wherein the second reference line has a second reference line index having a non-zero value.

6. The method of claim 1 , wherein a first tap number of the edge smoothing filter is different than a second tap number of the edge preserving filter.

7. The method of claim 1 , further comprising:

signaling a reference line index before intra prediction modes, wherein the signaled intra prediction modes excludes planar modes, and wherein the reference line index indicates which reference line is used for intra prediction.

8. A device for selecting an intra interpolation filter for multi-line intra prediction based on a reference line index for decoding of a video sequence, comprising:

at least one memory configured to store program code;

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

identifying code configured to cause the at least one processor to identify a set of reference lines associated with a coding unit;

first applying code configured to cause the at least one processor to apply both an edge smoothing filter with only positive or zero coefficients and an edge preserving filter to reference samples included in a first reference line, of the set of reference lines, that is adjacent to the coding unit to generate a first set of prediction samples based on the first reference line being associated with a first reference line index; and

second applying code configured to cause the at least one processor to apply only the edge preserving filter but not the edge smoothing filter to reference samples included in a second reference line, of the set of reference lines, that is non-adjacent to the coding unit to generate a second set of prediction samples based on the second reference line being associated with a second reference line index.

9. The device of claim 8 , wherein the edge smoothing filter comprises a linear interpolation filter having only positive or zero filter coefficients, and

wherein the edge preserving filter comprises at least one of a linear interpolation filter having at least one or two negative filter coefficients, a non-linear filter, or a bilateral filter.

10. The device of claim 8 , wherein the first reference line has a first reference line index having a value of zero.

11. The device of claim 8 , wherein the edge preserving filter comprises at least one of a Cubic filter, a discrete cosine transform (DCT)-based filter, a polynomial-based interpolation filter, a bilateral filter, and a Hermite interpolation filter.

12. The device of claim 8 , wherein the second reference line has a second reference line index having a non-zero value.

13. The device of claim 8 , wherein a first tap number of the edge smoothing filter is different than a second tap number of the edge preserving filter.

14. The device of claim 8 , further comprising:

signaling code configured to cause the at least one processor to signal a reference line index before intra prediction modes, wherein the signaled intra prediction modes excludes planar modes, and wherein the reference line index indicates which reference line is used for intra prediction.

15. A non-transitory computer-readable medium storing computer program code for selecting an intra interpolation filter for multi-line intra prediction based on a reference line index for decoding of a video sequence that, when executed by one or more processors of a device, cause the one or more processors to at least:

identify a set of reference lines associated with a coding unit;

apply both an edge smoothing filter with only positive or zero coefficients and an edge preserving filter to reference samples included in a first reference line, of the set of reference lines, that is adjacent to the coding unit to generate a first set of prediction samples based on the first reference line being associated with a first reference line index; and

apply only the edge preserving filter but not the edge smoothing filter to reference samples included in a second reference line, of the set of reference lines, that is non-adjacent to the coding unit to generate a second set of prediction samples based on the second reference line being associated with a second reference line index.

16. The non-transitory computer-readable medium of claim 15 , wherein the edge smoothing filter comprises a linear interpolation filter having only positive or zero filter coefficients, and

wherein the edge preserving filter comprises at least one of a linear interpolation filter having at least one or two negative filter coefficients, a non-linear filter, or a bilateral filter.

17. The non-transitory computer-readable medium of claim 15 , wherein the first reference line has a first reference line index having a value of zero.

18. The non-transitory computer-readable medium of claim 15 , wherein the edge preserving filter comprises at least one of a Cubic filter, a discrete cosine transform (DCT)-based filter, a polynomial-based interpolation filter, a bilateral filter, and a Hermite interpolation filter.

19. The non-transitory computer-readable medium of claim 15 , wherein the second reference line has a second reference line index having a non-zero value.

20. The non-transitory computer-readable medium of claim 15 , wherein a first tap number of the edge smoothing filter is different than a second tap number of the edge preserving filter.