COLOR QUANTIZATION OF IMAGES

Many image display devices allow only a limited number of colors to be simultaneously displayed. Usually, this set of available colors, called a color palette, may be selected by a user from a wide variety of available colors. Such device restrictions make it particularly difficult to display natural color images since these images usually contain a wide range of colors which must then be quantized by a palette with limited size. This color quantization problem is considered in two parts: the selection of an optimal color palette and the optimal mapping of each pixel of the image to a color from the palette. This paper develops algorithms for the design of hierarchical tree structured color palettes incorporating performance criteria which reflect subjective evaluations of image quality. Tree structured color palettes greatly reduce the computational requirements of the palette design and pixel mapping tasks, while allowing colors to be properly allocated to densely populated areas of the color space. The algorithms produce higher quality displayed images and require less computations than previously proposed methods. Error diffusion techniques are commonly used for displaying images which have been quantized to very few levels. This paper studies problems related to the application of error diffusion techniques to the display of color images. A modified error diffusion technique is proposed for resolving these problems. The new error diffusion technique is shown to be easily implemented using the tree structured color palettes developed earlier.