Can you emmit particles from only white areas on a mesh?

I know it is possible to use the intensity of a texture channel to control the TIMING of particle emissions (as explained on this page here, about 3/4 of the way down the page http://www.blender.org/cms/New_Particle_options_a.721.0.html )

What I want is to have particles emmited from only the white areas of the mesh, over the course of the entire animation…anyone know if this is possible? I can’t use vertex groups because the white and black texture will be moving, to drive the animation.

Yes, kind of. I think.

“Use Texture GRADient and set the Nabla value.”

and

“A very important thing is setting Blenders animation system. You have the option to choose between: Int ,RGB and Grad. Int is default settings for particle systems. In option RGB Blender calculate speed and movement of particles according color of texture. Red is for x component of speed, Green for Y and Blue for Z component. In this tutorial we use last option: Grad. “Grad” means the gradient of the texture. With help of mathematics Blender calculates the speed vector from the texture. Composed with high a high keys value this gives beautiful twisting effect. But be careful, you must make many adjustments to find the “right” value for nice twisting! Nabla is set to 0.014. Nabla button select dimension of texture area where the gradient is calculated.All you need is to apply some material to particles (snow texture) and animate it!”

If you add a wood texture and set Grad with TexEmit and add a slight Texture value you’ll have stripes of particles going with the normal on half the texture and oposite the normal on the the other stripe. So if you use a black and white material with no gray you might be able to make things work. It depends on wether or not the under side of the mesh is visible. Gray colors will cause things to go squirely.