Mask generation for multi-layer image decomposition

Abstract

To improve compression efficiency and decompressed image quality, images are decomposed into separate layers each containing a limited number of image element types (text, line art, or photographic). Each layer can then be compressed separately with a process that is optimal for the type of image element included in the layer. Images are decomposed into foreground, background, and mask layers. The value of a pixel in the mask layer designates whether the spatially corresponding pixel of the recomposed image will be selected from the spatially corresponding pixels of either the foreground or the background layer. The present invention provides a method of determining the value of a pixel for a mask layer. The image is partitioned into large and small sub-images or blocks. A sub-image mask is created for each sub-image by sorting pixels of the sub-image into clusters centered on the luminance of pixels of a pair of pixels of maximum luminance gradient. The value of a pixel in the mask layer is determined by a relationship of the sub-image mask values of the pixels larger and smaller sub-images of which the pixel being evaluated is a member.