Export single frame darkened (Bug?)

[guju]

Take an mp4/h264/YUV420 movie
https://c.gmx.net/@1155842887640945833/o46DBll3QqiL0KZkxjFo4g

Open with MPC-HC or VirtualDub2
Export 1st frame (PNG)
Open movie with ADM.
Export 1st frame.
Result:
ADM frame is significantly (Not randomly, but overall) darker.

When recoding the movie with same settings (h264/YUV420/crf18)
the difference seems to be the same/ maybe stronger.

This is kind of importance as doing a detailed comparison
I often use frame export.

[eumagga0x2a]

There are multiple pixel format and color range conversions involved, at least mpv screenshot seems to agree with JPEG export from Avidemux here (on macOS), but yes, it is generally a mess.

[guju]

Thank you  - after all I do know now what's going on.

[guju]

@eumagga0x2a:

This might be an issue for you, as in my tests ADM generates the same (wrong) colors as ffmpeg.
In short: unsufficient flags for swscale.

EDIT:
Maybe not use swscale at all but zscale?