This is Rock's unique flair for experimental design, approaching a problem from several directions until his conclusion seems inevitable. The subjects were not aware of these calculations of perceptual geometry, but only of the results of the calculations. Perception seems direct and immediate, but Rock showed with this and a host of other experiments that complex processing of information, requiring several steps, often precedes the conscious percept. Similar principles govern other aspects of perception. Perceived lightness of a surface depends not only on the intensity of light entering the eye from that surface, but also on the perceived orientation of the surface, and on the other surfaces that are visible.
Perceived size depends on perceived distance and visual context as well as retinal size.
Observers are conscious only of the lightness or the size, not of their dependence on prior calculations of other variables. The component processes are unconscious because they would only get in the way of a faithful perception of the properties of the world. By indirect perception Rock means that processing must occur before we can recognize objects, surfaces, people. He contrasts his view with the direct perception view of J. J. Gibson, for whom invariants are detected by some sort of resonating process. But Gibson's followers sometimes exaggerate his stance.