Photo Mosaic movie brush question

[Chris Northcross]

So I've a photo mosaic movie brush that I've created and I'm not getting the result I want.  I have an image with alpha that I'm using as a source image (well, its a movie actually but it has alpha.)

 

This is what the image looks like: 

 

Anyways, I'd like the brush to map to the areas with full on alpha and leave the full off alpha areas of the image unaffected.  Essentially, fill in the white areas and don't draw on the black areas:

 

What is currently happening is that the photo mosaic movie brush. maps one frame to the background and then draws the rest of the image like I'd expect it to.

 

Not sure how to drop out the background so that the painted image of the girl sits on a black ground rather than an image of the movie brush?  I tried turning on the Path End Src Alpha Mask setting to restrict the brush drawing into the alpha channel only but that did not seem to have the desired effect.  I also set the layer alpha to the source alpha before drawing, but again, it did not create the desired effect.

 

As always, thank you in advance for any help.

[Synthetik]

Why not just set the selection to the source alpha, invert the selection, turn on masking, and erase the canvas to black. That would take you existing photo mosaic image and then set the background to black.  Seems way easier than trying to restrict the photo mosaic generation process to alpha enabled pixels only. Or am i missing something you're trying to achieve here.