JVET-L0415 Tile groups for VVC [R. Sjöberg, M. Damghanian, M. Pettersson (Ericsson)]

This contribution proposes that tiles are adopted into VVC and that slices in VVC consist of an integer number of complete tiles instead of consisting of an integer number of complete CTUs.

The following was proposed:

• Adopt tiles into the VVC draft (HEVC tiles or flexible tiles as in JVET-K0260)
• Change the name of slices to tile groups. Keep the slice header but call it tile group header
• Replace the slice address by a tile group address in the tile group header
  • This means that the existing slices become tile groups that always contain an integer number of complete tiles
  • This also means that raster-scan CTU slices are no longer supported
• Add a syntax element num_tiles_in_tile_group that specifies the number of tiles there is in the tile group. This is used both for the decoder to know when the tile group ends and to know how many tile pointers there are in the tile group header
• Remove the end_of_slice_flag syntax element, instead the end of the tile group is given by the tile group address and the num_tiles_in_tile_group code word.

This is proposed as a starting point for tiles.

The number of tiles in a tile group is established in the NAL unit level syntax rather than at the parameter set level.

Entry points are proposed to be provided for every tile, which makes it necessary to know how many tiles will be in the “tile group”.

This proposes to adopt tiles into VVC (which had already been agreed earlier in the meeting).

Changing “slice” to “tile groups” would be needed (editorial).

Using a tile group address was discussed.

This was similar to JVET-L0306.

It was initially agreed to support multiple tiles in a tile group (otherwise tiles would be forced to be larger than necessary). As a starting point, a tile group is a string of tiles starting at a tile address in raster order and ending where indicated by a tile-level ending flag that is otherwise similar to the HEVC CTU-level more_data_in_slice_flag. Software implementation in a timely manner is required. Text is per JVET-L0306.

In the Sunday morning plenary (see section 11.1), it was agreed that instead of ending each tile with a more_data_in_group_flag, we would indicate the number of tiles in the tile group header and provide entry points to the start of each tile. This was further discussed and confirmed Thursday 11 October 1245 (GJS).

Various aspects remained to be further studied – potentially rectangular shape, potentially non-grid layouts, etc., TBD.

This was to be just a starting point – with the raster order aspect having no presumptive status.