Optical EngineeringPerception and interpretation of high-pass filtered images
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Digital high-pass filtering is used frequently to enhance details in scientific, industrial, and military images. High-pass filtered (HPF) images also are used both to illustrate and test models of visual perception. The visual system appears to interpret HPF images in the context of a multiplicative model of high-frequency reflectance and low-frequency illumination whenever possible. HPF images can be treated as a form of two-dimensional amplitude modulation signals. The low-frequency information, which is coded in the modulation envelope, disappears with the carrier if low-pass filtered. The envelope may be retrieved (demodulated) using one of many possible nonlinear operations followed by a low-pass filter. The compressive nonlinearity ofthe visual system is shown to suffice for demodulating such images. Simulations show that HPF images cannot be used to reject the hypothesis that illusions and grouping phenomena are due to low-frequency channels.