Transparency issue - can't rebuild beauty in Nuke

[leabum]

I have a scene with many transparent/semitransparent objects like trees and I do not seem to be able to rebuild the beauty render with my passes 100% inside of Nuke. I subtracted my rebuild from the original beauty to see the differences and they only occur around transparent areas. Is there a setting I dont know of in Vray or is my Nuke nodetree wrong?

[Morbid Angel]

Try unpremult all elements before rebuilding, then copy alpha at the end and premult again.

[leabum]

Thanks fr your reply.
Yeah I tried that, but the lighting AOV does not get unpremultiplied correctly it seams compared to the unpremultiplication of the GI AOV. I double unpremultiplied the lightingpass for curiosity reasons, but then I get white mattelines obviousely. I am still learning so please correct me if the lighting AOV is supposed to look like that after unpremultiplication, but for my eye there is too less difference compared to the premultiplied version.

[Morbid Angel]

Can you post this exr frame for me to take a look?

[leabum]

Hi thank you very much for doing that. Here you have the exr together with a nuke script. (Nuke 10)
https://www.dropbox.com/sh/ic8zpgwb1...Bli8i5j_a?dl=0

[Morbid Angel]

So the issue is not the alpha as it is the 2sided material in your leaves. Since there is no render element for isolating the 2 sided mtl, when you rebuild the comp it actually is missing it. What you need to do is take your final rebuild result and subtract it from the beauty exr, this will give you 2sided only, which you then plus into the lighting as a separate graph.

[leabum]

Okay did not know about that one, but thanks for the workaround. But as you can see there is also an issue with the transparency of the road in the centre of the frame. This one is just a simple VrayMTL with a Map for the Opacity. Is the same true for that case?

[Morbid Angel]

Yeah I'm not sure about that. The alpha is not premulted correctly for some reason. Perhaps some one from chaos can chime in...