Luminance and Colour Cues in Perceptual Transparency

A series of experiments was carried out to determine the dependence of the perception of transparency on colour relationships in a four-region pattern comprising two non-overlapping regions, an overlapping region, and a background. The proportion of trials where the pattern was perceived as transparent, and the relative layering in depth of the two perceived surfaces, were determined as a function of the luminance of colour of the overlapping region while those of the non-overlapping regions were kept constant. It was found that perceptual transparency could arise from displays where the four regions were isoluminant but different in colour. The function obtained for isoluminant colour-varying patterns was quite similar to that obtained for isochromatic luminance-varying patterns. In both types of patterns, the background condition strongly affected perceptual transparency and perceived depth order in a similar way. These results suggest that perceptual transparency for both the isochromatic and the isoluminant pattern was specified in a common manner by contrast relations in a certain region of the image, and was not always constrained by the physics of transparency. The dependence of perceptual transparency on the contrast relation may be accounted for by mechanisms incorporating the filling-in process associated with antagonistic luminance and opponent-colour responses.