When rendering hair with the raytracer, mental ray uses the ray origin to generate a temporary, flat surface which is as ray-facing as possible while lying on the hair axis. A corresponding normal is generated. When rendering with the rasterizer, the hair geometry is converted to camera-facing geometry, and intersected in the same manner as other geometric types. Note that a switch from the latter to the former is performed if both the rasterizer and the raytracer are active. Hair intersections cannot happen on the same hair strand twice in a row, thus avoiding surface acne from this switch. This does have the consequence that individual hair strands cannot cast shadows on themselves.
MasterZap
09-16-2008, 05:56 AM
Is the adaptivity based on some contrast threshold in screen space?
Yes.
2) Does the number of specular FG reflections and refractions as defined in the render settings only apply to the primary FG rays traced into the hemisphere or does it also apply to the initial FG points projected into the scene so that they can go through multiple specular bounces/refractions?
The "reflection" and "refraction" FG bounces apply to the primary bounces in the scene, to how "deep" it creates FG points in the prepass. The FG rays *themselves* only bounce diffusely, and are guided by the diffuse bounces amount.
3) Is interpolation at render time between FG points done in object or screen space?
World space, pretty much, but there are different modes. The "automatic" (default in 3ds Max) basically uses the "N nearest in worldspace".
MR 3.5. As I understood they are beneficial for reducing the number of final gather rays and they can better handle surfaces with bump mapping. Any other benefits?
And do they replace Ward's Irradiance Gradients or is it just an add-on?
They are very similar to Ward's Irradiance Gradients.
They don't really reduce the number of rays, but they reduce the number of FG *points* needed to resolve a bumpy surface. Now, it creates the FG using the un-bumped normal, and stores it directionally, and at render time applies the bump to look up into this directional storage.
Back in mental ray 3.3 it created FG based on the bumped normal, stored a simple color, so to resolve a bumpy surfaec, adaptivity had to kick in and create a ton of super-detailed FG points, which was very bad for performance.