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adding 2 alpha causing non mathmetical result.

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christian kim
adding 2 alpha causing non mathmetical result.
on Sep 15, 2010 at 12:47:38 am

Good morning community!

been having bit of problem with alpha channels.
did a research on the topic, no answer on the forum.

basically problem i am facing is..

255 in alpha means 100% solid.
0 in alpha means 0% solid (transparent).

so my understanding is..

if i had a pure RED layer, with alpha of 200.
below, if i had a pure RED layer, with alpha of 55.
they should produce SOLID pure RED layer (alpha 200+55 = 255).

however, when i try, it produces slight transparent pure RED layer (alpha less then 255).

i am using after effects to comp renders from 3d package, so i do get very accurate alpha, which should comp together to form a solid.
however, when i comp them in after effects, i am getting this less then solid fringing.

my guess is that this might be caused by gamma issue (maybe even though the alpha channel is gamma 1.0, as in real pixel data), but after effects assumed it to be gamma corrected, thus linearlising already linear footage, thus shifting the numbers down?

but i could not find any option to tell after effects to use linear comping when using alpha channels.

would be very thankful if someone can shed a light on this matter!

thanks community in advance : ) looking forward to any response : )


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christian kim
Re: adding 2 alpha causing non mathmetical result.
on Sep 15, 2010 at 1:23:04 am

just to add few more notes.

finding all this very interesting. comping with opacity is one of the most basic things to do in PSD or AE.

however..

if you have one layer with opacity 70%.
another layer with opacity 30%.

resulting layer is actually not solid. infact its a transparent layer.
this is the behaviour both in AE and PSD.

and for some odd reason, its making sense. as in i can understand why it does that on tip of my thoughts, just can't really explain mathmetically for me to understand.

basically it would be great! if any GURU can explain in mathmetical or any other terms why the transparency is compted like that.

At the end of the day, what i am trying to do is to add a layer with 70% opacity and 30% opacity to a 100% opacity image. is that possible?

thanks again for your time : )


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Todd Kopriva
Re: adding 2 alpha causing non mathmetical result.
on Sep 15, 2010 at 1:47:11 am

If you have a material that lets 50% of the light through and then you put that over another pane of the same material that lets 50% of the light through, how much of the original light gets through?

25%

If you want alpha values to add, then use the Alpha Add blending mode.

This is all explained here.

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Todd Kopriva, Adobe Systems Incorporated
Technical Support for professional video software
After Effects Help & Support
Premiere Pro Help & Support
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christian kim
Re: adding 2 alpha causing non mathmetical result.
on Sep 15, 2010 at 1:53:11 am

Thank you So much Todd!

you are absolutely right. if i take a step away from it. it all makes sense. just when looking at the actual values and try to do simple math it made me tunnel visioned.

thank you for the link! will read into it.


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