Sharpening image motion based on the spatio-temporal characteristics of human vision

Images of moving objects in films often appear normal or even sharper than they actually are, a phenomenon called motion sharpening. We sought to clarify which spatio-temporal frequency components of a moving image are sharpened when a pattern is moving. We applied various spatio-temporal filters to moving natural images and evaluated the perceived sharpness of motion by comparing them to a stationary image. On each trial, subjects adjusted three parameters of the still image: overall luminance contrast, the slope of the amplitude function in the spatial frequency domain, and cutoff spatial frequency. We found that the motion sharpening could be described by the relative increase in the amplitude of the higher spatial frequency components. Spatially low-pass filtered movies induced a motion sharpening, but spatially high-pass filtered movies were perceived to be blurred. The strongest motion sharpening was observed when image frames were spatially band-reject filtered. When temporal filters were applied, perceived sharpness became stronger when the movies were temporally band-reject filtered. A high-pass temporal filter drastically reduced the perceived sharpness of images. Our results demonstrate that the perceived contrast of higher spatial frequency components in moving images is enhanced by the interaction between different spatio-temporal frequency channels in the motion sharpening process. This suggests that it is possible to compress and enhance moving images by removing higher spatio-temporal frequency information.