Garbled output when doing colorspace conversion with zscale, since recent changes

[Marth64]

Summary of the bug: Yesterday, a user on IRC #ffmpeg presented a sample image and said that when they were trying to re-encode it as a looping H265 video with some colorspace conversions, the output was garbled. The specific claim was that the resulting video was a half-black image, hiding most of the original source picture, and that the dynamic color range is lost. I am focusing this ticket on the half-black image part of the issue since the output is not watchable.

I was able to help the user verify this behavior and confirmed it was a bug, at least had them to roll back to an older version where this behavior does not exist.

This must have been recently introduced, since when I run from a build on 12-27 I was not able to reproduce the issue. So sometime between 12-27 and 1-4, this bug was introduced.

The user was kind enough to share the source image and give permission to post it here. Attached files:

• colors2.exr - image from user
• output(latest).mkv - buggy output
• output(6.0.1).mkv - expected output

How to reproduce:

ffmpeg -loop 1 -t 10 -i colors2.exr -filter:v 'zscale=rangein=full:primariesin=709:transferin=709:matrixin=709:range=full:primaries=2020:transfer=smpte2084:matrix=2020_ncl' -pix_fmt:0 yuv420p10 -colorspace:0 bt2020nc -color_primaries:0 bt2020 -color_trc:0 smpte2084 -color_range:0 pc -c:v libx265 -x265-params lossless=1 OUTPUT.mkv

[Marth64]

Issue is resolved