Issue with post-multiplying volume matte

[ChristopherC]

Hi!

 

I've got a scene with a box and a volume. I am trying to split the rendering of each object into a different pass.

 

 

 

In the `/out` network from the attached scene, the `composite_premult_matte` node regroups a first setup of render nodes, each rendering a specific object while setting the other(s) as “forced matte”. When adding the resulting passes in compositing, I get back the beauty as expected.

 

Now I've set up `composite_extracted_matte` which this time extracts the mattes into the alpha layer of different files. For each “nomatte” render pass, the object(s) not to be rendered are set to “phantom”. When compositing the whole in Nuke, I don't get the same result as the beauty.

 

I tracked it down to this Nuke's graph below, where post-multiplying the matte of the volume doesn't seem to match with the pre-multiplied version: `volume_nomatte x volume_matte != volume`.

 

 

 

Note that this issue seems to happen only when dealing with volumes (transparent?) objects. Is there something that I'm missing within my rendering setup?

 

 

Cheers!

 

composite_volume.hipnc