Apparatus and methods for converting continuous grey tone images into high resolution halftone images utilize an iterative, multi-level, multi-resolution error convergence process. The continuous grey tone image is first binarized. At each level of the process, the binarized image is compared with the grey tone image over a larger window of pixels. Within each window, selected binarized image pixels are tested for possible change in binary assignment. The binary assignment of a given test pixel is changed if the change would result in a lower average error over the entire window. By varying the selection of test pixels, the process can provide clustered dot patterns and dithering.

Halftone imaging method and apparatus utilizing pyramidol error convergence

A new halftone algorithm is described. The algorithm is designed for implementation on a parallel architecture in order to provide fast, progressive coding of moderate-resolution images. The design is based on a multiresolution, hierarchical, pyramidal structure. At each pyramid level, the binarized image is compared with the original, gray-tone image over a successively larger window of pixels for calculation of a weighted averaged error. Within each level, selected binarized pixels are tested for possible changes in the binary assignment. The binary assignment is changed if the change results in a lower average error over the entire window. Varying the selection of test pixels can cause the same process to provide clustered-dot patterns and dithering. A comparison of performance with the best implementation of the error-propagation algorithm is presented visually. Quality is compared also in terms of isotropy of the texture and the appropriate blue-noise characteristics in areas of uniform gray tone. The benefits of this algorithm are realized with moderate-resolution display of the order of 512 dots × 512 dots. The processing can be carried out on smaller blocks since the results can be combined without any visible seams or edge effects.

Multiresolution, error-convergence halftone algorithm.

Physicians frequently need to see retinal, biopsy, and other images, produced originally by medical diagnostic equipment and then processed by computers. At present this is done using either time-consuming photography or expensive video equipment. We have used digital halftoning as a method for fast communication of images both on print and on computer terminal screens. This method quickly produces a good-quality halftone rendition of a grey-scale image. These images are suitable for display and printout on inexpensive devices that normally do not have grey-scale capability. The algorithm is based on a previously published error-propagation technique. We improved the algorithm by including a factor that accounts for the difference in size between light and dark points on various devices. The algorithm is extended to devices that have two bitplanes (VT240), and the execution and transmission times are reduced. At Tufts-New England Medical Center in Boston, this program has been used in processing and reporting the results of muscle and nerve biopsies. At the Eye Research Institute in Boston, it has been used to report the results of retinal visual field mapping. This technique has a wide range of applications. It allows "image processing" to be done on computers that have no traditional image-processing hardware. It allows several users to operate simultaneously on time-shared systems that have only a single image-processor. Images are displayed on 1 or 2 bitplane devices (LA50 printers or VT240 terminals). It allows image transmission over long distances --replacing video communication equipment with RS232 cables and modems.

Medical Image Communication Using Halftone Algorithms

This paper describes an improved digital screening technique to convert a continuous-tone image into a halftone (binary) image for pictorial image reproduction. The technique first generates a digital screen in gray scale (e.g., from 0 to 255) based on a set of mathematical equations; and then it adaptively thresholds the screen at each pixel using threshold value equal to the pixel gray intensity of input image to produce the output image which contains standard halftone dots. The screened image holds as many gray shades (e.g., 255) as in the input image by yielding different halftone dot sizes so that the binary image looks as if it has gray scales. The screen can be generated for either linear or nonlinear transformations from image gray tones into spatial sizes of halftone dots in order to preserve or to enhance low frequency information (i.e., average gray tone within the area of a halftone cell), respectively. In addition, the technique preserves sharp edges in the input image by producing asymmetric halftone dots. This technique works on a wide range of screen frequency (i.e., halftone dots per inch) and scan frequency (i.e., pixels per inch). Scanned and processed images are illustrated.© (1989) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Screening Pictorial Images with Gray Level Reproducibility

This paper presents a set of 128 halftone patterns on an 8 × 8 block size. The patterns have been designed to produce an acceptable gray-scale rendition when the halftone is seen from a distance and, at the same time, to allow comparisons to be made among the intensity values of individual pixels at a closer look. A Pascal program is also presented that produces halftones on an HP Laserjet+ laser printer, and sample halftones are shown. Finally, a method is proposed that prevents images digitized with a 4:3 aspect ratio from being distorted when halftones are printed at the usual 1:1 aspect ratio.

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128 Halftone patterns for printing gray-level images on a laser printer

A versatile and useful personal computer based procedure to generate halftone representation of digitized image data is presented. This feature provides limited interactive image processing capability for a basic personal computer work station without having to acquire a video monitor and a digital-to-analog converter board. The generated halftone images can be displayed on the standard dot-addressable graphics monitor, and/or they can be printed on regular paper via an associated printer. The halftoning procedure is based on an error diffusion technique in which dots are produced randomly, one at a time, based on both the local gray level value and some accumulated measure of the error due to previous assignments. The key elements in halftoning, such as dynamic, spatial, and display medium resolutions, are defined, and the mathematical correspondence among them is established.

Personal Computer Based Image Processing With Halftoning

Perspectives of a few challenging problems in the digital processing of low resolution halftone images are presented. The topics covered include screen selection, tonal reproduction control, and the scaling and resolution conversion of halftone images. Particular emphasis is placed on a new method of digital scaling and resolution conversion of electronically stored low resolution halftone images. Several simulation examples are given.

Hsieh S. Hou

Papers

Personal Computer Based Image Processing With Halftoning

Digital Processing Of Low Resolution Halftone Images