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Histogram equalization (HE) is a simple and effective method for contrast enhancement as it can automatically define the gray-level transformation function based on the distribution of gray levels included in an image. HE fails to produce satisfactory results for a broad variety of low-contrast images because HE does not use the spatial features included in the input image. This paper proposes a novel contrast enhancement method using not only the conventional histogram but also the differential gray-level histogram of the input image. The differential gray-level histogram contains edge information from the input image. Edge information is one of the important spatial features of the image. We can extract the regions that are to be emphasized by using both the conventional histogram and the differential gray-level histogram. The input histogram is modified by the histogram of the extracted regions. The gray-level transformation function is derived using the modified histogram. [ABSTRACT FROM AUTHOR]

Bit-length of digital images is decided by the Analog-to-Digital (AD) converter. The bit-length expansion technique is necessary for high-quality displays such as the liquid crystal display (LCD) and the plasma display. Bit-length expansion is also necessary for low-bit length images, which are degraded by the pseudo-contour. In this paper, we propose a bit-length expansion method from M-level to N-level [N=3(M-1)+1]. This operation means that two quantization levels are introduced between existing levels. The bit-length expansion method results in the estimation problem of an error image between an M-level's image and an N-level's image. The error image is the pattern image which consists of three values. We show the estimation method of the three values pattern image and that the proposed method can remove the pseudo-contour of digital images through several application results. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 92(10): 32–40, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10105 [ABSTRACT FROM AUTHOR]