1. In an image encoding method for compressing image data representing plural pixels of plural nonzero image tones, the improvement comprising: designating a current pixel to be encoded and defining a context region that includes multiple context pixels that are adjacent the current pixel, each of the context pixels having an image tone; quantizing the context region according to a pattern of unique image tones among the context pixels in the context region; and encoding the current pixel with reference to the quantization of the context region.