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).