Measuring perceptual contrast in uncontrolled environments

Contrast is one of the most relevant perceptual and quality factors in digital images and measuring it is not a trivial task. We have carried out an online psychophysical experiment to register perceived contrast. The results from the observers indicate that color images are rated higher than their respective greyscale one indicating that contrast is influenced color. A two-sided sign test at 5% level confirms this hypothesis. A comparison with a lab controlled environment experiment has been carried out in order to investigate possible differences. A statistical analysis of the two experiments indicate that the mean ratings of the observers are not significantly different. A decrease in correlation of previously developed contrast measures can be noticed and a comparison of the correlation coefficients indicate that measuring contrast in uncontrolled environments can be significantly different than measuring in a lab controlled environment.