Image quality assessment plays an important role in image processing applications. In many image applications, e.g., image denoising, deblurring, and fusion, a reference image is rarely available for comparison with the enhanced image. Thus, the quality of enhanced images must be evaluated blindly without references. In recent years, many no-reference image quality metrics (IQMs) have been proposed for assessing digital image quality. In this paper, we first review 21 commonly employed no-reference IQMs. Second, we apply these measures to Quickbird images with three different types of general content (urban, rural, and harbor) subjected to three types of degradation (average filtering, Gaussian white noise, and linear motion degradation), each with 40 degradation levels. We evaluate the robustness of the IQMs based on the criteria of prediction accuracy, prediction monotonicity, and prediction consistency. Then, we perform factor analysis on those IQMs deemed robust, and cluster them into several components. We then select the IQM with the highest loading coefficient as the representative IQM for that component. Experimental results suggest that different measures perform differently for images with different contents and subjected to different types of degradation. Generally, the degradation method has a stronger effect than the image content on the evaluation results of an IQM. The same IQM can provide opposite dependences on the level of degradation for different degradation types, and an IQM that performed well with one type of degradation may not perform well with another type. The training-based measures are not appropriate for remote sensing images because the results are highly dependent on the samples employed for training. Only seven of the 21 IQMs were found to fulfill the requirements of robustness. Edge intensity (EI) and just noticeable distortion (JND) are suggested for evaluating the quality of images subjected to average filter degradation. EI, blind image quality assessment through anisotropy (BIQAA), and mean metric (MM) are suggested for evaluating the quality of images subjected to Gaussian white noise degradation. Laplacian derivative (LD), JND, and standard deviation (SD) are suggested for evaluating the quality of images subjected to linear motion. Finally, EI is suggested for evaluating the quality of an image subjected to an unknown type of degradation.
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