[FFmpeg-user] RGB-YUV color shift

[Chris Bouchard]

> > Is your original problem only reproducible when encoding 
> > to prores or also if you encode with a yuv420p encoder?
> > (For example -vcodec mpeg4 -qscale 2)

I have found the same colour shift, and that using 
-vf colormatrix=bt601:bt709 will fix the colours to 
be almost 100% accurate for conversions from RGB to YUV. 
Let's say 98% accurate; most people wouldn't notice.

However there is definitely something else going on with ffmpeg's 
pixel format conversions: specifically 
when converting to yuv420p. I have noticed a very slight green tint 
being applied when you convert 
pixel formats from e.g. yuv422p to yuv420p. 

NO COLOUR SHIFT:
-pix_fmt yuv422p -vcodec libx264   

SUBTLE GREEN TINT:
-pix_fmt yuv420p -vcodec libx264


Whatever options you use a grey image will go slightly green when ffmpeg 
converts it to 420. The problem seems to get worse if you scale 
the video down as well as convert pixel formats, so it could be 
an issue in the scaler. Definitely something dodgy with the pixel format 
conversion to 420.

It's interesting that YouTube doesn't exhibit this problem, 
so their engineers have somehow got around it.  But other 
video sites like Vimeo and my ffmpeg encodes
 do have this issue... 

Any thoughts from experts would be very welcome!