Biomedical subjects

R G Erens

Publications and source records attributed to R G Erens.

4 recordsLinked to original sources

PubMed1995

Shape from shaded random surfaces.

The perception of surface relief from random shading patterns is measured by having observers adjust three-dimensional local probes, the projections of which are superimposed on the image. Three observers perform four settings of 91 probes on each of 14 images. These images are generated by calculating the Lambertian reflectance of a random superposition of elliptical Gaussian hills and valleys illuminated by a single distant light source as well as by ambient light. Neither the surface reflectance equation nor the light source direction is conveyed to our observers in any way. Mathematically, this "pure" shape-from-shading problem has highly non-unique solutions. Perception of a well-defined, stable shape therefore implies that the ambiguity is resolved, i.e. a gauge is fixed. We analyse the surface ambiguity or gauge freedom which is left unconstrained by pure shading information and we investigate possible ways of restricting it. Statistical analysis of the curl component of the field of probe settings reveals that the settings are significantly consistent with an underlying perceived surface. In spite of the large theoretical ambiguity in the stimuli, the settings are reproducible and show considerable inter-observer agreement. Even the correlation of the settings with the real surfaces is surprisingly large. If the settings are compared to the real surface normals, one finds a series of biases, the strongest of which is that the global surface slant is systematically underestimated, even in those cases where ending occluding contours or high-contrast luminance ridges, indicative of "almost" contours, are present in the image. Another bias then is that the corresponding rims on the surface are seen as roughly parallel to the picture plane.

Cues↗

PubMed1993

Estimating the gradient direction of a luminance ramp.

We report on the extent to which human observers are able to indicate the gradient direction of a luminance ramp. In our experiment modulation depth ranged from 1 to 64% and field sizes subtended 0.5 to 47.5 degrees of visual angle. Observers are not able to indicate the gradient direction for modulation depths below 6%. For values above this threshold, the angular standard deviation in the responses decreases proportionally with the logarithm of the modulation depth and is about 11 degrees for a modulation depth of 64%. The angular standard deviations for supra-threshold contrast are slightly increased for the field sizes of 0.5 and 47.5 degrees but are constant over a range of field sizes of 1 to 25 degrees. Thus, size invariance holds well for this range of field sizes.

Contrast Sensitivity↗

PubMed1993

Estimating local shape from shading in the presence of global shading.

In theory, global shading may help with the estimation of local surface structure from shading (e.g., in specifying the illuminant direction). Empirically, we do not know whether human observers combine the information given by the local and global shading to estimate local shape. Observers had to indicate the orientation of a local elongated perturbation with or without global shading information provided by a background surface. Our psychophysical results show the following: 1. Observers do not estimate the orientation of the local perturbation more accurately with global shading information than they do in the absence of such information. 2. Responses depend dramatically on the inclination between the illuminant direction and the viewing direction. For an inclination of 20 degrees, observers indicate more or less the orientation of the local ridge; however, for an inclination of 40 degrees, they indicate either the direction of the illuminant or an orientation close to the shadow edge of the perturbation. Most subjects show some combination of these behaviors. This behavior is not altered by global shading information. We conclude that in our paradigm, global shading information does not aid the estimation of local shape.

Female↗

PubMed1993

Perception of local shape from shading.

Theoretically, metric solid shape is not determined uniquely by shading. Consequently, human vision has difficulty in categorizing shape when shading is the only cue. In the present research, subjects were required to categorize shaded quadric surfaces. We found that they were rather poor at this task; they confused hyperbolic and elliptic (both convex and concave) shapes easily. When a cast shadow visually indicated the direction of the illuminant, they were able to notice the concavity or convexity of elliptic shapes. However, they still confused elliptic and hyperbolic ones. Finally, when an animated sequence of eight intensity patterns belonging to one quadric shape had been displayed, the subjects were able to categorize the quadrics. However, the results are still quite moderate. Our experiments indicate that local shading structure is only a weak shape cue when presented in the absence of other visual cues.

Algorithms↗