There was a problem to make a shadow and reflection of the object on invisible plane using Mental Ray's Matte/Shadow/Reflection material. After browsing and reading stuff I've found a good Jeff Patton's idea for Maya which I adopted for 3DS Max.
So, as usual use a plane with Matte/Shadow/Reflection material. To make it invisible use Environment background Switcher map on it's Camera Mapped background slot and on Environment slot of a scene, for example I had an HDRI map and gray background. Gray background is generated with Composite map, though the colour of the back can be set to any colour, read on to know why.
Do not forget to enable Process background and environment map in Exposure Control to avoid black plane (due to difference in exposure control).
May 21, 2013
This is a continuation of my attempt to make Trapcode Form particles respond the main composition camera motion. So they act almost like 3d objects as you see. The trick is in connecting precomposed particle camera to main composition camera plus many more little tricks added in this version. Particles were created in AE also.
Unfotunately it's not possible to use Sprites and Shading with this because these options make particles distorted for some reason (probably because the precomp is used with transormation collapse flag).
Another disadvantage of this is that it's not possible to use multiple particles source, at least i haven't found the way to do it (yet). Since the main comp and particle's cameras are connected in time it doesn let me use Time Samples to their full. I tried to trick it using another copy of a particle (with variations) in the same particle precomp (butterfly) with its own camera connected to main comp camera but with scripted time offset and using Split Clip mode, but it didn't work.
For this version i used a pair of butterflies in one precomp with slightly different positions and motion parameters.
at May 21, 2013