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After Effects - How to make a AE Solid Not Contribute to Alpha Channel

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Seth Borne
After Effects - How to make a AE Solid Not Contribute to Alpha Channel
on Jan 10, 2013 at 2:28:51 pm

Got a question that is stumping me.

How do you get an AfterEffects Solid to show up in the RGB channel, but to NOT show up in the Alpha Channel of a composition?

I created this small graphic:



So what i want is a color solid as a background that shows up in the rgb channel, but does not show up in the alpha channel.

I have tried many different channel effects, but none of them have been able to do what I need.

Any thoughts?

Thanks!


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Roland R. Kahlenberg
Re: After Effects - How to make a AE Solid Not Contribute to Alpha Channel
on Jan 10, 2013 at 3:09:10 pm

The Composition's background color is what you need. In your specific case, set the comp's background color to red.

HTH
RoRK

Intensive mocha & AE Training in Singapore and Other Dangerous Locations

Imagineer Systems (mocha) Certified Instructor
& Adobe After Effects ACE/ACI (version 7)


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John Cuevas
Re: After Effects - How to make a AE Solid Not Contribute to Alpha Channel
on Jan 10, 2013 at 3:10:20 pm

You can't add something to your composition and have it not show up in the RGB channel. You could change your comp settings "cntl + k" and change the background color. That won't show up in the alpha channel.

Johnny Cuevas, Editor
Thinkck.com

"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.


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Seth Borne
Re: After Effects - How to make a AE Solid Not Contribute to Alpha Channel
on Jan 10, 2013 at 4:30:19 pm

Thanks Guys!

The solid in this situation is a stand in for either a solid (which i can use the composition background, thanks for the suggestion) or most likely a gradient, or maybe an image backplate. In either of those cases, how do i get them to show in the RGB without showing in the alpha?


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Roland R. Kahlenberg
Re: After Effects - How to make a AE Solid Not Contribute to Alpha Channel
on Jan 11, 2013 at 12:17:53 am

Other than using the Background Color, what you want is not possible. I suggest that you duplicate the comp, in the Project Panel, and create sufficient duplicates of the comp to suit your various needs.

HTH
RoRK

Intensive mocha & AE Training in Singapore and Other Dangerous Locations

Imagineer Systems (mocha) Certified Instructor
& Adobe After Effects ACE/ACI (version 7)


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Darby Edelen
Re: After Effects - How to make a AE Solid Not Contribute to Alpha Channel
on Jan 11, 2013 at 4:47:22 am

[Seth Borne] "The solid in this situation is a stand in for either a solid (which i can use the composition background, thanks for the suggestion) or most likely a gradient, or maybe an image backplate. In either of those cases, how do i get them to show in the RGB without showing in the alpha?"

This is possible, with two limitations:

1) your background image/gradient/video must be exactly the size of your composition

2) you must render with a straight alpha (not premultiplied)

Place your background image on the bottom of your comp, apply the Shift Channels effect and set Take Alpha From to "Full Off." You can preview what your RGB channels will look like with the RGB Straight preview in the comp viewer. When you render make sure that you render with a Straight alpha.

Different viewers may support this in different ways but you can check your results by bringing the render back into AE and telling it to ignore the alpha.

Darby Edelen


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Seth Borne
Re: After Effects - How to make a AE Solid Not Contribute to Alpha Channel
on Jan 11, 2013 at 4:07:01 pm

Thanks for the suggestions everyone!

@Darby: Wow! That works perfect. But it certainly is something that has to be done in a very particular way.

Is there anyway you could describe how you figured this out?


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Darby Edelen
Re: After Effects - How to make a AE Solid Not Contribute to Alpha Channel
on Jan 11, 2013 at 8:19:52 pm

[Seth Borne] "Is there anyway you could describe how you figured this out?"

Well, I knew that a straight alpha should technically maintain RGB values outside of the alpha channel. I wasn't sure if it would only maintain those values within the range of non-zero values in the alpha channel.

I tested this set up in AE before rendering with the Straight Alpha preview and it seemed to work. I tried a render and everything came out as expected.

Then I decided to try a non-comp sized solid and the AE preview as well as the render failed to exhibit the expected behavior. Scaling the BG layer in AE led me to find that it has to be exactly the size of the comp for this to work or it will essentially be ignored.

Darby Edelen


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