Showing papers by "Eli Peli published in 1992"

Limitations of image enhancement for the visually impaired.

Abstract: Image enhancement as an aid for the visually impaired may improve visibility of TV programs and provide portable visual aid. This paper describes the current techniques for image enhancement and their underlying models. The limitations of the various techniques and of potential methods of implementation are high-lighted. Initial work in this area was based on a linear model. The finite dynamic range available in the video display and contamination of the enhanced image by high spatial frequency noise limited the model's usefulness. I propose a method to address some limitations of the original model that considers the nonlinear response of the visual system and requires enhancement of subthreshold spatial information only. This modification may increase the dynamic range available by decreasing the range previously used by the linear models to enhance visible details. However, for the modified technique to be most effective, the enhancement has to be continuously tuned, based on the patient's visual loss and the spatial frequency content of the displayed images. The implications of these limitations for the potential implementation in TV are discussed. Implementation of an image-enhancing visual aid in a head-mounted, binocular, full-field, virtual vision device may cause substantial difficulties. Patient adaptation may be difficult due to head movement and interaction of the vestibular system response with the head-mounted display. An alternate, bioptic design is proposed in which the display is positioned above or below the line of sight to be examined intermittently, possibly in a freeze-frame mode. Such implementation is also likely to be less expensive, enabling more users access to the device.

Display nonlinearity in digital image processing for visual communications

Abstract: The luminance emitted from a cathode ray rube (CRT) display is a nonlinear function (the gamma function) of the input video signal voltage. In most analog video systems, compensation for this nonlinear transfer function is implemented in the camera amplifiers. When CRT displays are used to present psychophysical stimuli in vision research, the specific display nonlinearity usually is measured and accounted for to ensure that the luminance of each pixel in the synthetic image properly represents the intended value. However, when using digital image processing, the linear analog-to-digital converters store a digital image that is nonlinearly related to the displayed or recorded image. The effect of this nonlinear transformation on a variety of image-processing applications used in visual communications is described.

Differences in tests of aniseikonia.

Abstract: The New Aniseikonia Test (NAT), a hand-held direct-comparison test using red/green anaglyphs, has several potential advantages as a screener. We compared the validity of the NAT to that of the Space Eikonometer in three experiments: (1) aniseikonia was induced by calibrated size lenses in a doubleblind study of 15 normal subjects; (2) habitual aniseikonia was measured with both instruments in four patients; and (3) eight of the normal subjects were retested with a computer-video simulation of the NAT. The NAT underestimated induced aniseikonia by a factor of 3 in the normal subjects and underestimated habitual aniseikonia in four patients. The Space Eikonometer correctly measured the magnitude of induced aniseikonia in the normal subjects. The simulation test did not show underestimation in the eight normal subjects. We could not attribute the NAT's underestimation of aniseikonia to the red/green anaglyph method, printing error, psychophysical method, or the direct-comparison test format. We speculate that the NAT induces a different sensory fusion response to aniseikonia than do the other tests, and that this altered sensory fusion response diminishes measured aniseikonia. We conclude that the NAT is not a valid measure of aniseikonia. Invest Ophthalmol Vis Sci 33:20632067,1992 More than one million cataract operations are performed annually in the United States 1 , with most patients receiving intraocular implants. Of these pseudophakic patients, 41% may have symptoms attributable to aniseikonia, 2 making this condition a

Perception and interpretation of high-pass filtered images

Abstract: 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.