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Alpha channel issues

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Stephen WisemanAlpha channel issues
by on Jun 10, 2014 at 9:08:51 am

Hi All

This is a problem that's been annoying me for a while across many projects, maybe someone can help. When I export graphics work with an alpha from After effects and then place into FCP 7, there is always a slight trace of the comp background (or something) showing through - especially around the edges of the graphics and where there is blurring/fading to transparency. I've used the QT animation codec and also just tried pro res 4444 which seems a bit cleaner but I can seem to get it perfect. I've attached two images,where the blue object is moving with motion blur. Both same codec, the only difference is that the after effects comp settings on one had a black background and the other a white background. Despite this being an alpha with millions colours+ it seems to be drawing information from the comp background where transparency is involved. I could make the comp background the same blue as this but there are other colours involved later on in the animation.
Has anyone else come across this or got a fix?

Thanks for your help


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Walter SoykaRe: Alpha channel issues
by on Jun 10, 2014 at 1:56:16 pm

FCP7 treats all video with alpha as straight alpha by default, but you are rendering with pre-multiplied alpha.

Right-click the clip in the browser in FCP7 before you edit it into a sequence, then choose Item Properties... Format. In the format tab, locate the line for alpha (currently says Straight). Right-click that and choose "Black."

If you have already cut the video into your timeline, you'll have to do this on the clip(s) in the timeline, too.

Walter Soyka
Designer & Mad Scientist at Keen Live [link]
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
@keenlive [twitter]   |   RenderBreak [blog]   |   Profile [LinkedIn]

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Stephen WisemanRe: Alpha channel issues
by on Jun 10, 2014 at 3:00:12 pm

Thank you for your help Walter! Works like a dream.

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