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Alpha channel and black background

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Bernhard Walzl
Alpha channel and black background
on Mar 28, 2014 at 6:36:01 pm

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


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Dave LaRonde
Re: Alpha channel and black background
on Mar 28, 2014 at 10:32:02 pm

what is the codec for the NON-alpha-channel version, and how are you going about combining the two in your illustration?

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Bernhard Walzl
Re: Alpha channel and black background
on Mar 28, 2014 at 11:06:08 pm

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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Walter Soyka
Re: Alpha channel and black background
on Mar 30, 2014 at 12:24:11 am

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


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Bernhard Walzl
Re: Alpha channel and black background
on Mar 30, 2014 at 12:38:30 am
Last Edited By Bernhard Walzl on Mar 30, 2014 at 12:40:39 am

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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Walter Soyka
Re: Alpha channel and black background
on Mar 30, 2014 at 1:02:24 am

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


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Bernhard Walzl
Re: Alpha channel and black background
on Mar 31, 2014 at 6:57:20 pm
Last Edited By Bernhard Walzl on Mar 31, 2014 at 7:00:31 pm

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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Dave LaRonde
Re: Alpha channel and black background
on Mar 31, 2014 at 9:34:29 pm

My question: why are there two distinctly different shades of gray in your picure?

Sorry, but I'm unable to download the project. My fault, not yours.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Bernhard Walzl
Re: Alpha channel and black background
on Mar 31, 2014 at 9:36:03 pm

Because through the middle runs a solid black layer in the background. the upper and lower bit are kept transparent.


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Walter Soyka
Re: Alpha channel and black background
on Mar 31, 2014 at 10:53:38 pm

[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


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Bernhard Walzl
Re: Alpha channel and black background
on Apr 1, 2014 at 12:23:17 am

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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Walter Soyka
Re: Alpha channel and black background
on Apr 1, 2014 at 12:39:45 am

[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


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Bernhard Walzl
Re: Alpha channel and black background
on Apr 2, 2014 at 7:56:44 pm

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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