Histogram specification of 24-bit color images in the color difference (C-Y) color space

Histogram equalization and specification have been widely used to enhance information in a gray scale image, with histogram specification having the advantage of allowing the output histogram to be specified as compared to histogram equalization, which attempts to produce an output histogram that is uniform. Unfortunately, expanding histogram techniques to color images is not very straightforward. Since humans are sensitive to chromatic changes, care must be taken to ensure that incorrect colors are not produced. Additionally, expanding the one-dimensional histogram used in gray level histogram techniques to a joint histogram (usually of three variables representing the primary colors of red, green, and blue) can yield specified histograms which have no physical meaning, hence making it difficult to determine the set of histograms required for a desired enhancement. In this paper, we describe a method of extending gray level histogram specification to color images by performing histogram specification on the luminance, saturation, and hue components in the color difference C-Y color space. These methods take into account the relationship between the luminance and saturation components while yielding specified histograms that produce natural-looking results.