Alfa Channel- Compositing Problem

[pixela]

Hello,
I have a problem with alfa channel.
I have a treeline at the back part of my image.
When I want to composite different sky, I have problem with the alfa channel. Because the image is rendered with sky1, is anti-aliased with sky1 and when I change sky, I get a halo around the treeline.

( You can see the problem on this post:
http://www.chaosgroup.com/forums/vbu...ad.php?t=47213
Dusk images have this problem )


What is the proper way to solve this situation?
How do people deal with this during compositing stage?
Or do I have to render each sky?

[rivoli]

if you're saving premultiplied outputs (and you almost certainly are), you should be rendering against a black background. then you can comp whatever backplate you like.

[pixela]

Thanks for your fast reply!

So rendering with a black background will solve problem?
There will not be a black halo this time?

[rivoli]

it shouldn't be there, no. what app are you using to do the comping?

[pixela]

Actually I didn't use any compsiting-specific software but just Photoshop.

[rivoli]

which is well known for its awful way to deal with premultiplication.. unfortunately.
but rendering against black should work all right though. you may also wanto to have a look at this thread:

http://www.chaosgroup.com/forums/vbu...ad.php?t=46356

[pixela]

Thanks a lot Rivoli for your great help