基於臨界可視色彩差異量之彩色影像壓縮研究

人類的眼睛是彩色影像最終的接收器，然而，人類視覺系統(Human Visual System; HVS)僅具備有限的靈敏度去分辨差異微小的彩色信號。因此，對於人類視覺來說，彩色影像中一定存在著相當程度的視覺多餘量(perceptual redundancy)。精準地估測視覺多餘量並有效地利用它，將有助於改善彩色影像處理上資料壓縮及其他各種應用的效能。在本論文中，我們將提出一個視覺模型，它可以用來估測彩色影像在任何色彩空間中存在於每一個像素的視覺多餘量。不論是空間領域或頻率領域，所提出的視覺模型以視覺敏感度臨界值的形式估測每一個彩色像素在任何色彩空間的視覺多餘量。為了驗證本論文所提出的視覺模型，我們利用該模型估測所得之視覺多餘量去改良兩個目前廣泛使用的標準影像編碼器，並透過模擬去驗證它們的編碼效率是否可以獲得改善。在空間領域中，我們以JPEG-LS編碼器之接近無損壓縮模式做為改善標的，其方式是使編碼過程中產生的編碼誤差成為視覺多餘量的一部分，並在RGB色彩空間去壓縮彩色影像。在頻率領域中，我們以JPEG2000編碼器做為改善標的，其方式是使位元率控制過程中所產生的視覺可視失真最小化，並在YCbCr色彩空間去壓縮彩色影像。模擬結果顯示，兩個標準影像編碼器在經過視覺化調校後，其效能皆優於未經調校過之標準影像編碼器，並以較低之位元率獲得相同的重建影像視覺品質。在本論文之後半段，我們提出了一個彩色影像視覺壓縮器之設計方式，同時，將視覺模型估測所得之視覺多餘量整合其中；除了影像壓縮上的應用外，在論文的最後更進一步地將視覺多餘量運用於數位浮水印技術。實驗結果皆證實我們所提出的視覺模型皆有助於彩色影像處理在不同應用領域上的效能。

關鍵字

人類視覺系統；視覺多餘量；視覺性壓縮

並列摘要

Human eyes are ultimate receivers of color images, while the human visual system (HVS) has limited sensitivity in discriminating color signals of small differences. To the human vision, there must exist in color images a certain amount of perceptual redundancy. The accurate measurement and effective exploitation of this perceptual redundancy will help to improve the efficiency in processing color images for data compression and many other applications. This thesis presents a visual model for measuring the perceptual redundancy inherent in each pixel of the color image in any color space. The model estimates the perceptual redundancy for each color pixel as a visibility threshold of color difference in any color space and in spatial or frequency domain. To justify the proposed visual model, two existing image coders are modified to take advantage of the perceptual redundancy and simulated to inspect if their coding efficiency is improved. In the spatial domain, the JPEG-LS coder in the near-lossless compression mode is modified to make coding errors part of the perceptual redundancy in compressing color images in the RGB space. In the wavelet domain, the JPEG2000 coder is refined by minimizing the perceptible distortion involved in the rate control of the compressed image in the YCbCr space. Simulation results show that, in both cases, the performance of the perceptually tuned coder is superior to that of the un-tuned coder in terms of the bit rate required for achieving the same visual quality. Furthermore, the perceptual redundancy is integrated into the design of a proposed perceptual compression scheme and the application of the digital watermarking technique for color images. Experimental results demonstrate that the proposed color visual model will greatly help to increase the performance of various applications on color image processing.

並列關鍵字

perceptual redundancy ； perceptual compression ； human visual system

參考文獻

[1] N. Jayant, “Signal compression: technology targets and research directions,” IEEE J. Select. Areas Commun., vol. 10, pp. 314-323, June 1992.

[2] N. Jayant, J. Johnston, and R. Safranek, “Signal compression based on models of human perception,” Proc. IEEE, vol. 81, pp. 1385-1422, Oct. 1993.

[3] J. Mannos and D. Sakrison, ”The effects of a visual fidelity criterion of the encoding of images,” IEEE Trans. Inform. Theory, vol. 20, pp. 525-536, Jul. 1974.

[4] D. L. McLaren and D. T. Nguyen, “Removal of subjective redundancy from DCT-coded images,” IEE Proc.- I, vol. 138, pp. 345-350, Oct. 1991.

[5] B. Tao, “On adaptive quantization and rate control for MPEG video coding environment,” Sarnoff Corporation, Princeton, NJ, Tech. Rep., Aug. 1996.

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基於臨界可視色彩差異量之彩色影像壓縮研究

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