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Re: Gamma problem with 2vuy compressor / ICM session

Note: I compared the ImageDescriptions that are being generated from
the ICM for 2vuy and yuv2.
The yuv2 compressor generates no image description extensions.
The 2vuy compressor gerenates both a 'colr' and 'gama' image
description extensions. ‘colr‘ is filled with type:nclc, primaries:6,
function:2 (means unknown), matrix:2, ‘gama‘ is filled with 2.2

I believe 'colr' takes precedence over 'gama', but if "function" is unknown then probably nclc defaults to 2.2 or maybe QT is smart enought to use 'gama' instead.
Probably doesn't solve your problem :-(
Have you tried deleting the 'colr' extension just to see how the video looks when 'gama' prevails?? 

Now, this is interesting:

I exported the WRONG looking 2vuy video that was created by my ICM session with QuickTime Player 7.1.5, again to 2vuy. The resulting file has NO ImageDscriptionExtensions (no gama and no colr) and looks perfect, without any gamma shift! So the problem is definitely the caused by the extensions.

Now I tried changing the 'gama' value of the description created by the ICM, indeed, if 'transferFunction' is set to 2 (unknown) the 'gama' setting does have an influence on the movie. What's weird is that I changed 'gama' to several values ranging from 0,5 to 3,5 and none of the modified movies looks right. As a last desperate try I then set 'gama' to 0.0, and magically all looks good.
Also, if I remove either 'gama' or 'colr' or both, the movie looks good!

So what have I learned, I'm not sure. Maybe I found a bug? Maybe someone can clarify me what is going on here?

Thanks for pointing me in the right direction Jan.
Mark


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 >Re: Gamma problem with 2vuy compressor / ICM session (From: "Jan E. Schotsman" <email@hidden>)



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