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Problems with alpha

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charlesb
Problems with alpha
on Apr 20, 2007 at 6:31:13 pm

Ok this might seem like a stupid question:

This problem has been racking our brain for months.

I have an alpha channel that we exported from a 3D software. The alpha channel comes from a shadow. This means that it contains grey information etc...

How would you go about compositing this alpha channel in photoshop?
Could I remove the black?

Or to the opposite:
If I was to put an image behind this alpha, would it be possible to see it while retaining the shadow information?

Here is an example of the image: http://www.boileau.cc/client/creativecow/logo-3D-passes.jpg

I can select this shape, but I can't keep the shadow information.

I hope I explained this clearly.



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wuzelwazel
Re: Problems with alpha
on Apr 22, 2007 at 4:04:24 am

[charlesb] "How would you go about compositing this alpha channel in photoshop?
Could I remove the black?

Or to the opposite:
If I was to put an image behind this alpha, would it be possible to see it while retaining the shadow information?"



I'm not sure what the goal is, but if you just want to recreate the shadow you had in your 3D program in Photoshop then you could make a selection from the alpha channel (ctrl or cmd-click it) and use Layer > Layer Mask > Reveal Selection on a new layer. Then fill that layer with whatever color your shadow should be (black probably?).

I hope that's what you meant by compositing. An alpha channel can be used for effects as well (such as using one as a height map in Lighting Effects or to control a Lens Blur). Let me know if there's a more specific effect you're trying for.



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Boyd Hawkins
Re: Problems with alpha
on May 8, 2007 at 2:36:49 am

How do you turn the black sectio of the alpha image to be transparent. Or how can I make black transparent in any image?

Maybe a similar question....how do the green screens work in Photoshop?


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