i'm masking stuff but there are some artifacts. here's a screen shot:

http://filebox.vt.edu/users/jizietz/seethroughmask.jpg

i drew red arrows pointing to the artifacts. i've got a mask over the rectangle on the left, and things are scrolling into view from right to left. unfortunately, things are showing up outside of the mask. is this because my background color isn't white? any ideas here?