Journal of the Korea Academia-Industrial cooperation Society (한국산학기술학회논문지)

The Korea Academia-Industrial cooperation Society (한국산학기술학회)
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A Novel Method of Determining Parameters for Contrast Limited Adaptive Histogram Equalization

대비제한 적응 히스토그램 평활화에서 매개변수 결정방법

  • Min, Byong-Seok (Department of Digital Electronic Communications, Chungcheong University) ;
  • Cho, Tae-Kyung (Department of Information & Telecommunications Engineering, Sangmyung University)
  • 민병석 (충청대학교 디지털전자통신과) ;
  • 조태경 (상명대학교 정보통신공학과)
  • Received : 2012.10.19
  • Accepted : 2013.03.07
  • Published : 2013.03.31
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Abstract

Histogram equalization, which stretches the dynamic range of intensity, is the most common method for enhancing the contrast of image. Contrast limited adaptive histogram equalization(CLAHE), proposed by K. Zuierveld, has two key parameters: block size and clip limit. These parameters mainly control image quality, but have been heuristically determined by user. In this paper, we propose a novel method of determining two parameters of CLAHE using entropy of image. The key idea is based on the characteristics of entropy curves: clip limit vs entropy and block size vs entropy. Clip limit and block size are determined at the point with maximum curvature on entropy curve. Experimental results show that the proposed method improves images with very low contrast.

히스토그램 평활화는 영상의 밝기 분포를 변화시킴으로써 화질을 향상시키는 방법으로 다양한 분야에서 응용되고 있다. 전역적인 방법은 영상 밝기의 전체적인 분포를 균등 분포로 변환함으로써 영상의 밝기가 과도하게 변하는 단점을 갖고 있다. 이를 해결하기 위한 방법으로 K. Zuierveld가 제안한 대비 제한 적응 히스토그램 평활화(CLAHE)가 실용적으로 널리 사용되고 있다. 이 방법에서는 블록단위의 처리를 위한 블록 크기와 대비 제한을 위한 매개변수 등 두 개의 매개변수가 히스토그램의 평활화 성능을 결정하는데, 이것들을 결정하는 구체적인 알고리듬은 없으며 실험적으로 시행착오학습 통해 결정한다. 본 논문에서는 영상의 엔트로피에 기반해서 CLAHE의 매개변수인 블록 크기와 대비제한 매개변수를 결정하는 새로운 방법을 제안한다. 제안한 방법은 CLAHE를 자동화할 수 있으며, 전체적으로 어두운 영상이나 밝은 영상에 적용한 결과 전역적인 방법에 비해 주관적 화질 개선의 효과를 나타내었다.

Keywords

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