Direct decorrelation stretch technique for RGB colour composition

This paper introduces a very simple method to achieve a saturation (decorrelation) stretch without coordinate transformation, the Direct Decorrelation Stretch (DDS) technique. In RGB (red, green and blue) colour coordinate space, a colour vector can be expressed in terms of an achromatic component and a chromatic (pure colour) component. A direct decorrelation stretch of a RGB colour image is achieved by decreasing the magnitude of the achromatic component of the colour vector for the pixel being processed. Initially, the R, G, and B digital numbers (DNs) for each colour pixel are examined and the lowest value (it could be the red, green or blue) is selected. A fixed (user controlled) proportion of the selected lowest DN is then subtracted from each of the original R, G and B DNs of this pixel. The result of the subtraction, based on RGB pixel minimum values, is an image with reduced achromacity. This corresponds to increased saturation. The DDS method consists of simple arithmetical operations within individual bands and does not use coordinate transformations. Mathematical analysis and experimental image processing show that DDS performs a decorrelation stretch which resembles, in principle, that based on a Hue, Saturation and Intensity (HSI) transformation. DDS is a simpler and more computationally efficient approach to decorrelation stretching.