Render only through object

[Sagroth]

Is there a way to render just what is behind the certain geometry?

Particularly, I've got a smoke that turbulate around the geometry and I want to get two separate passes: first - smoke matted by this geometry and second - just smoke behind it that was matted in the first pass.

[jacob clark]

Is there a way to render just what is behind the certain geometry?

Particularly, I've got a smoke that turbulate around the geometry and I want to get two separate passes: first - smoke matted by this geometry and second - just smoke behind it that was matted in the first pass.

We've had luck in the past rendering a shadow pass from camera of the object in question. Then projecting that map back through the camera, and using a bit of logic in a illuminace loop shadow to only render what is inside the shadow...

[Sagroth]

If I understand correctly this technique, that doesn't solve "in front of/behind geometry" issue.

Actually, in maya/mentaray I can assign transparent shader to my object and turn off "primary visibility" for the rest of the scene (keeping Visible in Transparency) - and that's what I'm looking for in houdini/mantra. I can make the rest of the scene "Phantom" but only trace function reveals it in my object - is there a way to make the same just with a plain opacity?

[Jason]

If I understand correctly this technique, that doesn't solve "in front of/behind geometry" issue.

Actually, in maya/mentaray I can assign transparent shader to my object and turn off "primary visibility" for the rest of the scene (keeping Visible in Transparency) - and that's what I'm looking for in houdini/mantra. I can make the rest of the scene "Phantom" but only trace function reveals it in my object - is there a way to make the same just with a plain opacity?

MentalRay is always raytracing, even for transparency. This makes it easier to do than with micropolygon mode where simple transparent objects don't send out rays from behind them. You could write a (very very simple) shader hich raytraces further along the I vector and puts the result into an export parameter - for AOV outputting goodness.

[Sagroth]

Yeah, I've made a simple shader with dir = I in Trace vop and it does exactly what I need.

But I can just plug a simple "constant 0 0 0" into Of and see my scene in transparency without Trace at all (as long as those objects are visible). The idea was to get rid of Trace (not to deal with trace limits and stuff where geometry folds and so on) since transparency actually works without it, just found no way to get rid of directly seen parts of the scene.