Color image enhancement with exact HSI color model

While enhancing the intensity or saturation component for high-quality color image enhancement, keeping the hue component unchanged is important; thus, perceptual color models such as HSI, HSL and HSV were used. Hue-Saturation-Intensity (HSI) is a public color model, and many color applications are commonly based on this model; however, the transformation from HSI model to RGB model usually generates the out-of-gamut problem after modifying intensity and saturation in the HSI model. Moreover, the saturation component is always increased or decreased following the change of intensity component no matter what the attainable saturation range is. In this paper, we propose accurate formulas for the color transformation between RGB and the proposed HSI color model, called the exact HSI (eHSI) color model, to resolve the out-of-gamut problem directly as well as automatically adapt the saturation range; that is, the saturation component can be enhanced or reduced according to the attainable maximum saturation range. In experiments, we demonstrate how to sacrifice a little contrast to improve the image saturation based on the proposed eHSI color model.