Abstract
Improved gray-scale (IGS) quantization is a known method for re-quantizing digital gray-scale images for data compression while producing halftones by adding a level of randomness to improve visual quality of the resultant images. In this paper, first, analyzing the IGS quantizing operations reveals the capability of conserving a DC signal level of a source image through the quantization. Then, a complete procedure for producing a multi-level halftone image by IGS quantization that can achieve the DC conservation is presented. Also, the procedure uses the scanning of source pixels in an order such that geometric patterns can be prevented from occurring in the resulting halftone image. Next, the performance of the multi-level IGS halftoning is evaluated by experiments conducted on 8-bit gray-scale test images in comparison with the halftoning by error diffusion. The experimental result demonstrates that a signal level to be quantized in the IGS halftoning varies more randomly than that in the error diffusion halftoning, but not entirely randomly. Also, visual quality of the resulting halftone images was measured by subjective evaluations of viewers. The result indicates that for 3 or more-bit, in other words, 8 or more-level halftones, the IGS halftoning achieves image quality comparable to that by the error diffusion.
Highlights
Digital halftoning is a technique to re-quantize a digital image to fewer bits while preventing the image appearance from being corrupted by producing continuouslooking tones, that is, so-called halftones [1]
In a halftoning method, some kind of signal is added to each pixel of a source image and the resulting signal is re-quantized to fewer bits
The source images used in the experiments include vertical and horizontal ramp images shown in Figures 4(a) and (b), respectively, and two natural scene images Barbara and Lena shown in Figures 4(c) and (d), respectively
Summary
Digital halftoning is a technique to re-quantize a digital image to fewer bits while preventing the image appearance from being corrupted by producing continuouslooking tones, that is, so-called halftones [1]. For being printed with a black-and-white printer, an 8-bit gray-scale image is to be re-quantized to one bit or two possible levels. Halftoning with two levels is still indispensable in printing. In a halftoning method, some kind of signal is added to each pixel of a source image and the resulting signal is re-quantized to fewer bits. These signals determine the resulting image appearance. A variety of halftoning methods to generate the additional signals can yield good halftone quality in the resulting images that have been proposed
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