Color Noise Analysis

Graininess is one of the important image quality metrics in the photographic quality. In this paper, we will report an investigation of our evaluation model for color graininess. In this evaluation model, overall image noise is predicted as a graininess index (GI) to the average lightness for each patch image, and the GI is calculated from two image noise metrics, the lightness noise (LN) and the chromatic noise (CN). These are estimated respectively from fluctuation of the brightness component and of the chromatic component in the image. Concerning this evaluation model, we made a subjective evaluation experiment in order to find the optimum weighting factor for CN. The correlation coefficient between the subjective evaluated levels and the optimized GI improved greatly compared with the case where CN is not considered, or the weighting factor for CN is zero. Using this optimized GI, we evaluated the color graininess for several hardcopy images with different printing types, like silver halide, electrophotography, ink jet, and so on, and we studied the perception of noise on color images.