Is there a way to setup object render parameters so that an object is invisible but still casts GI? Or eg. invisible but receives shadows, or invisible but casts shadows?

I know that in maya you can setup these stuff in render section - receives reflections, visible in reflections, casts GI, receives GI etc.

I think In the Houdini you should open Edit Parameter Interface window from your special Object and add your option like “visible in xxx” in the “For Rendering” Tab
:?

tried looking for the parameters to add from mantra, couldnt find em

Of course for hide object in the Render time ,But render it in the Reflection ,You can turn on Phantom option in the Rendering section from your specify object :?

Yeah but but i'm spceficially interested in invisible but cast GI and invisible but receive shadows

What is invisible but receives shadows look like?

I think andrej22 is talking about a shadow matte material.

Shadow eg - lets say I've got a box and a sphere which casts a shadow on a box and I want only the sphere and the shadow(which respects the shape of a box) visible in my render. Shadow matte shader is probably the solution though, I'm gonna try it out.

cast GI eg - lets say I got a background plate, an image of a mountain and lets say I wanna model a house which I would “put” on a mountain and I want the color of the mountain to bleed on the house, cast GI on the house. BUT, the mountain is just a background plate which I don't want in my renders, I'd add it later in comp. So I'd like to project the mountain image on a mountain geometry which I'd make just for the purpose of casting GI on my house. Basically I wanna have an invisible object that bleeds color onto my visible geometry. I hope that explains it a bit better now. Now, I know the workaround for this, I could just render it all with the mountain background plate projected onto geometry and use house objects alpha to cut it out in post, but I kinda wanna avoid that if possible and just render out the house which receives GI from invisible(during render time) geometry.

Hi andrej,

For the second one, I'm doing exactly same thing in production shots right now where I need plate features to light and reflect in my CG geometry. So yes, I'm modelling the feature, then projecting UVs from camera, and then I just assign it a basic Surface Model shader with everything off but emission, where I put my plate exrs. And then I just check Phantom property on the object (if it's not there, you can add with Edit Parameters).

Works like a charm!

I just tested it out. Actually “phantom” according to the manual makes an object invisible but casts shadows… but it also casts GI after I ran few tests. I didn't even have to use emission. Just made a mantra surface with diffuse RED, made a grid and a sphere sitting on top of it. Applied the red mantra surface to the sphere and turned on “phantom” on it and it was obviously casting shadows AND GI, I noticed the red color bleeding on th grid from the sphere.