Help with material wrapper

[djanim8]

I'm not understanding the material wrapper. I've assigned it to objects in the scene, and tried setting the alpha contribution to 1, 0 and -1, but the areas where the object are in the scene are never black in the alpha channel. The other objects that are behind the wrapped object aren't being rendered, but the alpha channel isn't representative of hidden objects.

What am I missing? I'm just trying to get one object to block out the other parts of the render to composite later.

This is an example. The rendered image doesn't show the particles behind where the boat is (boat has the mtl wrapper on it), but the alpha channel says they are there.

[nikolay.kusht]

In your scene the boat object should probably have the Matte surface On and Alpha contribution set to -1.

If you still have issues and you cannot achieve the desired results, send us a test scene with more details so we could investigate this further at support@chaosgroup.com.