The visual perception of 3-D shape from multiple cues: are observers capable of perceiving metric structure?

Three experiments are reported in which observers judged the three-dimensional (3-D) structures of virtual or real objects defined by various combinations of texture, motion, and binocular disparity under a wide variety of conditions. The tasks employed in these studies involved adjusting the depth of an object to match its width, adjusting the planes of a dihedral angle so that they appeared orthogonal, and adjusting the shape of an object so that it appeared to match another at a different viewing distance. The results obtained on all of these tasks revealed large constant errors and large individual differences among observers. There were also systematic failures of constancy over changes in viewing distance, orientation, or response task. When considered in conjunction with other, similar reports in the literature, these findings provide strong evidence that human observers do not have accurate perceptions of 3-D metric structure.