Department of Electrical & Computer Engineering Signal and Image Laboratory (SaIL) The University of Arizona®

Past Research

Effects of Coordinate Systems on Color Image Processing

Student: Christopher C. (Chuen-Chi) Yang

We investigated the effects of color coordinate systems on the processing of digital color images. This work involved several subprojects: luminance quantization error in histogram modification, techniques for bypassing color coordinate transformations, and saturation clipping in histogram modification.

Luminance quantization error in histogram modification

For color images, histogram modification is usually applied to the quantized luminance component. However, the luminance quantization error can be significantly magnified by the transformation function, leading to distortion in the processed image. The propagation of the quantization error was analyzed theoretically to determine the worst-case error. The relationship between the number of luminance quantization levels and the output quantization error was derived. Experimental results with histogram equalization demonstrated that the use of a higher-resolution histogram leads to reduced distortion as well as a "flatter" output histogram.

Efficient techniques for bypassing color coordinate transformations

In many applications of color image processing, only modification of the luminance component is desired. However, the commonly used coordinate systems, such as HSI, LHS, and YIQ, are not perceptually orthogonal; that is, luminance modification can cause perceptual shifts in the hue and saturation. We performed a theoretical analysis of this phenomenon. Efficient techniques were developed for bypassing the costly coordinate transformations when only the luminance or only the saturation is to be modified. Experimental results using histogram equalization supported the theoretical analysis.

Saturation clipping in histogram modification

In histogram modification of the luminance component of color images, the gamut limitation of the color coordinate system is an important issue. The processed luminance value of a pixel can fall outside the gamut of the color space. Traditionally, in such cases the luminance is clipped to keep the color vector within the gamut. But this procedure can reduce the effective contrast in the image. Instead, we have proposed clipping the saturation of the color vector. For most images, the change in saturation is not so perceptible as the change in luminance. Experimental results demonstrated that improved contrast was achieved by this approach.

Publications:

  1. Christopher C. Yang and Jeffrey J. Rodriguez, "Efficient Luminance and Saturation Processing Techniques for Color Images," J. of Visual Communication and Image Representation, vol. 8, no. 3, Sept. 1997, pp. 263-277. [ PDF ]

  2. Christopher C. Yang and Jeffrey J. Rodriguez, "Saturation Clipping in the LHS and YIQ Color Spaces," in Color Imaging: Device-Independent Color, Color Hard Copy and Graphic Arts, J. Bares, Ed. Proc. SPIE, vol. 2658, 1996, pp. 297-307. Presented at the IS\&T/SPIE Intl. Symp. on Electronic Imaging '96: Science \& Technology, San Jose, CA, Jan. 27 - Feb. 2, 1996. [ PDF ]

  3. Christopher C. Yang and Jeffrey J. Rodriguez, "Efficient Luminance and Saturation Processing Techniques for Bypassing Color Coordinate Transformations," in Proc. IEEE Intl. Conf. on Systems, Man and Cybernetics, vol. 1, Vancouver, Canada, Oct. 22-25, 1995, pp. 667-672. [ PDF ]

  4. Jeffrey J. Rodriguez and Christopher C. Yang, "High-Resolution Histogram Modification of Color Images," Graphical Models and Image Processing, vol. 57, no. 5, Sept. 1995, pp. 432-440. [ PDF ]

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