How to render a Matte object?

[allenh]

At the risk of over-explaining: a "Matte" object as I'm using the term describes a piece of geometry that 'cuts a hole' through any object its in front of; usually used to hold out geometry for compositing purposes.

What is the correct way to create matte objects (or to otherwise hold out geometry) in Octane?

[juanjgon]

I suppose that the best workflow to render mattes is to use render layers. There are several topics about it in the forum:
viewtopic.php?f=101&t=58609


Thanks,
-Juanjo

[allenh]

Hi Juanjo;

This method more or less works, with one exception:

I have a reflective/refractive object that I would like to mask for compositing purposes. I can add the "matte" geometry to my scene, and use Render Passes as you describe above to hold out the geometry the way I need. But: the renderable geometry seems to reflect/refract the matte object itself.

So this means I need to decide what shader to put on it. I'm using the OSL Projection trick mentioned elsewhere in this forum to project a background plate onto my matte geometry. To do this, I connect an RGB texture to the Emissive slot of the diffuse material, and set Diffuse itself to be 0.

What I see now is sort of a bright 'halo' around my object; it's as if my geometry is being pre-composed with the matte/emissive geo before the final image is written

I'll try to share some images here that explain the problem: