[FFmpeg-trac] #8779(ffmpeg:new): Zscale transform to ST 2084 EOTF results in black video output

[FFmpeg]

#8779: Zscale transform to ST 2084 EOTF results in black video output
-------------------------------------+-------------------------------------
             Reporter:  zambelli     |                     Type:  defect
               Status:  new          |                 Priority:  important
            Component:  ffmpeg       |                  Version:  git-
             Keywords:  zscale,      |  master
  libzimg, z.lib, smpte2084,         |               Blocked By:
  st2084, hdr                        |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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 == Summary of the bug: ==
 Sometime between releases 4.2.2 and 4.2.3 a bug was introduced into Zscale
 filter which causes the video output of the filter to be reduced to black
 when transforming video using SMPTE ST 2084 (PQ) EOTF.

 == How to reproduce: ==
 Source video:
 https://hulu.box.com/shared/static/5o419x2dag93wx5f0msc69gbljfydri1.mp4

 ffmpeg version git-2020-06-28-4cfcfb3
 built with gcc 9.3.1 (GCC) 20200621
 {{{
 ffmpeg.exe -y -i zscale_test_source.mp4 -vf
 zscale=t=linear:w=640:h=360,format=gbrpf32le,zscale=t=smpte2084:p=bt709:m=bt709,format=yuv420p10le
 -vcodec ffv1 zscale_test_smpte2084.avi
 }}}
 Sample output:
 https://hulu.box.com/shared/static/6a8yjs5mel2scv63vvpmrn0438fqbiil.avi

 If t=smpte2084 is replaced with another EOTF, such as bt709, the filter
 produces the correct output:

 {{{
 ffmpeg.exe -y -i zscale_test_source.mp4 -vf
 zscale=t=linear:w=640:h=360,format=gbrpf32le,zscale=t=bt709:p=bt709:m=bt709,format=yuv420p10le
 -vcodec ffv1 zscale_test_bt709.avi
 }}}
 Sample output:
 https://hulu.box.com/shared/static/uuy3gfhg6mwflrqmhvb2bvkl7009nlf4.avi

 The bug does not repro in FFmpeg release 4.2.2. To the best of my memory
 while working with nightly builds, this Zscale transform worked correctly
 until at least early May 2020.

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Ticket URL: <https://trac.ffmpeg.org/ticket/8779>
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