Bitmapdata and Overlap Problem

[mstx]

Hi, I want to create a game where user draws over a dashed path. The dashes are group objects and user draws on a bitmapdata. At the end I check if the dashes and the bitmapdata drawing matches. But the problem is bitmapdata drawing is document size and overlaps all dashes. How to ignore the transparent areas in the drawing? Or is there any different way for drawing and checking for overlap?

Thanks