Color image fidelity metrics evaluated using image distortion maps

Several color image fidelity metrics are evaluated by comparing the metric predictions to empirical measurements. Subjects examined image pairs consisting of an original and a reproduction. They marked locations on the reproduction that differed detectably from the original. We refer to the distribution of error marks by the subjects as image distortion maps. The empirically obtained image distortion maps are compared to the predicited visible difference calculated using (1) the widely used root mean square error (point-by-point RMS) computed in uncalibrated RGB values, (2) the point-by-point CIELAB Delta E-94 values (CIE, 1994) and (3) S-CIELAB Delta E-94, a spatial extension of CIELAB Delta E metric. The uncalibrated RMS metric did not predict the perceptual image distortion data well. The point-by-point CIELAB Delta E-94 metric provided better predictions, and the S-CIELAB metric, which incorporated the spatial color sensitivity of the eye, gave the most accurate predictions. None of the metrics provided an excellent fit to the data. Image areas with poor predictions were concentrated in regions containing large negative local contrast. When these areas were excluded from our data analysis, both S-CIELAB and CIELAB predictions had much better agreement with the perceptual data. This suggests that the next step in improving color image fidelity metrics is to redefine color difference formula such as CIELAB Delta E-94 in terms of local contrast. (C) 1998 Elsevier Science B.V. All rights reserved.