Pixel Removal

Was hoping for some starting points and suggestions for this project.

I want to start with a live camera feed of a dancer, and a still image of a plain colour, in this case it will be grey.

I want any pixel that has movement occuring in it, from the feed of the dancer, to remove the corresponding pixel from the image.

i.e. the image will be projected onto a screen, with the camera capturing the dancer pointing in the same direction. When the dancer walks across the projection screen, her movement will fade out the image as she moves across.

Our application of this will be a performer sweeping with a broom. Because her movement will delete the image, it will appear she is sweeping away the image.

I think I may have to do something with a matrix, but has anyone got any starting points for me?

One idea I had was to turn the original camera feed into a whole bunch of pixels. When there is movement the pixel will turn white. I can then use that change to correspond to the deleting of the image. Any ideas?