I'm using the environment room approach more and more with a square room and the environment map shader (usually with a gradient) to take renders of objects… in the past I used to use a structure around the object and leave one side open to get the object against a transparent background for later compositing, I'm wondering is there a way to, after doing a render, remove an object from the render so it just shows the object I want against transparency?

I'm not really sure this matters for the end quality of the render but being able to do the initial render against a background helps me see it better (since if the object of focus has dark colors around the edges you can't tell where it ends and the nothingness starts) thanks for any help on this

Is this what you're looking for?
http://www.sidefx.com/docs/houdini10.0/nodes/shop/v_matte [sidefx.com]

There is also the “Phantom” parameter on Geometry objects
http://www.sidefx.com/docs/houdini10.0/nodes/obj/geo [sidefx.com]

I'm having a hard time understanding your question.

Thanks for the response.. I think one of those may work.. I need to look into what they do in more detail… basically this is what I want to do:

render an object within a box so that the raytrace hits all sides of the box during the render but then to remove one side of the box within the final output of the render so that the object shows over transparency. (so that the object can be composited into another scene)

Are you talking about mattes? There is a checkbox for that in the render tab of the object node.

Yeah I think mattes is what I am thinking of.. thanks, I didn't know what it was called.