[FFmpeg-devel] [PATCH] avformat/movenc: stop guessing colr atom values

[Michael Bradshaw]

On Mon, Mar 30, 2020 at 4:48 PM Carl Eugen Hoyos <ceffmpeg at gmail.com> wrote:

> I believe there is some historical evidence that this guessing is needed
> to get visually correct output in many cases.


I haven't found any evidence of that. This guessing has been part of the
code since the write_colr flag was originally added. Unfortunately I don't
see the original patch in the ffmpeg-devel archives so I don't know what
discussions there were around it originally. But I would counter with the
following:

   1. The user can guess if they want using
   the -color_primaries/-color_trc/-colorspace flags.
   2. If ffmpeg does want to guess, this is the wrong place to do it. There
   are various parts of ffmpeg that deal with color primaries/trc/space. The
   right way to guess this info is higher up in the pipeline where it can be
   consistently applied throughout the pipeline and bitstream. Doing it
   locally here is just begging for one part of ffmpeg to make one assumption
   and this part to make a different assumption, which would be even worse.
   3. Building on point 2, this is an especially bad place to guess this
   because the color info is a property of the input, not the output.

If this really breaks things for people (which I highly doubt it will) they
can always send a patch with a better and more robust implementation. But I
feel strongly that we should not stabilize the write_colr flag with this
auto-guessing logic. We really shouldn't have people depend on it and
should encourage them to use the -color_primaries/-color_trc/-colorspace
flags themselves for better color guessing.