Andrea J. van Doorn

Perturbation Study of Shading in Pictures

Pictorial relief was measured for a series of pictures of a smooth solid object. The scene was geometrically identical for all pictures, but the rendering was different. Whereas all pictures were monochrome full-scale photographs, they were taken under different illuminations of the scene, the source being frontal and displaced towards either the upper left, the upper right, the lower right, or the lower left. It was found that different illuminations led to significantly different, systematic alterations of pictorial relief. It is concluded that though shape constancy under changes in illumination might be said to rule in the first rough approximation, the deviations from true constancy are indeed both significant and systematic. Different from either stimulus-reduction or cue-conflict paradigms, this ‘perturbation analysis’ shows that shading is used as an important source of information even if the particular illumination appears to be ignored at first blush. For all subjects, brighter parts in the stimulus were consistently interpreted as being nearer in pictorial space, both for the global layout and for the subsidiary relief.

Noise Limited Performance Of The Visual System.

We present results of recent experiments concerning the processing of signals in the visual system under conditions limited by both natural (Poissonean) and artificial white noise. The results for natural noise limited conditions show that at any location in the visual field sampling apertures of different size are simultane-ously present. The dominant aperture size decreases with increasing irradiance. In the temporal domain it appeared that presentation of the signals over a longer period than 1 sec. did not improve the performance anymore. In the second part results of experiments with static and moving artificial noise are presented. It appears that the visual system is capable of processing almost all of the information in the image in order to set its threshold limited by noise. Detection of coherent movement of details within dynamic white noise can be performed with a signal to noise ratio of 1%. Models which describe these phenomena are discussed and their possible impact on image processing is suggested.