改良式直方圖等化應用於即時影像增強之低成本FPGA設計

Low Cost FPGA Circuit Design of Modified Histogram Equalization

羅一中(Yi-Chung Lo)

指導教授 ： 郭天穎

國立臺北科技大學/機電學院/電機工程系所/碩士(2005年)

https://doi.org/10.6841/NTUT.2005.00258

摘要

一般而言，動態影像對比增強的硬體實現技術皆需藉助圖框緩衝記憶體來儲存處理中的影像，然而硬體上使用圖框緩衝記憶體是要付出代價的，所以本論文提出了一個改良式直方圖等化的演算法，配合參照前一圖框的轉換函數，可以不用圖框緩衝記憶體即可完成動態影像的對比增強。
直方圖等化可以針對不同的輸入影像，累積其影像直方圖的分佈函數，自動地產生不同的轉換函數，因此被廣泛的應用在影像對比增強。但是當直方圖的分佈函數中某一個值特別大的時候，則會使產生的轉換函數斜率過大，換句話說就是使影像過度增強，這會使輸出影像太亮，清晰度反而下降。我們提出的改良式直方圖等化演算法利用直方圖非零統計量的平均值與標準差將這個特大的值過濾出來，這樣可以避免產生的轉換函數斜率過大，之後再利用一階移動平均濾波器將轉換數列中兩兩相鄰的數值加以平均，使得最後的轉換函數不僅可以過度的避免影像對比強化，也可增加影像的動態範圍，並且灰階變化平緩的區域不會因為增強而產生方塊效應。本研究中我們並將改良式直方圖等化演算法成功地實現在FPGA上，以證明其即時增強動態影像的能力。

關鍵字

即時影像處理 ； 直方圖等化 ； 影像增強 ； 現場可程式邏輯陣列

並列摘要

For real-time moving picture contrast enhancement, the existing methods usually require one or more frame buffers to store the intermediate output, which is expensive to implement on a practical hardware system. Thus, this thesis proposes a modified histogram equalization (MHE) algorithm combined with a backward frame translation table to exclude any need of frame buffers.
Conventional histogram equalization is widely adopted in contrast enhancement because of its automation in generating the transformation curve. It transforms the image based on the cumulative distribution function of histogram. However, there is a caveat to over-enhance the contrast when the auto-generated transform curve is too sharp due to some high peaks in the histogram. In this case, conventional histogram equalization may result in a harsh, noisy appearance of the output image. Our proposed MHE algorithm employs the mean and standard variation to pickup the high peaks in the histogram and modify histogram before making the translation table. Then the translation table was applied by a low pass filter to smooth on the contiguous values. This algorithm does not only avoid over-enhancement and increase dynamic range of the grey level, but also translates low spatial frequency area smoothly. The proposed algorithm was successfully implemented in an FPGA platform to demonstrate its effectiveness.

並列關鍵字

Histogram Equalization ； Contrast Enhancement ； Real-time Image Processing ； Image Enhancement ； FPGA

參考文獻

[1] R. C. Gonzalez and R. E. Woods, ”Digital Image Processing,” second edition, Prentice-Hall 2002.

[2] J. B. Zimmerman, S. M. Pizer, E. V. Staab, J. R. Perry, W. McCartney and B. C. Brenton, “An evaluation of the effectiveness of adaptive histogram equalization for contrast enhancement,” IEEE Trans. Medical Imaging, Vol. 7, Issue 4, pp. 304 - 312, Dec. 1988.

[3] T.-L. Ji, M. K. Sundareshan, H. Roehrig, ”Adaptive image contrast enhancement based on human visual properties” IEEE Trans. Medical Imaging, Vol. 13, Issue 4, pp. 573 - 586, Dec. 1994.

[4] K. K. Tae, K. P. Joon and S. K. Bong, “Contrast enhancement system using spatially adaptive histogram equalization with temporal filtering,” IEEE Trans. Consumer Electronics, Vol. 44, Issue 1, pp. 82 – 87, Feb. 1998.

[5] R. E. Walpole, R. H. Myers, S. L. Myers, and K. Ye, “Probability & Statistics for Engineers & Scientists,” seventh edition, Prentice-Hall 2002.