i honestly don't really see the need for mainPic at all , or why you are using copyPixels in this case. If i were you i would ditch that , and in your loop just use the draw method and the original display object as the source. And really you could simplify this whole thing by creating a sprite , with X number of sprites with rectangular fill's as children , set it as a mask, and move , tween its children. There's no need to do all these bitmap data conversions , its far more processor intensive then its needs to be.