Adaptive Selection Of Threshold Matrix Size For Pseudo-Gray Rendition Of Images

Abstract

Dither rendition of images is a well-known method to produce pseudo-gray scale images using bi-level output devices. Moreover, it reduces the data to 1 bit/pixel. For fixed sub-picture (matrix) size there is, however, a contradiction between resolution and the dynamic range of the gray scale. In this paper we show how this conflict may be overcome using variable resolution. For this purpose we introduce a detail-dependent segmentation of the image into variable-size subpictures. We describe first the construction of suitable dither matrices of size 2mx2m, with m = 2,3,4. The next section deals with adaptation criteria to obtain the appropriate pseudo-pixel size. A major aspect of the paper is the fast in-place computer method to break the full picture up into subpictures of appropriate size. This sub-picture size depends adaptively on the local "activity" of the picture. The method is demonstrated using 8-bit test pictures and shows good gray scale rendition and sufficient de-tail resolution.