alpha error with render layers

[sharktacos]

We are rendering out grass (vray fur) in separate render layer from the rest of our scene. The grass render layer also has the other objects in it, but with wrapper materials to give us holdout mattes for comp.

In the following image you can see that we are getting premult errors when combining the two renders in comp:



On the top we have just an over. This produces a white rim around the fence posts (circled in red).
On the bottom this is being fixed by raising the gamma of the alpha to 2.2, which fixes the white rim on the fence posts, but then creates an error on the dirt path (circles in red) where the grass stops.

So the solution of boosting the gamma of the alpha fixes one thing and breaks another...

[svetlozar.draganov]

Have all elements been rendered on a black-background?
Have you also tried to premult/unpremult elements in Nuke?
Would it be possible to send us the scene + nuke composition for investigation? It doesn't have to be the full scene, it could be simplified version which still holds the same issue.

[sharktacos]

Have all elements been rendered on a black-background?

yes.

Have you also tried to premult/unpremult elements in Nuke?

yes

Would it be possible to send us the scene + nuke composition for investigation? It doesn't have to be the full scene, it could be simplified version which still holds the same issue.

I emailed the scene to you (via the support email). Thanks for taking a look.

[sharktacos]

So I wanted to follow up on this in case others are have a similar issue. If you look at the image above in the OP you will see what looks like a premult error around the fence posts. This is actually GI bounce. The issue was that the ground under the grass (the first render layer) is quite bright, and so it bounces up into the fence posts. This results in the comp looking wrong. Darkening the ground so it is basically the same color as the grass above it solves the issue.

TLDR: user error. Vray is fine