Illustration of band-pass filtering and spatial distortion. Distortions introduced into the lower left quadrant of (a,b) two typical natural images are easily noticed, but could not be detected in band-pass filtered versions of the distorted image (d,e) at any spatial frequency or distortion frequency tested. However, distortions that are introduced (upper left quadrant) after band-pass filtering (c,f) are easily detected from the presence of spatial frequency artifacts outside the pass-band of the band-pass filtered image.