Android :: How To Merge Images?

in my application, i want merge serveral images dynamically accroding to the user's selection, and the merged image will be scrolled and zoomed.i extends View to OverlayImageView, which has the properties background Bitmap, layers Bitmap and Matrix. in the onDraw event, i draw the background Bitmap and layers bitmap sequently by the method canvas.canvas.drawBitmap(image, imgMatrix, null).however, if all the bitmaps are stored in memery, it leads to the Out Of Memory problem. so, i have to read the image in the onDraw event by the method BitmapFactory.decodeResource, but it's slow to response user's operation, for example, the scroll and zoom the image will leads to the onDraw event. so, does anyone have some good ideas?