Geometry Shader

[sascha]

I'm interested in using some sort of geometry shader in Mantra, a shader which creates the geometry at render run-time. Is that possible in Mantra?

[Jason]

I'm interested in using some sort of geometry shader in Mantra, a shader which creates the geometry at render run-time. Is that possible in Mantra?

15341[/snapback]

Yes it does.

Mantra calls them "Mantra procedurals", (like "Renderman procedurals") and these basically do the same thing as mental ray Geometry shaders.

If you look in the Render tab of any GeometryObject, and go to the Geometry parameter you can select Procedurals and so on. This is also where you find the "Render Polygons as Subdivision Surfaces" option. Once you've selected Procedurals, you'll see other parameters enabled which dictate which procedural to use.

I believe there is a video tut on the Vislab site which describes how to use HscriptInstance somewhere.

Hope this helps you out,

Jason

ps. If you're a programmer and want to write some in the HDK, look into the VRAY/ C++ classes.

[sascha]

Thanks. I found the tutorial video which shows some of the "geo instancing" functionality done with custom parameters. But the video you mentioned is another one I think, I could not find it on the side. Maybe if someone has a link...

I'm not so interested in instancing through the shader because I have not so good experiences if the master object is part of the scene in XSI and it makes no sense in my case. I want to parse a sequence of shapes (vectors, vertices, polygons and UVs) for some hundred objects from a file into the shader. Doing it in mental ray is much faster and stable compared to importing the entire data with custom operators in XSI for example. But I wonder how it is with Houdini. Will Houdini have so much trouble as XSI or will the Mantra procedurals solution be as fast as loading hundred of file sequence directly in Houdini?

[Jason]

It sounds like you're interested in the on-demand loading of geometry when a bounding box of the geometry is struck, right? This kind of procedural is not really found in the Mantra Procedural section - it's higher up in the Object's Geometry parameter, and it's called Render from file and its variant that only loads geometry when it strikes a bounding box for that geometry. Used correctly, you can render really huge scenes without a problem, especially if you combine it with instancing.