Just couldn’t force myself to study for an exam i have on friday so i thought i’ll be more useful doing something that bothered me for quite some time.
When You render a couple of render layers and try to mix them You’re probably going to get ridiculous seams (borders) all around a layer. It’s quite a simple oversampling problem, RGB shouldnt be touched when a sample with alpha 0 is found (meaning the ray didnt collide with anything).
Unfortunately Blender Internal mixes the sky color with the material color in such situation. So… if the sky was black (eg. sky rendering was turned off for a layer) You’ll get good alpha values but whenever alpha is below 1, rgb values are mixed with black which results in a nice border whenever oversampling encountered both sky and some object.
So I’ve created a simple Node group which multiplies rgb values back to the values they should have. And here are some results:
( without the node -> with the node)
And now, how to use the node. Quite simple actually:
Image - image to repair it’s rgb values, layer’s sky color must be black! (sky rendering turned off for the layer)
Fac - image’s alpha
Image - repaired image
And of course the .blend file
Hope You find the node useful
…now back to studying for the exam…
PS. this is just a work-around, the problem should be fixed with some changes in BI code