Perceptual Evaluation of Tone Mapping Operators

This thesis focuses on tone mapping operators and the perceptual differences between them when the tone mapped images are compared with their corresponding real-world views. To achieve this goal, two tone reproductions were implemented within the scope of this thesis. High dynamic range (HDR) images were acquired using the camera response curve recovery function of Robertson et al. for the perceptual experiment. After the perceptual experiment, multivariate statistical methods were used to analyze the set of data from the psychophysical experiment. Two tone reproductions, the Ashikhmin method and fast bilateral filtering introduced by Durand and Dorsey were implemented. Both of them are classified as local tone mapping techniques. Ashikhmin's method deals with the local adaptation level and local contrast separately. The bilateral filtering method is based upon the work of Tomasi and Manduchi and sped up by piece-wise linear and subsampling strategies. This algorithm divides an image into two layers (a base layer and a detail layer) in order to reduce contrast but preserve details. In order to create HDR images, the camera response curve was recovered by using the method of Robertson et al. Each HDR image was constructed from 15 images with different exposures and saved in the Radiance RGBE format. A psychophysical experiment was held with 14 human observers over seven tone mapped images on two scenes. The tone mapping operators are the followings: the linear tone mapping, the fast bilateral filtering by Durand and Dorsey, Pattanaik et al.'s method, the Ashikhmin method, the Ward method, the photographic tone reproduction of Reinhard et al., and the adaptive logarithmic mapping of Drago et al. In the psychophysical experiment, subjects were asked to compare each of the images to its corresponding real-world view and rate its overall brightness, contrast, detail reproduction in dark and bright regions, and naturalness. The set of data was analyzed by using multivariate statistical analyses for the main effect of the tone mapping operators. The result shows that those tone mapping operators are perceived very differently when compared with the corresponding real-world views. The multivariate analysis of variance (MANOVA) shows that means of the set of data are in a three-dimensional space but neither along a line nor on a plane. An interesting result shown by this experiment is that those operators are divided into global and local categories by Mahalanobis distances. The main effects of the tone mapping operators to each of the attributes were quite significant. Overall brightness provides the biggest difference among the tone mapping operators. The linear tone mapping operator was perceived with the highest brightness and Pattanaik, Ward, and Drago also had a higher amount of overall brightness than the others. Additionally, the result shows that although the main effect of the tone mapping operators for the details in bright regions is highly significant, for the details in dark regions is not. Correlations of all of possible combination of the attributes were tested. Regarding the naturalness, this research shows that none of the other attributes has a strong influence by itself. This may suggest that naturalness is influenced by a combination of the other attributes