Perceptual difference between `scale` and `zscale` after the OCIO filter

[johnjoumaa]

When following the FfmpegOcio.md guidance to convert from RGB (OCIO filter output) to YCbCr using the scale filter, I noticed a perceptible difference from the EXR source. Swapping scale for zscale resolved it and much closer to the source in my testing.

Example:

ffmpeg -y -framerate 24 -start_number <STARTFRAME> -i SOURCEFRAMES.%05d.exr \
   -c:v prores_ks -profile:v 1 -pix_fmt yuv422p10le \
   -vf "ocio=input=ACEScg:output=ACEScct:format=rgb48,zscale=matrix=709:primaries=709:primariesin=709:transfer=unspecified:transferin=unspecified:rangein=pc:range=limited" \
   -movflags +faststart+write_colr -vendor apl0 OUTPUTFILE.mov

I don't have CLA clearance setup yet, so sharing my branch here instead: ffmpeg-ocio-zscale-command on my fork, which adds a zscale note + alternate examples to FfmpegOcio.md alongside the existing scale-based ones. Happy to edit, have someone pick it up, or I can open the PR myself once CLA is sorted.

Let me know what your prefer,

Cheers!