Shader effect only outer part of object

[gubbfet]

Hi,
I have a problem, I can't find out how to do this effect. I want to add "dust" to a tyre in C4D corona.
But I just want it to effect the "outer" part of the object. The part of tyre that only has contact with the ground. See attached image.

Any suggestions?

/Karl

[Cinemike]

Easiest solution: Use selection tags and just add the dust there.
Want some transition? Use a vertex map as a mask (example attached).
A more beautiful and "random in fall-off" solution would be controlling the mask with fields.

HTH
Michael

[gubbfet]

Thanks!
I got the hand of the two first solutions.
I have been trying with the fall off but don't really understand how to do this. Any tips?

/Karl

[Cinemike]

Quick example attached.
The best way would probably be painting a mask or creating a vertex map by hand. Most control.

If you need more information, please ask. No guarantee I will still answer in 2019, though ;)

Good luck,
Michael

[gubbfet]

Thanks Michael!