OK, this is a tricky one. (at least for me it is).

I have this rain falling in a canyon and the camera is a crane shot. Now the canyon is actually made out of a light colored sand, so when the rain hits, besides little splashes I want to have the sand get darker in those spots. I've got it working that on the collision event, and particle with an instanced object is created it that spot with no movement, and I plan to get it looking right in the composite. This part all works fine.

Here's where I'm stuck. The little instanced planes are always pointing up. This would work well if the canyon were flat, but it's not. When a rain drop hits the walls, the splotch still points up rather than having the appropriate perspective. What I think I need to do is figure out a way to read the tangent at which the drop hit the wall, and the set the aim direction of the instanced particle to be perpendicular to it. This is the part I have no idea how to do.

My guess is no one here will know either, but I figured it was worth a shot.

Thanks,
-dann