After Effects:Converting An Object's 3D Coordinates Into 2D Screenspace

L = thisComp.layer("Null 1"); fromComp(L.toComp(L.anchorPoint))

"Hiding" Particles Based on Texture Map

After creating a texture map on my grid, I read the Texture Map color's brightness and tested if it was Less Than or Equal to .5.  If it was, I "hid" the particles by scaling them to 0,0,0. 

I couldn't figure out how to delete the points; if you could, I'm not sure it would work correctly with an animated texture.

Also note, I scaled the texture projection down slightly to avoid incorrect values around the edge of the image.

Hopefully this helps.