Rendering 3-D High Dynamic Range Images: Subjective Evaluation of Tone-Mapping Methods and Preferred 3-D Image Attributes
High dynamic range (HDR) images provide superior picture quality by allowing a larger range of brightness levels to be captured and reproduced than traditional 8-bit low dynamic range (LDR) images. Even with existing 8-bit displays, picture quality can be significantly improved if content is first captured in HDR format, and then is tone-mapped to convert it from HDR to the LDR format. Tone mapping methods have been extensively studied for 2-D images. This paper addresses the problem of presenting stereoscopic tone-mapped HDR images on 3-D LDR displays and how it is different from the 2-D scenario. We first present a subjective psychophysical experiment that evaluates existing tone-mapping operators on 3-D HDR images. The results show that 3-D content derived using tone-mapping is much preferred to that captured directly with a pair of LDR cameras. Global (spatially invariant) and local (spatially variant) tone-mapping methods have similar 3-D effects. The second part of our study focuses on how the preferred level of brightness and the preferred amount of details differ between 3-D and 2-D images by conducting another set of subjective experiments. Our results show that while people selected slightly brighter images in 3-D viewing compared to 2-D, the difference is not statistically significant. However, compared to 2-D images, the subjects consistently preferred having a greater amount of details when watching 3-D. These results suggest that 3-D content should be prepared differently (sharper and possibly slightly brighter) from the same content intended for 2-D displaying, to achieve optimal appearance in each format. The complete database of the original HDR image pairs and their LDR counterparts are available online.