Trees and Alpha problem.

[cg_Butler]

Hi all.

 

I need your help with something, fairly urgent!

 

I am using some evermotion trees in a render and I have told them to not be visible to the camera, but they are still visible for reflection purposes. In a few places they are causing problems. In the RGB channel it appears parts of the objects are black. Upon inspecting the Alpha channel these black areas are the background.

 

Why would the trees in front of the building be causing the background to be visible, through the building?

 

I've not experienced this before. Please see the renders for the problem below.

 

I would expect the Alpha channel to be solid white where the building is. They have no fancy matte settings in the Vray properties.

 

They are not proxies, just standard geometry.

 

Tree RGB

 

Alpha Channel

 

Any ideas?

Edited April 19, 2013 by cg_Butler

[Krisztian Gulyas]

Try putting them in a new layer and hiding them + turn off the rendering of that layer.

[cg_Butler]

Worth a try! I'll let you know what happens.

 

EDIT............As I thought, no joy. It renders exactly the same.

Edited April 19, 2013 by cg_Butler

[j_forrester]

In your Vray "Global switches" try setting your Secondary rays bias to 0.001

 

Also try turning off "recieve shadows" in the trees object properties.

[beestee]

You might check the tree materials for a VRayMtlWrapper, as those can contain matte settings. This seems unlikely though since it is only bits and pieces of the trees.

[cg_Butler]

Neither of these options had an effect to get rid of the problem unfortunately. Thanks for the suggestion though chaps.

 

I have also applied a standard vray mateial to the problematic trees and they still do it. Told them not to cast or receive shadows and even set their alpha contribution to -1 and they still do it.

 

Time to have a play with every setting i can think of! I'll let you know what I find, if anything...........

Edited April 22, 2013 by cg_Butler

[Stephen Thomas]

Is it a problem with viewing backface of the leaves? Though I'm not sure why that would show through to the background. Check backface cull isn't ticked in object properties. Clutching at straws a little here

[cg_Butler]

I tried that option also.........still no different.

 

I'm stuck now. I do have to paste the trees over the top so it's not such a problem but it means i have to render these regions with the trees turned off so i get a proper background, and with them off I'm not getting the correct reflections in the glass of my building. It's rather annoying!

[Krisztian Gulyas]

If you want I can send you a (high poly) tree model and try with that. Just tell me if you need it.

[beestee]

Maybe it would be easier to get a matte of the glass objects and add the tree relections in post, unless you are doing an animation which could make it more complex.

[cg_Butler]

I could do that but it's not ideal. I've never had this problem with any trees i've used before, although I've never used these particular trees.

 

I'm just rendering it, then turning the trees off and rendering the problematic regions for now. Until I can get to the bottom of this problem!