FFMpeg scale filter makes video lose color

[FEWP]

Easily reproduceable with many source videos. Once you scale a video (ffmpeg -i source -vf scale=x:y) the colors looks a bit washed out. It's not related to the compression used since I converted it to lossless HuffyUV and still got the same color issue. I tried both x264 and x265 without rescale and the colors are preserved. If it's some kind of color interpolation error you might see some videos get more saturated and some videos get more washed out, but with all the videos I tested, they're consistently more washed out.

This is on an ffmpeg version N-90656-ga26c9fdd1b which is a recent nightly.

Screenshots attached. source is the source obviously, x264noscale is an x264 compressed frame without scaling, huffyscaled is Huffyuv lossless frame scaled.

Is this a known issue?

[sneaker]

How are you creating the screenshots? Often this is because players assume different colormatrixes for different resolutions (BT.601 vs BT.709) not because of any problem with the scaler. (I haven't actually tested this on your screenshots.)

[FEWP]

sneaker, I'm using MPC-HC "Save Image". If that's the case, I assume if I scale down, then scale up, the colors will be as it was originally. I can try that out.

Edit: I did the above and the downscale-upscale output retained the original image. I never knew this.

Edit 2: MPC-HC has a BT.601 -> BT.709 shader that appears to correct the issue

[pandy]

you should measure colour levels - this is only objective way to verify if issue exist in ffmpeg or somewhere else - for example waveform filter can be used to objectively verify video.

https://trac.ffmpeg.org/wiki/WaveformMonitor
https://trac.ffmpeg.org/wiki/Vectorscope
https://trac.ffmpeg.org/wiki/Histogram