[silin]

ну да, не универсально, да и кривовато
надежнее будет по-простому копировать с альфа-маской

public class Main extends Sprite 
{
	private var bmd:BitmapData = new BitmapData(400, 200, true, 0xFF8080C0);
	private var brush:Sprite = new Sprite();
	private var brushContainer:Sprite = new Sprite();
	private var filter:ColorMatrixFilter = new ColorMatrixFilter(
		[ 1, 0, 0, 0, 0,
		0, 1, 0, 0, 0,
		0,  0, 1, 0, 0,
		0, 0, 0, -1, 255]);
 
	public function Main():void 
	{
		var r:Number = 20;
		brush.graphics.beginFill(0xFFFFFF);
 
		/*//а можно и с 'мягкими' краями
		var mtrx:Matrix = new Matrix();
		mtrx.createGradientBox(2 * r, 2 * r, 0, -r, -r);
		brush.graphics.beginGradientFill(GradientType.RADIAL, [0xFFFFFF, 0xFFFFFF, 0xFFFFFF], [1, 1, 0], [0, 0xCC, 0xFF], mtrx);
		*/
		brush.graphics.drawCircle(0, 0, r);
 
		addChild(new Bitmap(bmd));
		brushContainer.addChild(brush);
		addChild(brushContainer);
		stage.addEventListener(MouseEvent.MOUSE_MOVE, stage_mouseMove);
	}
 
	private function stage_mouseMove(e:MouseEvent):void 
	{
		brush.x = mouseX;
		brush.y = mouseY;
		var maskBmd:BitmapData = new BitmapData(400, 200, true, 0x0);
		maskBmd.draw(brushContainer);
		maskBmd.applyFilter(maskBmd, maskBmd.rect, new Point, filter);
 
		bmd.copyPixels(bmd, bmd.rect, new Point() , maskBmd, new Point());
	}
}