CasparCG
Mixer is not gamma-correct
Hi,
It looks like the basic operations in the mixer (like scaling) are not working in linear light as they should; in particular, see http://www.4p8.com/eric.brasseur/gamma.html. In particular, this matters when doing crossfades between light and dark media.
Simple test: Download http://www.4p8.com/eric.brasseur/gamma-1.0-or-2.2.png as media/gamma.png, set up a 720p consumer and do
LOAD 1 gamma PLAY 1 gamma MIXER 1 FILL 0 0 0.2 0.1777777
Result included, if maybe a bit offensive. :-)
The right way to fix this is to convert all data from sRGB (or whatever colorspace you assume) on the way in, do operations in floating-point and then convert back on the way out.
Simply using SRGB texture format in OpenGL for both input texture and the frame buffer instead of RGB should solve the problem.
I noticed today that this also makes CasparCG output incorrect premultiplied alpha. If you have a white (255,255,255) at 50% alpha, it should be (188,188,188) after premultiplication, not (128,128,128), since all blending, and thus also the premultiplication of alpha, should happen in linear gamma and only be converted to sRGB for final storage. This causes out-of-gamut values and halos when fed into a processing pipeline that actually understands sRGB correctly.
Using straight alpha is a workaround, although perhaps not a very satisfying one. Note that alpha is always linear (never gamma-corrected, since it wouldn't make any sense), so storing 50% alpha as 128 is completely fine.
@5opr4ni can you analyze this? i.e. make it reproducible so we can fix it at some point.