particle geometry instancing question

[mji]

Hi,
I'm attempting to use a particle system to do hardware instancing on a piece of geometry. That part works just fine... but I'd also like to be able to rotate the instanced geometry to face the direction in which each particle is moving. I know how to use instancing to do rotations... I just can't figure out a way of calculating this rotation on a per-particle basis. Any suggestions?
Thanks,
Michael

[malcolm]

I have a feeling you'll need to use a GLSL MAT for this. Use a SOP to CHOP to get the v[3] attribute, which is the velocity of the particle (direction and speed). Then pass that into the GLSL MAT as a texture buffer under the 'arrays' page. Using this vector in the vertex shader you need to determine the rotation for the instance (from (0,0,-1) to the v[3] vector), which is the tricky part. There is a function whose name I can get tomorrow which may help you, or you may find something good online about building a rotation matrix from two vectors which may solve the issue.

[mji]

Ah, I see... this looks a bit trickier than I had originally thought... I think I'm going to try something else for now, but might have to revisit this later. Thanks!