JVET-M0875 Request for flexible unit size tile with implementation friendly restriction [T. Ikai, Y. Yasugi (Sharp), G. Bang (ETRI), Y.-W. Chen, X. Wang (Kwai Inc.), M. Coban (Qualcomm), C.-C. Lin (ITRI), P.-H. Lin (Foxconn), A. Ichigaya (NHK), K. Kawamura (KDDI), K. Kazui (Fujitsu), R. Sjöberg (Ericsson), R. Skupin, K. Sühring, Y. Sanchez, T. Schierl (HHI), L. Zhang (Bytedance)] [late]
This contribution proposes flexible unit size tile with certain restrictions, allowing tiling with tile size unit that is not a multiple of the CTU size. In the tile BoG, the implementation difficulty for less than CTU size and loop filter control for 360° video has been discussed. In this contribution, it is asserted that CTU alignment is too restricted to enable important tile use cases. It is also argued that the proposed specific restrictions alleviate the concerns implementation cost increase.
In JVET-M0066, the main part of this proposal had reportedly been implemented, tested and cross-checked. The software and working draft had been uploaded at 2018-12-28 05:28:56 and this was announced in the JVET main reflector at 2018-12-29.
The contribution reported the following points as the main difficulty for hardware implementation which could affect cost:
- Throughput capability, i.e. Partial size CTUs could need the same time processing time as full size CTUs in pipeline processing.
- Memory bandwidth / compression, i.e. temporal motion vector or other information needs to be transferred between internal memory and external memory via burst transfer.
- Address generator, i.e. address generator needs more flexible calculation which depends on tile position.
The contribution proposed that the minimum unit granularity (TileUnitSizeY) be required to be 32 or larger.
A participant commented that if the 32x32 restriction is imposed, the desired functionality can be obtained with no special support in the standard, at a small loss of coding efficiency – by using 32x32 as the CTU size.
It was also commented that the 32x32 granularity is still restrictive and may not align with natural content boundaries.
The proponent showed some test results that indicated that the penalty of using 32x32 CTUs was large. Other participants questioned whether that penalty could really be that large. The data had not been cross-checked.
A participant commented that pipelining implementation requires fetching and processing data in large chunks with a regular structure that this would interfere with.
There were strong concerns about this expressed by some participants. It was said that the 32x32 CTU case is not as difficult because these come in strings.
Further study was encouraged. It was planned to ask AHG13 to measure the effect of CTU size on coding efficiency.
Flexible tiling (4)
Option 3 of JVET-M0261 is effectively another way to achieve the flexible tiling functionality, through dividing pictures into rectangular sub-pictures and each sub-picture may refer to its own PPS and may hence have its own tile partitioning.