Video Compression

VideoNerd

There are two modes of tiling: 
  • Pure Tiling:  a frame is divided into tiles and all tiles share same slice header, offsets to each tile are signaled in the slice header. Therefore there is no need to put start codes to indicate tile starts.
  • Slice-encapsulated Tiling:  a frame is divided into tiles and each tile is encapsulated in its own slice (i.e. each tile is preceded by its own slice header, incl. the start code).                                 Examples:

            the option ‘–slices tiles’ in kvazaar encoder makes slice encapsulation tiling:    

    kvazaar -i container_352x288.yuv –input-res 352×288 –tiles 2×2 -p 30 –slices tiles –mv-constraint frametile –bitrate 500000 -o test2x2.h265 

SVT-HEVC also supports slice-encapsulated tiling, there is the switch  ‘-tile_slice_mode 1’ which makes each tile to be encapsulated in its own slice:

     SvtHevcEncApp.exe -i test1080p.yuv -w 1920 -h 1080 -n 100 -fps 60 -tbr 10000000 -sao 1 -umv 0 -lad 0 -ua-delm 1 -rc 1 -pred-struct 0 -tile_row_cnt 2 -tile_col_cnt 2 -tile_slice_mode 1 -b test2.h265
The second mode (Slice-encapsulated) is relevant when it’s required to compose video from different input streams. So, each tile is actually a self-contained frame from the corresponding input stream. Slice headers are not necessarily be aligned (they can have different initial QPs), although SPS and PPS of all input streams should be aligned.
Tiles 2×2

Tiles 2x1_2 (two vertical tiles and the right tile is divided into two sub-tiles

The Slice Encapsulated Tiling mode enables to create the above collages without re-encoding of input stream, only by manipulation of high-level syntax. The author has a corresponding python script to create various collages.

The requirements to the input streams are: 

1) coherent resolutions

2) same SPS/PPS

3) width and height of each input stream should be divisible by 64.

4) no WPP or Tiles (hierarchical tiling not supported by HEVC).

5) Motion vectors should not cross picture boundaries, i.e. SPS flag motion_vectors_over_pic_boundaries_flag = 0  (in kvazaar there is the switch ‘ –mv-constraint frametile’).

 

 

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