View Full Version : Compositing Renders?

10 October 2005, 10:01 PM
I've been looking around a few tutorial on how to do it, but havn't really come up with much. A bunch on how to creat alphas and such. All my renders of course have alpha channels. I'm sure its very simple. I'm not familiar with photoshop when it comes to alpha channels and masking.

I just want to be able to comp a render onto any background based on the alpha mask. I normally do this in Shake, which does it with a simple Over node. If I need to premultiply my images, how is that done in PS.



10 October 2005, 11:01 PM
Ok I'm getting it. Just like Shake, you just have to tell the layer to actually use the alpha.

Selected alpha and used the selection as a Layer Mask. Got it.:thumbsup:


Ian Jones
10 October 2005, 01:03 PM
Looks like you've got it sussed. An extra tip: Select your mask then goto channels tab and you should see your mask as an additonal channel which if you select it and turn off RGB you can easily edit, copy, paste whatever to it.

10 October 2005, 05:54 AM
Here's how to do it "old school" . . .

10 October 2005, 07:18 AM

comes along with an example file for Photoshop

10 October 2005, 10:54 AM
jfrancis -

Thanks for posting that. I was actually thinking of doing a basically identical "how to deal with pre-multiplied alphas in photoshop" tutorial myself, but now I see that there is one already!

(The point is, if you just use your alphas as layer masks, you'll sometimes get some dark edges where things are layered together. That tutorial shows how to fix that problem, even though it takes two layers per element to do it right.)


10 October 2005, 11:59 AM
nice quick and clear tute there :D

10 October 2005, 01:42 PM
Thanks, everyone!

Yes, I think if you use layer masks with premultiplied 3D CG over black you may get something similar-looking to this:

10 October 2005, 04:33 PM
"remove black matte" can shift the edge line...not sure i've tried it with blurred objects though

10 October 2005, 05:34 PM
"remove black matte" can shift the edge line...not sure i've tried it with blurred objects though

I think you might find trouble with motion blur, as well as with soft, cloudy objects that fit their mattes.

The workflow I suggest above works in the general case.

10 October 2005, 11:56 PM
Jeremy -- you might find this interesting --

I got a comment on my blog on this page . . .

In which a commenter brings to my attention a photoshop-specific workflow which is probably the way to go:

"Am I wrong in that if one renders premultiplied on either a Black or White, then using photoshops':
will do this same thing in one shot?

I've been doing this for 3 years with no issues."

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