Hi there,

I am attempting to write an expression which puts motion vectors into rgbPP for regular Maya particles.

I got as far as this:

```
//multiplier
float $mult=0.5;
//velocity
vector $vel=particleShape1.velocity;
float $xvel=$vel.x;
float $yvel=$vel.y;
float $zvel=$vel.z;
//rgbPP
particleShape1.rgbPP=<<($mult*$xvel),($mult*$yvel),0>>;
```

which does indeed do the job, just as long as the camera is pointing exactly down the z-axis. Fine, but of limited use. Now I need a bit of help with some vector transformation maths.

Ideally I want to get the motion vectors to work from my camera, whatever direction it is pointing.

I think I can get the unit vector to the camera:

```
//camera
vector $camPos=`getAttr camera1.translate`;
float $xCamPos = $camPos.x;
float $yCamPos = $camPos.y;
float $zCamPos = $camPos.z;
vector $camUnitAngle = `unit<<$xCamPos,$yCamPos,$zCamPos>>`;
float $xCamUnitAngle=$camUnitAngle.x;
float $yCamUnitAngle=$camUnitAngle.y;
float $zCamUnitAngle=$camUnitAngle.z;
```

(is that right???)

I think I need to multiply the inverse of this unit vector by the particle’s velocity but I am not sure how to do that. (I should have paid more attention to my maths teacher in that hot summer of 1976…) Any help would be gratefully received !

I should also let you know that I have no Python at all (yet)

Thanks

Dan