[FFmpeg-trac] #9772(avfilter:new): colorspace filter fails on trc=linear

[FFmpeg]

#9772: colorspace filter fails on trc=linear
-------------------------------------+-------------------------------------
             Reporter:  Llyw         |                    Owner:  (none)
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:  avfilter
              Version:  unspecified  |               Resolution:
             Keywords:  colorspace   |               Blocked By:
  conversion linear gamma            |
  resampling                         |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
Comment (by Llyw):

 I did some more digging around on my own and please correct me if I'm
 wrong, but it seems that the current situation regarding gamma-aware
 resampling is as follows:

 swscale's `scale` filter is apparently hard-coded to assume a gamma of
 2.2, see,
 https://github.com/FFmpeg/FFmpeg/blob/n5.0.1/libswscale/utils.c#L1614, so
 my above `colorspace=[...],scale=[...],colorspace=[...]` filter chain
 would absolutely not do what I want and it's probably a good thing that
 the bug reported in this issue prevents me from using such a filter chain.

 z.lib/zimg's `zscale` filter can do resampling in a linear gamma domain
 but requires explicit conversion to a linear transfer characteristic using
 up to three zscale filters, see comments in https://github.com/sekrit-
 twc/zimg/issues/141. While the performance is not as good as the scale
 filter in the release/5.0 branch, there seems to have been significant
 improvements in git-master.

 So while I still think that you will probably want to investigate the
 `colorspace` issue with `trc=linear`, it seems that for gamma-aware
 resampling my best bet is to wait for the performance improvements for
 `zscale` being available in a release branch and using that instead, with
 one `zscale` instance converting to a linear transfer characteristic, the
 next one doing the resamling, and the final one converting to the output
 color encoding.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/9772#comment:5>
FFmpeg <https://ffmpeg.org>
FFmpeg issue tracker