# A Perceptually Weighted Rank Correlation Indicator for Objective Image Quality Assessment

## TL;DR

This paper introduces a perceptually weighted rank correlation indicator for objective image quality assessment that emphasizes high-quality images and perceptually similar pairs, providing a more accurate evaluation of IQA algorithms.

## Contribution

It proposes a novel weighted rank correlation measure that accounts for perceptual importance and subjective uncertainty, improving the assessment of IQA metrics.

## Key findings

- Better correlation with human perception in IQA tasks
- Enhanced ability to rank high-quality images accurately
- Robustness across degraded and enhanced images

## Abstract

In the field of objective image quality assessment (IQA), the Spearman's $\rho$ and Kendall's $\tau$ are two most popular rank correlation indicators, which straightforwardly assign uniform weight to all quality levels and assume each pair of images are sortable. They are successful for measuring the average accuracy of an IQA metric in ranking multiple processed images. However, two important perceptual properties are ignored by them as well. Firstly, the sorting accuracy (SA) of high quality images are usually more important than the poor quality ones in many real world applications, where only the top-ranked images would be pushed to the users. Secondly, due to the subjective uncertainty in making judgement, two perceptually similar images are usually hardly sortable, whose ranks do not contribute to the evaluation of an IQA metric. To more accurately compare different IQA algorithms, we explore a perceptually weighted rank correlation indicator in this paper, which rewards the capability of correctly ranking high quality images, and suppresses the attention towards insensitive rank mistakes. More specifically, we focus on activating `valid' pairwise comparison towards image quality, whose difference exceeds a given sensory threshold (ST). Meanwhile, each image pair is assigned an unique weight, which is determined by both the quality level and rank deviation. By modifying the perception threshold, we can illustrate the sorting accuracy with a more sophisticated SA-ST curve, rather than a single rank correlation coefficient. The proposed indicator offers a new insight for interpreting visual perception behaviors. Furthermore, the applicability of our indicator is validated in recommending robust IQA metrics for both the degraded and enhanced image data.