Color banding when downscaling from UHD HDR10 to FHD - bug in scaler?

[Woodstock]

Originally Posted by poisondeathray

Can you stream copy a few frames from the source mp4 and post it ?

Thank you, it was your request that brought me one step forward. I didn't want to copy the full UHD frames, so I cropped out a part which significantly shows the banding after down scaling. Surprisingly it now showed the banding without any scaling, just with cropping.

After a few tests I found that ffmpeg is innocent: The player causes the color banding! So I have to do some research and then maybe post in the Software players forum.

Just some info already here: I use PotPlayer with madVR 92.10. For unknown reasons it shows color banding if the source video is 2176x1224 or smaller, but not if it is 2240x1260 or bigger. The output video size doesn't matter. The madVR debug OSD shows the same correct values for both. According to my knowledge (MediaInfo) both test videos are identical apart from the size. Both videos were created with the same ffmpeg and x265 settings from the same UHD source. Both videos look the same in VLC, but VLC shows washed out colors as it doesn't properly support HDR.

[sneaker_ger]

Are you using DXVA native? Is dithering in madvr disabled?

[Woodstock]

Originally Posted by sneaker_ger

Are you using DXVA native? Is dithering in madvr disabled?

I already tried many settings, including with and without DXVA, and different dithering algorithms. They all showed the banding.