CGTalk - Adding passes gives black edges.

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MartinDay

02-23-2009, 10:42 AM

Hello,

I have a character rendered in two passes... Ambient pass and Sun pass.
I'm putting the Sun pass on top of the Ambient in Add blend mode and it gives me black edge around. Both passes have premultiplied alpha in the footage interpretation options.

What I have found is that AF needs a background (or solid) underneath these layers to get the alphas correctly. So, let's say. If I set the viewport bg color to white I'm getting this black edge but when I place a white solid layer under the footage the edges are fine.

Now lets say I want to pre-compose my character layers to have them in a group with all corrections etc. so the result is white solid and Character comp above... black edge :/

When I have a closer look on the alpha channels it seems that AF adds them together - makes less transparent, which makes showing black pixels, even tho I'm using Add or Screen blend modes... Preserve Transparency or using Stencil Alpha reduces the effect but does not get rid of it completely :/

I could probably get around just by chocking the matte but it will not work when I render with motion blur.

Has anyone had the same problem? Any solutions?

BTW... I'm using AF 7.0 Pro

Thanks

scrimski

02-23-2009, 11:16 AM

Change the passes alpha channel interpretation.

MartinDay

02-23-2009, 11:28 AM

I have tried all the options and it does not make any difference.
I've tried "Ignore" and then duplicating one pass, setting to "Straight" and using its alpha for both other passes but still the same result.
In my understanding "Premultiplied" should couse no problems and it kinda doesn't as long as I use one pass only. Adding two passes together always gives me that $%@*(# :/

Cheers

mart1jn

02-23-2009, 02:27 PM

What sometimes helps for me is :
add a white solid underneath
add a layer with the alpha on top with blending mode: stencil alpha.

Cheers

MartinDay

02-23-2009, 02:33 PM

Well... yep...

I've tried that. As I said, it reduces the effect but does not solve completely. Probably with smaller motion blur I could live with it.

Thank you anyways
Cheers

mart1jn

02-23-2009, 02:57 PM

Sorry, I obviously did not read your post completely.

You said though, that when you add a white solid underneath the black lines are gone. Can't you use that and cut it out with an alpha?

Just out of curiousity, In what programm did you render and did you render straight color or premultiplied?

cheers

MartinDay

02-23-2009, 03:06 PM

I've tried that as well :)
Basicaly any attempt of matting gives the same result. Preserve Transparency, Stencil Aplha, Set Matte or Track Matte. All reduce the effect but still there is something not right.
I've tried saving the files as straight as well as premultiplied and still the same.
File format does not seem to change anything as well.

and... renders come from MAX.

I will try to post an example comp so maybe it will give more ideas.

Mylenium

02-23-2009, 03:23 PM

Both passes have premultiplied alpha in the footage interpretation options.

Good for them. But were they rendered premultiplied? and if so, against what background color? The default black?

What I have found is that AF needs a background (or solid) underneath these layers to get the alphas correctly.

Well, where there is no RGB data, there can be no blending operations. That's logical, is it not? The comp background is 100% transparent and does not contribute to processing operations other than providing a fill color for pixels that are not 100% opaque.

When I have a closer look on the alpha channels it seems that AF adds them together - makes less transparent, which makes showing black pixels, even tho I'm using Add or Screen blend modes...

Blending operations only affect RGB channels, naturally the Alpha channel is retained, in case of Add mode that of the bottom layer. If it was interpreted incorrectly, the RGB values of pixels that were previously partially covered up by the layer on top would naturally be more prominent, once you lift the veil, in a manner of speaking.

Anyway, you can dance around the matter as much as you want - something is wrong with your Alpha interpretation, so check again.

Mylenium

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