  Objective image quality metrics try to estimate the perceptual quality of the
given image by considering the characteristics of the human visual system.
However, it is possible that the metrics produce different quality scores even
for two images that are perceptually indistinguishable by human viewers, which
have not been considered in the existing studies related to objective quality
assessment. In this paper, we address the issue of ambiguity of objective image
quality assessment. We propose an approach to obtain an ambiguity interval of
an objective metric, within which the quality score difference is not
perceptually significant. In particular, we use the visual difference
predictor, which can consider viewing conditions that are important for visual
quality perception. In order to demonstrate the usefulness of the proposed
approach, we conduct experiments with 33 state-of-the-art image quality metrics
in the viewpoint of their accuracy and ambiguity for three image quality
databases. The results show that the ambiguity intervals can be applied as an
additional figure of merit when conventional performance measurement does not
determine superiority between the metrics. The effect of the viewing distance
on the ambiguity interval is also shown.
