Although models of transparency have focused on articulating the conditions that lead to the perception of transparency, there is a related problem that is not usually explicitly addressed; namely, specifying how the visual system determines whether a scene (or a region of a scene) is in plain view. There is a growing body of data demonstrating that the visual system uses contrast relationships to compute both the presence of transparent surfaces as well as their quantitative properties (such as transmittance).
2 But how does the visual system determine that a given image region is actually a portion of a scene in plain view rather than a higher contrast region viewed through a partially transmissive medium? Anderson (
1999,
2003a) proposed that the visual system employs a
transmittance anchoring principle (TAP) to determine when a transparent surface is present. Intuitively, the TAP states that the visual system treats the highest contrast image regions as regions in plain view and only infers the presence of transparent surfaces if there are spatial or spatio-temporal (Anderson, Singh, & Meng,
2006) perturbations in the contrast magnitude along contours, surfaces, or textures. More specifically, Anderson et al. (
2006) showed that the visual system uses the region of highest contrast as a
transmittance anchor, i.e., as the normalization factor used to compute the transmittance of transparent surfaces in lower contrast regions of the image. Note that the TAP only applies to partially occluded contours since contrast variations along an occluding contour are consistent with surfaces in plain view behind an occluding edge. Thus, there are a number of aspects of the contrast variations in
Figure 1 that contribute to the percepts of transparency and shifts in perceived lightness of the circular targets. First, the geometric continuity of the diagonal and circular contours defines the figural unity of both the transparent layer and the discs. These geometric properties are identical in the two halves of
Figure 1, so the difference in the perceived lightness, transparency, and depth in these images must be due to the photometric differences in the displays. The preservation of contrast polarity along the continuous contours in this figure is consistent with either contour arising from a transparent surface or illumination change. However, the strong reduction in contrast magnitude of the circular contour provides information that these contours are obscured by a light (left image) or dark (right image) transparent surface. Finally, the TAP states that the highest contrast portions of the contour segments are perceived in plain view. This corresponds to contours belonging to the dark portions of the circles in the left image, and the light portions of the circles in the right image, giving rise to the striking difference in perceived lightness of the discs in the two images. This shift in perceived lightness arises from a reversal in the portions of the discs that are seen in plain view. In the limiting case, this principle reduces to conditions of occlusion, and portions of the circular figures are no longer visible (see
Figure 2).