[FFmpeg-user] convert from bt601 to bt709

[Bob Maple]

> Could it be that the color atom are not valid from ffmpeg? Can we set them
> manually?

Unfortunately ffmpeg doesn't write the 'colr/nclc' atom with primaries &
matrix information - which may or may not actually affect you in this
case because Avid has their own way of doing things in their Windows/Mac
Quicktime codecs..

Avid's DNxHD Quicktime codecs are a screwed-up mess themselves and will
give different results depending on how the file was encoded.  They have
an additional flag which gets stored (for reasons I don't understand)
defining if the original input was RGB or 709 levels (0-255 or 16-235
respectively in 8-bit) and will inexplicably change the way the codec
decodes depending on how it is set.  I think it's in an atom called
ACLR;  I doubt ffmpeg writes this either, and I'm not sure what the
codec's default behavior is if it's missing (though it sounds like it
could behave as if it's set to 709, which the Quicktime codec then
scales the data to 16-235 even though it already IS 16-235 in the file..)

I'll have to do some tests and refresh my memory of what all is wrong..

Bob