CGTalk - Alpha channel problem

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fuxy

11-27-2004, 08:39 PM

I found problem with alpha channel (or raytracer). See pictures:
RGBA image:
http://web.axelero.hu/fuxy/forum/AP_RGBA.jpg

Alpha channel:
http://web.axelero.hu/fuxy/forum/AP_Alpha.jpg

I used only sky object with frontal map as background, main light as sun and distant light as skylight.

If the BG color is black alpha works but when BG goes to white the polys of the leaf and branch effect on alpha channel.

maybe it is join to other reflection problem:
http://www.cgtalk.com/showthread.php?t=189931

My Soluton to compositing
Thats why I use multipass layers, set the sun to separate pass, and add the shadow pass to pass list. Then I merge the the alpha pass(multiply) and invert shadow pass in Photoshop and use as mask to compositing picture.

Question:
why goes wrong the alpha?
Is it my fault or program bug?
Has anybody easier method to get clear alpha?

thanks to comments
fuxy

rikke

11-28-2004, 03:05 PM

This is a common problem when using radiosity in combination with a sky object and alpha textures. I'm not really familiar with your multipass solution (sounds great, must try this too), but an other way to try avoiding this problem is by 1)using GI spheres instead of skies

2)or to make sure there is an object between the alpha textures and the sky. Meaning, that you don't get to see the sky whilst looking 'through' the alpha because it's obstructed by an object. For some reason (that could be explained by more experienced members), the radiosity calculation does the right thing in this case.

Maybe something to fix for release 10.0?? :)

fuxy

11-28-2004, 07:14 PM

Thanks rikke, I will try it.

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