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TIFF with Alpha creates gamma shift when rendering Uncompressed 10-bit

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Jorn Bergmans
TIFF with Alpha creates gamma shift when rendering Uncompressed 10-bit
on Nov 3, 2011 at 10:24:40 am

Hello Creative Cows,

I am currently working on a project where I'm importing a TIFF sequence with a masked-out background into a FCP project. (i.e. the TIFF does have a background, but it is masked out in QT as straight alpha. When opening in Photoshop I still see the background that is present in the TIFF file)
My sequence is an Uncompressed 10-bit timeline, and we render out in Uncompressed 10-bit.

When looking at a single TIFF file in preview (spacebar), importing the TIFFs into QT and creating a (reference) movie out of this TIFF sequence, or importing that movie into FCP (7), the alpha channels work perfectly fine; my title is shown and you can see the background right through it.
However, when I render this sequence out of FCP as Uncompressed, as soon as the title comes in I see a pretty heavy gamma-shift; the image turns a lot lighter. As soon as the title TIFF ends, the image goes back to normal.
If I render out a file in a codec with an alpha channel -such as ProRes 4444 or Animation- I do not get any gamma shifts. (However for our workflow purposes I should be rendering an Uncompressed file. )

I figured this was due to Uncompressed not being able to support an alpha channel, but my output is a 'flat' image; just RGB. And FCP seems to support the alpha pretty well, seeing as my titlecard is still transparent in the render. (it doesn't 'ignore' the alpha when rendering)
It's just that the whole background lights up a few notches as soon as the titlecard comes in.

I don't see any reason why the alpha of 1 clip would affect the gamma of my video. Does the alpha channel get re-calculated with a different gamma setting during the render? And if so, why?


I'm currently trying a workaround creating a 'flat' render in AE by importing the TIFF sequence directly and importing a small insert from the background video to render out an insert with titlecard as Uncompressed and importing that into FCP, which should work, but sort of negates the point of having a transparent TIFF in the first place...

Would you guys have any ideas / tips / tricks to get this fixed?
Any comments would be much appreciated.


Sincerely,

Jorn Bergmans


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Rafael Amador
Re: TIFF with Alpha creates gamma shift when rendering Uncompressed 10-bit
on Nov 3, 2011 at 3:05:18 pm

Hi John,
First to say is that, although supported, TIFF is not an favorite FC format. Seems that PNG always causes less issues.
I don't think is any 10b unc or Alpha Channel matter.
Set Your sequence to "Superwhites" and check "Render all YUV material in High Precision".
Although your stuff is 8b RGB, the mapping/rendering will be done with higher accuracy.
rafael

http://www.nagavideo.com


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Brad Wright
Re: TIFF with Alpha creates gamma shift when rendering Uncompressed 10-bit
on Nov 3, 2011 at 6:06:37 pm

Are you using high precision YUV as your render settings in FCP? FCP's handling of color RGB has never been ideal.

Brad Wright is software engineer, so often hard to figure out what he is talking about. He is always happy to explain answers further.


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Jorn Bergmans
Re: TIFF with Alpha creates gamma shift when rendering Uncompressed 10-bit
on Nov 3, 2011 at 9:26:30 pm

Hey, thanks for the suggestions.

I know TIFF isn't a very nicely supported file, but that is what the animator sent us. Apparently a TIFF sequence is the best lossless output they can get out of Nuke or Fusion (not sure what they're using)

I will check out the YUV settings when I get in tomorrow, although I can't imagine that it would not be set to High Precision. I encountered this problem while on-lining a feature-length documentary at the post-production company where I work. It is not a personal project, so the stakes, and the settings, are set a bit higher than on my own shoddy little laptop ;)

Anyway the AE workaround fixed it, I made some small inserts importing the TIFF sequence directly instead of as a reference movie, and then exported Unc 10-bit videos from AE to paste back into the sequence.
(but I still want to check it out tomorrow cause this doesn't answer the puzzel ;) ) I'll let you guys know. Thanks again.


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