An investigation of perceived sharpness and sharpness metrics

Sharpness is an important attribute that contributes to the overall impression of image quality. As digital photography becomes more and more popular, digital photo enhancement has been a topic of great interest. In this paper, we investigate two issues related to digital photo sharpness. 1) How do we quantitatively measure the sharpness of a digital image? 2) What is the preferred sharpness of a digital image, and what is the relation between preferred sharpness and sharpness detection threshold? Both issues are of practical use to the digital photography market. First, we present the design and properties of three sharpness metrics to answer the first question. Next, we describe psychophysical experiments to investigate the second question. It is found that 1) the sharpness metric Digital Sharpness Scale (DSS) and Average Edge Transition Slope (AETS) are highly correlated to the perceived sharpness; 2) Both DSS and AETS predict sharpness equality with acceptable error; 3) the sharpness detection threshold is relatively consistent across subjects and across image contents, compared with the sharpness preference; 4) the average level of preferred sharpness is consistently higher than the detection threshold across image contents and across subjects, which implies that observers in general prefer a sharpened image to the original image; and 5) the preferred level of sharpness has a strong dependency on image content.