Abstract: A method and apparatus for enhancing an image for a display, first digitizes the image to produce a map of the image, this map containing pixels having 2.sup.n grey levels where n is the number of bits per pixel. The map of the image is then downsampled to produce a downsampled map of the image. Thresholding is then performed on the downsampled map of the image to reduce the number of bits per pixel from n bits to m bits to produce a thresholded downsampled map of the image, where m is less than n. The number of bits per pixel is restored from m bits to p bits, where p is greater than m, to produce a restored thresholded downsampled map of the image. The restored thresholded downsampled map of the image can be downsampled again to produce an image map suitable for display on a low to moderate cost display, such as a CRT, with grey scale capability. The displayed image has an enhanced appearance, since it is high fidelity with respect to the original.

Abstract: An adaptive spatial filter for digital image processing selectively performs spatial high pass filtering of an input digital signal to produce an output pixel value having an enhanced signal level using a gain factor that effects a variable degree of enhancement of the input pixel, so that pixels whose activity values are close to an iteratively adjustable activity threshold level will be emphasized significantly less than pixels whose associated activity values are substantially above the threshold level. The activity threshold level is selectively updated at the end of a scan line in accordance with prescribed spatial activity criteria, so as to effectively prevent discontinuities in the threshold level during line-by-line adjustment and to take into account how `noisy` the background is.

Abstract: An adaptive quantizer for digital image processing eliminates the influence of non-information anomalies, such as scratches, dust, etc. that may be present in an image in the course of determining the minimum and maximum signal levels LCHAR and HCHAR, respectively, and more accurately updates respective estimated image information level PREV.sub.-- HCHAR that effectively prevents discontinuities or `jumps` during line-by-line adjustment, thereby effectively obviating the problem of potential corruption of a local mean window and taking into account how `noisy` the background is.

Abstract: The invention is a high speed error diffusion process consisting of repetitive computational cycles in which a binary print value is determined from an error-diffused pixel in a continuous tone image. In each cycle, the computation of the previous pixel error exploits computations made during the previous cycle. Specifically, for a continuous tone image consisting of n-bit pixels, the n least significant bits of the previous pixel error are computed during the previous cycle and its sign bit is determined during the current cycle. The n least significant bits of the previous pixel error are the n least significant bits of the error-diffused pixel of the previous cycle. The sign bit of the previous pixel error is determined during the current cycle from the three most significant bits of the error-diffused pixel value of the previous cycle and from the binary print value of the previous cycle.