Rank learning on training set selection and image quality assessment
Machine learning (ML) techniques are widely used in recent no-reference visual quality assessment (NR-VQA) metrics by training on subjective image quality databases. In these metrics, the optimization function is constructed based on L2 norm of the distance between subjective image quality and predicted image quality. There are two problems in these L2 norm based methods: (1) human's opinion on subjective image quality rating is not reliable at fine-scale level. A small difference between subjective image qualities represented by mean opinion scores (MOSs) of two images may not truly reflect the real quality difference between these two images, but acts as noise. The optimization process should avoid such noise. (2) Generally, human's opinion on pairwise comparison (PC) for image quality is more reliable and believable than MOS. The importance of PC is ignored during the optimization process of existing ML-based studies, which are designed based on the numerical rating system. In this paper, we introduce image quality ranking concept to establish a new optimization objective instead of L2 norm optimization, and then a novel NR-VQA is constructed based on ranking learning. The proposed metric firstly suggests a reasonable training set for ML, which is ignored by existing ML-based NR-VQA. The ranking theory is adopted to build optimization function, which reflects the properties of PC over the numerical ranting system used by traditional NR-VQA. By ignoring the small difference between MOSs from two images during the optimization process, the proposed ranking-based NR-VQA can also well address the first problem from the existing related metrics. Experimental results show that the proposed ranking-based NR-VQA can obtain better performance over the state-of-the-art NR-VQA approaches.