The `--target-web` is not working really well with video filters.

[gphg]

Even though it explicitly set -pix_fmt yuv420p, FFmpeg is ignoring it.

Here is why: The -vf (video filter) graph resets the pixel format to match the source right before it hits the encoder. Because scale and fps process the native sRGB colorspace, these pass that sRGB data straight to libvpx-vp9, completely bypassing the -pix_fmt yuv420p flag.

To fix this, it need to force the color conversion inside the video filter chain itself using format=yuv420p as the very last step of -vf argument.

[gphg]

The main issue has to do with MP4 to WebM (VP9) encoding failed because colorspace

I am having a very rare case where a MP4 video has multiple color profile (three). I don't really understand this, but after workaround — the solution is required extra flags:

-colorspace bt709 -color_primaries bt709 -color_trc bt709

[gphg]

I fixed it by tweaking the produced FFmpeg command from --print instead of the Python source file. I am running several process in batches and this is going to be problem, so yeah. Priority: low.