1. A method for compositing a source image and a destination image using a mask image to produce a result image in a digital image processing system comprising: on a pixel-by-pixel basis, using the mask image to select between the source and destination images, where a black color value for a mask pixel selects the color value of the corresponding pixel of the source image, and a white color value for a mask pixel selects the color value of the corresponding pixel of the destination image, and where an intermediate color value for a mask pixel selects a weighted average between source and destination pixel color values, and writing said selected color values to corresponding pixels of the result image.