Maya/V-Ray - problem with overlaying edges/alpha

[FranciscoN]

Hey guys, I have a problem I can't seem to fix and it's related to alpha channels (I think) and V-Ray and/or Maya. So I need to render out all the parts of a model I'm making for an online configurator. There are a lot of model parts and colours, so the renders that Maya outputs, need to be the final renders as we don't have time for any amount of post work.

The thing is, when I go and test how the images stack, there's always a fine line between all of the parts, that breaks the illusion the model is a whole piece. I've investigated quite a bit but found no true fix for this. Here's what I'm talking about, composited in two differents applications.

Nuke shows the inline transparent edges quite well
http://imgur.com/dhzMoZ2

Photoshop does a better job, but it's still visible
http://imgur.com/A0AGRhj

I have all the pieces become alpha mattes so that the visible piece has the correct placement, and I'm using the vray MtlWrapper for that effect. The research I've done suggests this is an alpha premultiplication problem but V-Ray doesn't seem to have the unpremult / don't premult option. I've tried rendering it against a black/grey/white background but the problem never goes away.

Am I overlooking something really basic here or is this surprisingly complicated?

[FranciscoN]

The alpha looks like this by the way
http://imgur.com/KhzVc9l

[vlado]

First, you need to render your models against black background. Second, V-Ray produces always images with premultiplied alpha (so the compositing equation is color=imgColor+bg*imgAlpha). That's all.

Best regards,
Vlado