Matte Pass with only moving objects?

#1

I have 2 passes I’ve rendered. One is a clean “Ice” floor, the other is the broken Ice floor which eventually shatters when objects pass through it.

What I am trying to do, since there are thousands of little pieces which shatter, is create a matte file in which the pieces change color as soon as they start to move … so I can use that as a matte to bring on my shattered pieces making it look like they shatter as they start shifting around.

All of my shatter pieces start at 0,0,0 (object space) and I would basically like to build a shader that ramps them from black to white as they hit a couple units away from the origin.

Any ideas? I’ve been searching around and playing with sampler nodes and distance tools for hours, I can’t seem to get it.

Thanks in advance.

#2

Oh and I have access to Mental Ray and VRay.

#3

I was able to pull it off with a motion vector pass. I just keyed out the grey “non moving” parts of the pass and used the rest as an alpha matte to get the moving portions. Worked like a charm, looks beautiful :slight_smile:

#4

Seems like you could render them as matte the whole time, then do your cross-fade in comp?