Conditional signalling of reference picture list modification information

1. One or more computer-readable memory or storage devices having stored therein encoded data produced by operations comprising:

receiving video frames; and

encoding the video frames to produce the 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 one or more computer-readable memory or storage devices of claim 1 , wherein the flag is conditionally signaled as part of an RPL modification structure.

3. The one or more computer-readable memory or storage devices 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.

4. The one or more computer-readable memory or storage devices of claim 1 , wherein the syntax elements explicitly signaled in the bitstream specify multiple list entries of the RPL.

5. The one or more computer-readable memory or storage devices 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.

6. The one or more computer-readable memory or storage devices 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.

7. One or more computer-readable memory or storage devices having stored therein encoded data, the encoded data being organized for decoding according to operations comprising:

receiving the encoded data in a bitstream; and

decoding 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.

8. The one or more computer-readable memory or storage devices of claim 7 , wherein the flag is conditionally parsed from an RPL modification structure.

9. The one or more computer-readable memory or storage devices 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.

10. The one or more computer-readable memory or storage devices of claim 7 , wherein the decoding 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.

11. The one or more computer-readable memory or storage devices of claim 7 , 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.

12. The one or more computer-readable memory or storage devices of claim 7 , wherein the syntax elements explicitly signaled in the bitstream specify multiple list entries of the RPL.

13. A computer system comprising one or more processing units and memory, wherein the computer system implements a video encoder system comprising:

a buffer configured to receive video frames; and

a video encoder configured to encode the video frames to produce the encoded data in a bitstream, wherein the video encoder is configured to perform operations comprising:

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.

14. The computer system of claim 13 , wherein the flag is conditionally signaled as part of an RPL modification structure.

15. The computer system of claim 14 , 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 computer system of claim 13 , wherein the syntax elements explicitly signaled in the bitstream specify multiple list entries of the RPL.

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

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.

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

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.

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

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.

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

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.