>>> Help, Math wizards, you're my only hope.
> 
>> Try the attached.
> 
> Yeah baby! There's nothing like a copy/paste solution that I almost 
> understand, but does exactly what I want. ;)
> I can't wait to try it out on non-solid, non-spherical shapes.
> 
> Thanks!

:-)

What you're trying to do is transform some kind of "screen" coordinates 
into world coordinates. What I did was to calculate the directions of 
the "new" x,y,z axes and then use those to transform the original 
coordinate to the actual world coordinate:

 > #local newZ = vnormalize( Lookat-Camera );
The new Z axis direction should be from Camera to Lookat, this is what 
you want. vnormalize makes sure it's a unit length vector.

 > #local newY = y;
The new Y axis direction is just y to start (it will be fixed later).

 > #local newX = vnormalize(vcross(newY,newZ));
The new X axis direction is perpendicular to the Z and Y defined in the 
above two lines (ie to the right). X and Z are now correct, but Y needs 
to be adjusted to take account of Z probably not being horizontal (Y 
needs to be "tipped back" a bit).

 > #local newY = vnormalize(vcross(newZ,newX));
The new Y is now calculated as perpendicular to the Z and X just 
calculated. This is correct now, with all 3 perpendicular to each other.

 > #local bar = bar.x * newX + bar.y * newY + bar.z * newZ;
This line essentially takes the original xyz coordinates and transforms 
them using the new XYZ axes. There are neater ways to do this, but I 
thought this made it a little more clear what is going on.

Note the new coordinates are relative to the camera, so you still need 
the translate camera line.

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