Sunday, October 19, 2014

Displacement and Falloff

If you've used the Displacement option on a material with the  Mental Ray renderer then you've probably been frustrated when trying to use it within an animation. You can use the "View" option which will limit the effect to your current camera (or perspective view) and this works fine for still images. Over the last few years I have on occasion wanted to use Displacement to help create grass in an animation. Of course the "View" option will create all kinds of artifacts and flickering under this scenario, regardless of your render settings, as MR re-computes the Displacement effect each frame and it ends up just slightly different each frame. I have not had luck unchecking Displacement, being willing to eat the render overhead as the effect is computed for the entire scene.
You might think to try using a "Falloff Map" map to control the effect over distance as I did. However, for whatever reason, Mental Ray will not respect this and it still computes the effect over the entire surface. I've even tried using a black and white "Mask" Map to control the physical location of the effect on the surface and this fails also. Below is a sample rendering with this technique. In this case the Black diffuse colored grass is in the foreground and shows what should be the limit of the displacement effect, yet the effect continues on to the horizon as shown with the green colored grass.
I would have to assume that the "View" option in the renderer precludes the use of any other technique that might do the same thing.

I have yet to find an acceptable solution and it's sad because this technique can be used to quickly add detail where needed, but apparently only in still images. If you come across this post and have a solution, please let me know.