[FFmpeg-user] Another odd colourspace issue

Tim Nicholson nichot20 at yahoo.com
Tue Jul 10 13:34:27 CEST 2012

I have a programme that generates packed yuv test signals.

Using the programme I generate 100/0/100/0 colour bars and greyscale,
which I then convert to mov's using a command of the form:-

ffmpeg -pix_fmt uyvy422 -s $size -i $in_file -c:v copy -vtag 2uvy $out_file

I have created both SD (720x576) and HD (1920x1080) frame size files
using both 601 and 709 coefficients, as described at
http://avisynth.org/mediawiki/ColorBars_theory and verified by my own
calculations from first principles.

When I inspect the files on both FCP and Kdenlive waveform monitors
there is am marked difference in levels between the HD and SD versions
using the same colourspace. The 709HD and 601SD look correct, but the
709SD and 601HD look wrong. Based upon the errors in the green channel
it is clear that different coefficients are being used to decode than
were used to generate the original material, and these coefficients seem
to be being auto selected on the basis of the frame size.

The question is why? and what is doing the selection?
Is it both Kdenlive and FCP deciding on the basis of framesize, or is
ffmpeg adding some kind of colourspace marker to the file when it
rewraps to mov?

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