Houdini newbie here. I have two semi-transparent objects that I want to holdout from each other in two separate render passes to be added back together later in compositing. How do I go about this in Houdini/mantra?

My first attempt was to add the holdout object to the “Forced Matte” parameter of the mantra node. Then it was to turn on “Matte Shading” on the holdout geometry sop. In both cases, the holdout object is 100% held out, when what I really want is it to be held out only to the amount of its alpha.

Thanks for any pointers,

-Greg

I think you can do this with the matte shader. There you have to tweak the alpha. The matte switch seems to replace the material with a default matte shader:

vm_matte / object:matte = ('false')
Replaces any shaders with the default matte shader. The default matte shader is controlled using the vm_matteshader setting.

You can use takes so set up the passes.

Another way, the most versatile with transparent objects and volumes, is to use deep camera maps (http://www.sidefx.com/docs/houdini11.0/rendering/deepshadowmaps). [sidefx.com]

You are probably aware of this, but much like rsl, you'll need to work with both opacity and alpha to get the result you are after.

Hope that helps, cheers,

Koen