Structural Fidelity vs. Naturalness - Objective Assessment of Tone Mapped Images

There has been an increasing number of tone mapping algorithms developed in recent years that can convert high dynamic range (HDR) to low dynamic range (LDR) images, so that they can be visualized on standard displays. Nevertheless, good quality evaluation criteria of tone mapped images are still lacking, without which, different tone mapping algorithms cannot be compared and there is no meaningful direction for improvement. Although subjective assessment methods provide useful references, they are expensive and time-consuming, and are difficult to be embedded into optimization frameworks. In this paper, we propose a novel objective assessment method that combines a multiscale signal fidelity measure inspired by the structural similarity (SSIM) index and a naturalness measure based on statistics on the brightness of natural images. Validations using available subjective data show good correlations between the proposed measure and subjective rankings of LDR images created by existing tone mapping operators.