In this study, the authors investigated how noise affects sharpness perception. The authors probed the sharpness of black-and-white tree bark images with various noise levels. Overall, the sharpness decreased as the noise amount increased, while some observers seemed to perceive more sharpness. The authors next used one- and two-dimensional unifrequency patterns as stimuli in an attempt to reduce such variability in the judgment. The result showed that sharpness of higher-frequency stimuli decreased with increased noise, while that of lower-frequency stimuli increased at certain levels. From this result, the authors thought that noise reduces the sharpness at edges, but can sharpen the lower-frequency component or texture of the image. To prove this prediction, the authors experimented again with the image used in the first experiment. The perceived sharpness only decreased when noise was added to the edge regions, whereas it improved when noise was applied to texture. The authors consider that the interaction between noise and texture increases the perception of image sharpness.
Takehito Kurihara, Naokazu Aoki, Hiroyuki Kobayashi, "Analysis of Sharpness Increase by Image Noise" in Journal of Imaging Science and Technology, 2011, pp 30504-1 - 30504-7, https://doi.org/10.2352/J.ImagingSci.Technol.2011.55.3.030504