Conditional signalling of reference picture list modification information

1. In a computer system that implements a video decoder, a method comprising:

receiving encoded data in a bitstream; and

decoding the encoded data to produce reconstructed video, including:

determining a value of a variable that indicates a number of total reference pictures;

as part of processing for a slice header of a current slice, determining that a condition is satisfied, wherein the condition depends at least in part on the value of the variable that indicates the number of total reference pictures; and

responsive to the condition being satisfied:

parsing from the bitstream a first flag that indicates whether a first reference picture list (“RPL”) is modified according to first syntax elements explicitly signaled in the bitstream; and

responsive to the current slice being a B slice, parsing from the bitstream a second flag that indicates whether a second RPL is modified according to second syntax elements explicitly signaled in the bitstream.

2. The method of claim 1 , wherein the first flag and the second flag are parsed from an RPL modification structure.

3. The method of claim 1 , wherein a current picture includes the current slice, the number of total reference pictures accounting for short-term reference pictures used in decoding for the current picture and accounting for long-term reference pictures used in decoding for the current picture.

4. The method of claim 1 , wherein:

if the first flag indicates the first RPL is modified, the first syntax elements, which are explicitly signaled in the bitstream, specify multiple list entries of the first RPL;

otherwise, the first flag indicating the first RPL is not modified, the first syntax elements are missing from the bitstream;

if the second flag indicates the second RPL is modified, the second syntax elements, which are explicitly signaled in the bitstream, specify multiple list entries of the second RPL; and

otherwise, the second flag indicating the second RPL is not modified, the second syntax elements are missing from the bitstream.

5. The method m of claim 1 , wherein the decoding the encoded data further includes parsing from the bitstream another flag, the other flag indicating whether an RPL modification structure may be present in the bitstream.

6. The method of claim 5 , wherein the other flag is signaled as part of a picture parameter set.

7. One or more computer-readable media having stored thereon encoded data in a bitstream, the one or more computer-readable media being selected from the group consisting of volatile memory, non-volatile memory, and storage media, the encoded data being organized to facilitate decoding of the encoded data, to produce reconstructed video, by operations comprising:

determining a value of a variable that indicates a number of total reference pictures;

as part of processing for a slice header of a current slice, determining that a condition is satisfied, wherein the condition depends at least in part on the value of the variable that indicates the number of total reference pictures; and

responsive to the condition being satisfied:

parsing from the bitstream a first flag that indicates whether a first reference picture list (“RPL”) is modified according to first syntax elements explicitly signaled in the bitstream; and

responsive to the current slice being a B slice, parsing from the bitstream a second flag that indicates whether a second RPL is modified according to second syntax elements explicitly signaled in the bitstream.

8. The one or more computer-readable media of claim 7 , wherein the first flag and the second flag are parsed from an RPL modification structure.

9. The one or more computer-readable media of claim 7 , wherein a current picture includes the current slice, the number of total reference pictures accounting for short-term reference pictures used in decoding for the current picture and accounting for long-term reference pictures used in decoding for the current picture.

10. The one or more computer-readable media of claim 7 , wherein:

if the first flag indicates the first RPL is modified, the first syntax elements, which are explicitly signaled in the bitstream, specify multiple list entries of the first RPL;

otherwise, the first flag indicating the first RPL is not modified, the first syntax elements are missing from the bitstream;

if the second flag indicates the second RPL is modified, the second syntax elements, which are explicitly signaled in the bitstream, specify multiple list entries of the second RPL; and

otherwise, the second flag indicating the second RPL is not modified, the second syntax elements are missing from the bitstream.

11. The one or more computer-readable media of claim 7 , wherein the operations further comprise parsing from the bitstream another flag, the other flag indicating whether an RPL modification structure may be present in the bitstream.

12. The one or more computer-readable media of claim 11 , wherein the other flag is signaled as part of a picture parameter set.

13. A computer system comprising:

a buffer, implemented using memory, configured to receive video; and

a video encoder, implemented using one or more processing units and memory, configured to perform operations to encode the video to produce encoded data in a bitstream, wherein the operations include:

determining a value of a variable that indicates a number of total reference pictures;

as part of processing for a slice header of a current slice, determining that a condition is satisfied, wherein the condition depends at least in part on the value of the variable that indicates the number of total reference pictures; and

responsive to the condition being satisfied:

signaling in the bitstream a first flag that indicates whether a first reference picture list (“RPL”) is modified according to first syntax elements explicitly signaled in the bitstream; and

responsive to the current slice being a B slice, signaling in the bitstream a second flag that indicates whether a second RPL is modified according to second syntax elements explicitly signaled in the bitstream.

14. The computer system of claim 13 , wherein the first flag and the second flag are signaled in an RPL modification structure.

15. The computer system of claim 13 , wherein:

if the first flag indicates the first RPL is modified, the first syntax elements, which are explicitly signaled in the bitstream, specify multiple list entries of the first RPL;

otherwise, the first flag indicating the first RPL is not modified, the first syntax elements are missing from the bitstream;

if the second flag indicates the second RPL is modified, the second syntax elements, which are explicitly signaled in the bitstream, specify multiple list entries of the second RPL; and

otherwise, the second flag indicating the second RPL is not modified, the second syntax elements are missing from the bitstream.

16. The computer system of claim 13 , wherein the operations further comprise signaling in the bitstream another flag, the other flag indicating whether an RPL modification structure may be present in the bitstream.

17. The computer system of claim 16 , wherein the other flag is signaled as part of a picture parameter set.

18. The computer system of claim 13 , wherein a current picture includes the current slice, the number of total reference pictures accounting for short-term reference pictures used in decoding for the current picture and accounting for long-term reference pictures used in decoding for the current picture.

19. The computer system of claim 13 , wherein the operations further comprise:

evaluating results of motion compensation for which the first RPL is modified according to the first syntax elements;

evaluating results of motion compensation for which the first RPL is not modified according to the first syntax elements; and

modifying the first RPL so as to accomplish one or more of (a) reordering one or more reference pictures for more efficient addressing with reference indices, (b) removing one or more reference pictures based at least in part on frequency of use during encoding, and (c) adding one or more reference pictures based at least in part on frequency of use during encoding.

20. The computer system of claim 13 , wherein the operations further comprise:

evaluating results of motion compensation for which the second RPL is modified according to the second syntax elements;

evaluating results of motion compensation for which the second RPL is not modified according to the second syntax elements; and

modifying the second RPL so as to accomplish one or more of (a) reordering one or more reference pictures for more efficient addressing with reference indices, (b) removing one or more reference pictures based at least in part on frequency of use during encoding, and (c) adding one or more reference pictures based at least in part on frequency of use during encoding.