Restriction of prediction units in B slices to uni-directional inter prediction

1. A method for coding video data, the method comprising:

determining, based on a size characteristic of a prediction unit (PU) in a B slice of the video data being below a threshold, that the PU is restricted to uni-directional inter prediction;

generating a merge candidate list for the PU;

determining a selected merge candidate in the merge candidate list, wherein the selected merge candidate is a bi-directional merge candidate, wherein two different reference blocks are associated with motion information specified by the selected merge candidate; and

based on the PU being restricted to uni-directional inter prediction, converting the selected merge candidate from being a bi-directional merge candidate to being a uni-directional merge candidate by keeping a list 0 motion vector of the selected merge candidate and ignoring a list 1 motion vector of the selected merge candidate such that a predictive video block for the PU is generated based on no more than one of the two different reference blocks associated with the motion information specified by the selected merge candidate.

2. The method of claim 1 , wherein determining that the PU is restricted to uni-directional inter prediction comprises determining, based on a height or a width of a video block associated with the PU being below the threshold, that the PU is restricted to uni-directional inter prediction.

3. The method of claim 1 , wherein determining that the PU is restricted to uni-directional inter prediction comprises determining, based on a first dimension of a video block associated with the PU being less than a threshold and a second dimension of the video block associated with the PU being less than or equal to the threshold, that the PU is restricted to uni-directional inter prediction.

4. The method of claim 1 , wherein determining that the PU is restricted to uni-directional inter prediction comprises determining, based on a first dimension of a video block associated with the PU being less than a first threshold and a second dimension of the video block associated with the PU being less than a second threshold, that the PU is restricted to uni-directional inter prediction.

5. The method of claim 4 , wherein the first threshold is the same as the second threshold.

6. The method of claim 1 , wherein determining that the PU is restricted to uni-directional prediction comprises:

determining, based on a size characteristic of a coding unit (CU) associated with the PU being less than or equal to a particular size and the size characteristic of the PU being below the threshold, that the PU is restricted to uni-directional inter prediction, wherein the size characteristic of the CU is a height or a width of a video block associated with the CU.

7. The method of claim 6 , wherein the particular size is equal to eight and the threshold is equal to eight.

8. The method of claim 1 , wherein determining the selected merge candidate comprises:

determining, based on a candidate index value for the PU, the selected merge candidate in the merge candidate list;

determining motion information of the PU based on the selected merge candidate;

generating a predictive block for the current PU based on a reference block associated with the list 0 motion vector of the selected merge candidate; and

using the predictive block for the current PU and residual video blocks associated with transform units of a coding unit (CU) to reconstruct a video block of the CU.

9. The method of claim 1 , further comprising generating a bitstream that includes an encoded syntax element that indicates a position in the merge candidate list of the selected merge candidate.

10. The method of claim 1 , wherein the method is performed on a mobile computing device.

11. A video coding device that comprises:

a data storage medium configured to store video data; and

one or more processors configured to:

determine, based on a size characteristic of a prediction unit (PU) in a B slice of the video data being below a threshold, that the PU is restricted to uni-directional inter prediction;

determine that the PU is not restricted to uni-directional inter prediction if the size characteristic of the PU is not below the threshold;

generate a merge candidate list for the PU;

determine a selected merge candidate in the merge candidate list, wherein the selected merge candidate is a bi-directional merge candidate and two different reference blocks are associated with motion information specified by the selected merge candidate; and

based on the PU being restricted to uni-directional inter prediction, convert the selected merge candidate from being a bi-directional merge candidate to being a uni-directional merge candidate by keeping a list 0 motion vector of the selected merge candidate and ignoring a list 1 motion vector of the selected merge candidate such that a predictive video block for the PU is generated based on no more than one of the two different reference blocks associated with the motion information specified by the selected merge candidate.

12. The video coding device of claim 11 , wherein the one or more processors are configured to determine, based on a height or a width of a video block associated with the PU being below the threshold, that the PU is restricted to uni-directional inter prediction.

13. The video coding device of claim 11 , wherein the one or more processors are configured to determine, based on a first dimension of a video block associated with the PU being less than a threshold and a second dimension of the video block associated with the PU being less than or equal to the threshold, that the PU is restricted to uni-directional inter prediction.

14. The video coding device of claim 11 , wherein the one or more processors are configured to determine, based on a first dimension of a video block associated with the PU being less than a first threshold and a second dimension of the video block associated with the PU being less than a second threshold, that the PU is restricted to uni-directional inter prediction.

15. The video coding device of claim 14 , wherein the first threshold is the same as the second threshold.

16. The video coding device of claim 11 , wherein the one or more processors are configured to:

determine, based on a size characteristic of a coding unit (CU) associated with the PU being less than or equal to a particular size and the size characteristic of the PU being below the threshold, that the PU is restricted to uni-directional inter prediction.

17. The video coding device of claim 16 , wherein the particular size is equal to eight and the threshold is equal to eight.

18. The video coding device of claim 11 , wherein the video coding device decodes video data and the one or more processors are configured to determine the selected merge candidate based on a syntax element that indicates a position in the merge candidate list of the selected merge candidate.

19. The video coding device of claim 11 , wherein the video coding device encodes video data and the one or more processors are configured to output a syntax element that indicates a position in the merge candidate list of the selected merge candidate.

20. The video coding device of claim 11 , wherein the video coding device is a mobile computing device comprising at least one of a display configured to display picture data associated with the PU or a camera configured to generate the picture data associated with the PU.

21. A video coding device comprising:

means for determining, based on a size characteristic of a prediction unit (PU) in a B slice of the video data being below a threshold, that the PU is restricted to uni-directional inter prediction;

means for generating a merge candidate list for the PU;

means for determining a selected merge candidate in the merge candidate list, wherein the selected merge candidate is a bi-directional merge candidate, wherein two different reference blocks are associated with motion information specified by the selected merge candidate; and

means for converting, based on the PU being restricted to uni-directional inter prediction, the selected merge candidate to being a uni-directional merge candidate by keeping a list 0 motion vector of the selected merge candidate and ignoring a list 1 motion vector of the selected merge candidate such that a predictive video block for the PU is generated based on no more than one of the two different reference blocks associated with the motion information specified by the selected merge candidate.

22. A non-transitory computer readable storage medium that stores instructions that, when executed, configure one or more processors to:

determine, based on a size characteristic of a prediction unit (PU) in a B slice of the video data being below a threshold, that the PU is restricted to uni-directional inter prediction;

generate a merge candidate list for the PU;

determine a selected merge candidate in the merge candidate list, wherein the selected merge candidate is a bi-directional merge candidate, wherein two different reference blocks are associated with motion information specified by the selected merge candidate; and

based on the PU being restricted to uni-directional inter prediction, convert the selected merge candidate from being a bi-directional merge candidate to being a uni-directional merge candidate by keeping a list 0 motion vector of the selected merge candidate and ignoring a list 1 motion vector of the selected merge candidate such that a predictive video block for the PU is generated based on no more than one of the two different reference blocks associated with the motion information specified by the selected merge candidate.

23. The video coding device of claim 11 , further comprising a display configured to display the decoded video data.

24. The video coding device of claim 11 , further comprising a camera configured to capture the video data.