SubD's and MentalRay... Output Curves or Poylgons?

[frankb123]

Just curious if XSI's true curve SubDs are output as curve surfaces instead of being tesselated as poly faces and if MentalRay or any other connectable renderer can output XSI's SubDs as curves like RenderMan can (so that no matter how high the resolution nor how close the camera gets all edges are always perfectly smooth)?

Thanks for any feedback!

--Justin

[srvargas]

Mental Ray is a micropolygon renderer - so it tasselates evrything before rendering - SubD, NURBS, etc. As far as I know RenderMan is the only software to render directly SubdivisionSurfaces, but its subdees have nothing to do with the subdees in Maya or XSI. RenderMAn subdees are not comprised of polys (as in XSI) but are a standalone type of geometry

[frankb123]

Oy? What I read on XSI's feature list they listed SubDs under the Curves heading which made me think that XSI's SubDs were true higher order surfaces.

Now, in Maya Unlimited the SubDs are indeed curves, but they render as pre-tessellated polygons with Maya's own renderer... HOWEVER, if you output Maya's SubDs to RenderMan, they are transferred as curves and are not pre-tessellated at all. RenderMan renders them perfectly smooth, just like NURBS.

I thought that XSI was the same in concept, even if it can't transfer/export to RenderMan at this time. Am I wrong?

If I'm right, I was curious if MentalRay handled XSI's curve geometry as curves (perfectly smooth at any resolution)??

[shanesternstein]

"RenderMAn subdees ...are a standalone type of geometry"

That's not strictly true. From an interface perspective, that is correct, but when you get down to the actual implementation, you'll find Sub-Divisional Surfaces are B-spline Surfaces of Arbitrary Topology.

The reason I'm mentioning this is not to sound like a prick, but to stress the fact that Sub-Divisional Surfaces are indeed Surfaces. Surfaces which are continuously smooth yet can have non-rectilinear structure.

XSI and Maya (as far as I know) use approximations of these surfaces in the application and rendering. XSI has a parameter called "Mesh Subdivisional Level" which basically controls how close the resultant mesh gets to representating the true Sub-D Surface. A value of infinity would truly represent the surface (but would have an infinite number of polygons).