High quality/low complexity halftoning and color image enhancement

Sang Ho Kim, Purdue University

Abstract

We present three research topics for improving the perceptual quality of halftone images. The first two topics are for developing high quality halftone algorithms with the consideration of computational efficiency. The third topic is for enhancing continuous-tone images as a preprocessing step before printing them. In Ch. 1, using the iterative direct binary search algorithm, we compare the halftone texture quality provided by four different HVS models that have been reported in the literature. Choosing one HVS model as the best for DBS, we then develop an approximation to that model which significantly improves computational performance while minimally increasing the complexity of the code. By varying the parameters of this model, we find that it is possible to tune it to the gray level being rendered, and to thus yield superior halftone quality across the tone scale. We then develop a dual-metric DBS algorithm that effectively provides a tone-dependent HVS model without a large increase in computational complexity. In Ch. 2, we propose a novel halftoning method of hybrid LUT/screen halftoning which achieves both of high quality and compressibility using only point processes. Experimental results show that the halftone images from our hybrid LUT/screen algorithm have the quality comparable to those from direct binary search (DBS) algorithm with unprecedented performance of halftone compression. In Ch. 3, we consider the problem of restoring a noisy blurred image using an adaptive unsharp mask filter. Starting with a set of very high quality images, we use models for both the blur and the noise to generate a set of degraded images. With these image pairs, we optimally train the strength parameter of the unsharp mask to smooth flat areas of the image and to sharpen areas with detail. We characterize the blur and the noise for a specific hybrid analog/digital imaging system in which the original image is captured on film with a low-cost analog camera. A silver-halide print is made from this negative; and this is scanned to obtain a digital image. Our experimental results for this imaging system demonstrate the superiority of our optimal unsharp mask compared to a conventional unsharp mask with fixed strength.

Degree

Ph.D.

Advisors

Allebach, Purdue University.

Subject Area

Electrical engineering

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