Conditional signalling of reference picture list modification information

1. In a computing device that implements a video encoder, a method comprising:

receiving video frames; and

encoding the video frames to produce encoded data in a bitstream, wherein the encoding the video frames includes:

as part of processing for a slice header of a slice, evaluating a condition, wherein the condition depends on whether value of a variable is greater than 1, the variable indicating a number of total reference pictures; and

depending on whether the condition is satisfied, conditionally signaling in the bitstream a flag that indicates whether a reference picture list (“RPL”) is modified according to syntax elements explicitly signaled in the bitstream.

2. The method of claim 1 wherein the flag is conditionally signaled as part of an RPL modification structure.

3. The method of claim 1 wherein the encoding the video frames further includes:

evaluating results of motion compensation for which the RPL is modified according to syntax elements explicitly signaled in the bitstream;

evaluating results of motion compensation for which the RPL is not modified according to syntax elements explicitly signaled in the bitstream; and

modifying the RPL so as to reorder one or more reference pictures for more efficient addressing with reference indices.

4. The method of claim 1 wherein the encoding the video frames further includes:

evaluating results of motion compensation for which the RPL is modified according to syntax elements explicitly signaled in the bitstream;

evaluating results of motion compensation for which the RPL is not modified according to syntax elements explicitly signaled in the bitstream; and

modifying the RPL so as to remove one or more reference pictures based at least in part on frequency of use during encoding.

5. The method of claim 1 wherein the encoding the video frames further includes:

evaluating results of motion compensation for which the RPL is modified according to syntax elements explicitly signaled in the bitstream;

evaluating results of motion compensation for which the RPL is not modified according to syntax elements explicitly signaled in the bitstream; and

modifying the RPL so as to add one or more reference pictures based at least in part on frequency of use during encoding.

6. A computing device comprising memory and one or more processing units, wherein the computing device implements a video decoder system that includes:

a buffer configured to receive encoded data in a bitstream; and

a video decoder configured to perform decoding of the encoded data to produce reconstructed video frames, wherein the decoding of the encoded data includes:

as part of processing for a slice header of a slice, evaluating a condition, wherein the condition depends on whether value of a variable is greater than 1, the variable indicating a number of total reference pictures; and

depending on whether the condition is satisfied, conditionally parsing from the bitstream a flag that indicates whether a reference picture list (“RPL”) is modified according to syntax elements explicitly signaled in the bitstream.

7. The computing device of claim 6 wherein the flag is conditionally parsed from an RPL modification structure.

8. A computer system comprising:

means for, as part of processing for a slice header of a slice, evaluating a condition, wherein the condition depends on whether value of a variable is greater than 1, the variable indicating a number of total reference pictures; and

means for, depending on whether the condition is satisfied, conditionally signaling in or parsing from a bitstream a reference picture list (“RPL”) modification structure, wherein the RPL modification structure includes a flag that indicates whether an RPL is modified according to syntax elements explicitly signaled in the bitstream.

9. The computer system of claim 8 further comprising:

means for evaluating results of motion compensation for which the RPL is modified according to syntax elements explicitly signaled in the bitstream;

means for evaluating results of motion compensation for which the RPL is not modified according to syntax elements explicitly signaled in the bitstream; and

means for modifying the 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.

10. The computer system of claim 8 wherein the RPL is a first RPL, and wherein the RPL modification structure further includes a second flag that indicates whether a second RPL is modified according to syntax elements explicitly signaled in the bitstream.

11. The computer system of claim 8 further comprising:

means for signaling in or parsing from the bitstream another flag that indicates whether the RPL modification structure for the slice may be present in a bitstream, wherein the other flag is signaled as part of a picture parameter set.

12. The method of claim 2 wherein the RPL is a first RPL, and wherein the RPL modification structure further includes a second flag that indicates whether a second RPL is modified according to syntax elements explicitly signaled in the bitstream.

13. The method of claim 1 wherein the encoding the video frames further includes:

signaling in the bitstream another flag that indicates whether an RPL modification structure for the slice may be present in the bitstream, wherein the other flag is signaled as part of a picture parameter set.

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

15. The computing device of claim 7 wherein the RPL is a first RPL, and wherein the RPL modification structure further includes a second flag that indicates whether a second RPL is modified according to syntax elements explicitly signaled in the bitstream.

16. The computing device of claim 6 wherein the decoding of the encoded data further includes:

parsing from the bitstream another flag that indicates whether an RPL modification structure for the slice may be present in the bitstream, wherein the other flag is signaled as part of a picture parameter set.

17. The computing device of claim 6 wherein a current picture includes the slice, the number of total reference pictures accounting for short-term reference pictures used in encoding or decoding for the current picture and accounting for long-term reference pictures used in encoding or decoding for the current picture.

18. The method of claim 1 wherein the syntax elements explicitly signaled in the bitstream specify multiple list entries of the RPL.

19. The computing device of claim 6 wherein the syntax elements explicitly signaled in the bitstream specify multiple list entries of the RPL.

20. The computer system of claim 8 wherein the syntax elements explicitly signaled in the bitstream specify multiple list entries of the RPL.