[FFmpeg-user] Valid number of slices for libx264 and FFV1

Jerome Martinez jerome at mediaarea.net
Tue Apr 26 20:51:05 CEST 2016

Le 26/04/2016 à 08:49, Kal Sze a écrit :

> [...]
> And what is the valid number of slices when encoding as FFV1 level 3?
> The wiki says 4, 6, 9, 12, 16, 24, or 30; but I have not seen ffmpeg
> complain if I try to specify something like 25 slices for a 512x424
> video. Does ffmpeg just coerce the value to a valid one?

FFmpeg rejects the command if you provide a "not valid" (from FFmpeg 
point of view) slice number (e.g. 5);

In theory (if we refer to the specs), any count of slices is possible, 
depending of the encoder.

In practice:
- FFmpeg encoder tries to find a value compatible with some constraints 
(minimum 2 rows and 2 columns, rows count must not be > 2x columns 
count, columns count must not be > 2x rows count, and slices count can 
not be more than 64). so 25 is acceptable (5x5, OK), 26 is not (13x2 or 
2x13 are the only combinations, NOK)
- FFmpeg decoder accepts any slices count below 256

Note that it is slightly different than H264, slices are in macro-blocks 
sequence order only (no rows/cols).

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