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Alpha channel and black background
Hello!
I just came across something weird and I was wondering if one knows what is going on here:
I have a comp which is entirely transparent. I place a logo in a corner, which transparency is on 40%. I export it using the codec Animation, preserving the Alpha channel.
Then I do the same again but with a solid black layer in the background (the transparency of the logo is still on 40%).
Surprisingly, the results of the two logos will be different:
See this picture, to understand what I mean:
I understood that transparency will be interpreted black when there is nothing in the back, so I am quite surprised why the two results differ.
Thanks for sharing your ideas,
Bernhard -
10 Replies
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Both were encoded using the codec ANIMATION (plus alpha) with the same settings. The picture is done with photoshop to demonstrate the difference. But I can render it out by placing the solid black half behind the icon.
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Are you correctly interpreting the alpha when you re-import the test? You’d get wrong results if you rendered with pre-multiplied alpha but interepreted as straight alpha.
Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog – What I’m thinking when my workstation’s thinking
Creative Cow Forum Host: Live & Stage Events -
I have exported and viewed the exact same way.
After rendering out the first version, I have created a black solid in the back, then for the export, I have duplicated the old render entry to get the exact same result.
The picture I have taken is a screenshot what quicktime displays.Can someone reproduce this? I am happy to upload my after effects file…
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There is no image uploaded here.
If you collect your project and source footage, zip it up and upload it, I’ll take a look at it tonight for you.
Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog – What I’m thinking when my workstation’s thinking
Creative Cow Forum Host: Live & Stage Events -
Thanks Walter!
Please find here the AEP file: 7315_iampuzzled.aep.zip
//edit: sorry I just realized my default font was not a standard one, feel free to change it, it will produce the same result.
It is white text with 40% opacity. Through the center is a solid black in the background while the upper and lower part stays transparent. Rendered as Animation, preserving the Alpha channel, and when viewing in Quicktime it will look like this:
My question: Why.
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Because through the middle runs a solid black layer in the background. the upper and lower bit are kept transparent.
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[Bernhard Walzl] “when viewing in Quicktime it will look like this:
…
My question: Why.”Any question about why things look the way they do in QuickTime Player is virtually unanswerable.
If you bring this into an alpha-aware application, it will display correctly.
Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog – What I’m thinking when my workstation’s thinking
Creative Cow Forum Host: Live & Stage Events -
yes, VLC will play it correctly.
however, the problem started off that I have two videos: one is an intro and the other is a logo (which has video footage in the background). one has a black background, the other one is transparent whereas both use the animation codec.
when I put them in the timeline (premiere cs6) next to each other, export as h264, their grey tone will change.
So I am just trying to find out where the root of this problem sits, to solve this issue.
No need to propose me a workaround, i want to know why things are how they are.
Any help with this is highly appreciated.
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[Bernhard Walzl] “when I put them in the timeline (premiere cs6) next to each other, export as h264, their grey tone will change. So I am just trying to find out where the root of this problem sits, to solve this issue. No need to propose me a workaround, i want to know why things are how they are.”
Ah, I see now. The Premiere part was important!
I think that the issue here is that Premiere CS6 composites in a linear space when accelerated by GPU, but in a gamma-encoded space without GPU acceleration.
If you turn off GPU acceleration in Premiere (Project > Project Settings > General, then set the Renderer to “Mercury Playback Engine Software Only”) then you should not see this shift in gray tones.
Walter Soyka
Principal & Designer at Keen Live
Motion Graphics, Widescreen Events, Presentation Design, and Consulting
RenderBreak Blog – What I’m thinking when my workstation’s thinking
Creative Cow Forum Host: Live & Stage Events -
Thats right! Thanks Walter!
Very interesting indeed. Is there any news out there why this is happening from a technical point of view?
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