International organisation for standardisation organisation internationale de normalisation

5.13.4Switching granularity

5.13.4.1.1.1.1.1.1JCTVC-H0391 AHG15: Slice-Level Control of In-Loop Filter [R. Srinivasan, C. Ghone (TI)]

The current draft of HEVC and the HM5.0 code support turning off the loop-filter across all slices in a picture. However, for better video quality, it was suggested to be desirable to be able to turn off loop-filter across a subset of a few slices only. One such scenario is "gradual decoder refresh". Here, the video data is "refreshed" (compression is done independently of previous frames that might not be available to the decoder) one slice at a time. In this case, it is desirable to turn off the loop-filter only for a slice that is next to a "refreshed" slice, so that the "refreshed" slice does not depend on "non-refreshed" data. It was suggested that loop-filtering across slices should be allowed for all other data so as to not compromise on video quality. This was asserted to not be possible with the current draft of HEVC. This contribution proposed a syntax change to allow for control of the loop filters to be made available at each slice.

The contribution seemed to have been based on some prior version of the WD. The requested functionality was actually already in the current WD, except with respect to SAO, which is currently restricted to be static within the picture. No action was taken on this at the moment.

Decision: We should have similar control capability as in AVC – able at slice level to turn off deblocking entirely, or only at boundaries (top/left), or not turned off. When disabled at the SPS level, no filter control should be sent at the slice header level. When deblocking is turned off only at slice boundaries, ALF and SAO should also be turned off at slice boundaries.

5.13.4.1.1.1.1.1.2JCTVC-H0392 AHG15: Enable picture-level CU/TU size signalling in HEVC [N. Srinivasamurthy, K. Prasad, Y. Dutt, M. Zhou (TI)]

This contribution proposed to allow flexibility in Max CU and TU size for different pictures of a coded video sequence. CU and TU tree size is indicated in the syntax of the sequence parameter set. This means that inter and intra frames need to have the same minimum and maximum CU and TU size. It was asserted that often, in DSP and HWA implementation, one may prefer to use different CU and TU tree sizes for inter and intra frames, for performance and implementation reasons. In this submission, it was proposed that the syntax elements that control CU and TU tree size should be conditionally sent at the PPS level also.

It was asked whether there was substantial overhead for having "always splitting" operation with a particular LCU size. The full implications of the proposed flexibility seemed to need further study. Further study was encouraged.