Many computer graphics systems use pixels to define images.  The pixels are arranged on a display screen as an rectangular array of points.  Aliasing occurs because the pixels have a discrete nature.  Artifacts can appear when an entire pixel is given a light intensity or color based upon an insufficient sample of points within that pixel.  To reduce aliasing effects in images, the pixels can be sampled at subpixel locations within the pixel.  Each of the subpixel sample locations contributes color data that can be used to generate the composite color of that pixel.  However, some graphics systems may limit the amount of memory for storing subsample color data for each pixel.  Thus, such graphic systems must carefully select which color data are stored so that these systems can still accurately produce a composite color for each pixel. 
Hence, there is a need for a method and an apparatus that, for each pixel, can make color selections and replacements without introducing unsatisfactory artifacts into a displayed image.