Alpha channel interpretation issue Topic is solved

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Audiovisualisa
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Alpha channel interpretation issue

Post by Audiovisualisa » 22 Jul 2017, 17:57

Dear TVPaint community,
hopefully you can help me with an alpha export issue. I'll try to describe:

Basically, I've got a 2D handdrawn animation scene with characters running. As they touch the ground, they raise dust. I painted the dust clouds on extra layers in pure white. Then turned the alpha down to about 30% so the clouds look transparent. As the characters run off into the distance the clouds become more transparent.

Now I have to export the scene as TIFF sequence. The scene has NO background, because it will be composed in After Effects later on. The dust clouds partly overlap with the characters. So the parts that don't overlap with the characters have no background, and the final background should shine through them.

I exported the characters together with the dust clouds. But when they were composed with the backgound the dust suddenly turned grey (with transparency) instead of white (with transparency). Funny thing is that they are still white transparent where they overlap with the characters. But where there is no background they turned grey.

Looked like a premult issue for me at first. But I always tick the premult checkbox when exporting TIFFs. I checked it several times and even tried to save the dust clouds separately - but they continue to turn grey in After Effects. Also, in TVPaint, the clouds are set on blending mode "Color" so this, too, should cause no problems.

I could export them in 100% white with no transparency at all and adjust the alpha in After Effects. But it will be additional work, as I would have to animate the decreasing alpha again. And I still would love to know how to solve the issue in TVPaint. Or is After Effects causing this issue?

My PC has Windows 10 (64 bit). The version of my TVPaint is 11 (Professional, Education Edition)

Do you guys have any idea where the colour change might come from?

Many thanks in advance!
Lisa
Last edited by Audiovisualisa on 29 Jul 2017, 11:37, edited 1 time in total.

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schwarzgrau
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Re: Alpha export issue

Post by schwarzgrau » 22 Jul 2017, 19:34

Have you tried to export it as PNG? (Honestly I don't know if this could help, but usually I never had such problems with PNGs)
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slowtiger
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Re: Alpha export issue

Post by slowtiger » 22 Jul 2017, 19:52

I've just done a test.
TIFF: no difference between pre and nonpremultiplied, both with grey border.
PNG: "Not Premultiplied" gives you the desired effect, without a grey border.

So the file format matters!
TVP 10.0.18, Mac Pro Quadcore 3 GHz, 16 GB RAM, OS 10.11, QT 7.7.3

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Elodie
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Re: Alpha export issue

Post by Elodie » 24 Jul 2017, 04:43

Make also sure you import your images properly in Afyer Effects, for it won't always detect alpha channel ina proper way.
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Fabrice
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Re: Alpha export issue

Post by Fabrice » 24 Jul 2017, 07:58

indeed, when exporting in pre-multiply (or not) , the result depends on the software opening the files : does it handle (pre)multiply ?
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slowtiger
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Re: Alpha export issue

Post by slowtiger » 24 Jul 2017, 10:35

AFX had (last time I checked) the options Guess, Pre-Multiply, Without Pre-Multiply.
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Audiovisualisa
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Re: Alpha export issue

Post by Audiovisualisa » 29 Jul 2017, 11:36

Hello guys,

thanks for your quick replies. The problem is solved now. My team-mate and I finally found that the issue was caused when importing the TIFFs to Premiere where the alpha was misinterpreted.

We would have to switch the alpha options in the "interpret footage" settings in Premiere - the preset was set on "straight alpha" but with the confusing addition that it would use the alpha information from the files. We changed it to "premultiplied alpha" and this seems to work now. I think that I'll delve deeper into the alpha interpretation matter to fully understand what happened there.

But anyway, guys, thanks again and no TVPaint problems here. :)

All the best,
Lisa

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slowtiger
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Re: Alpha channel interpretation issue

Post by slowtiger » 29 Jul 2017, 11:39

Alpha can be tricky. I found the only way to be sure is stubbornly test each possible combination in the workflow, take notes, and then adamantly stick to the one which works.
TVP 10.0.18, Mac Pro Quadcore 3 GHz, 16 GB RAM, OS 10.11, QT 7.7.3

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