The second reason we perceive the distortion of the photograph in the photograph is that we are not free to choose which surface will control the process of compensation: In this picture, there is a primary surface and a secondary surface (perhaps unlike the example in Figure 7.5, in which there may be two surfaces equally demanding of compensation). Presumably, there are more cues that tell us that the primary surface is a representation of a scene, such as perceptibility of surface texture and of a frame, than exist for the photograph represented in it.

		Fig.7.7 Plan of Ames distorted room.

Although we have made some progress in our inquiry into the robustness of perspective, we have yet to understand how the visual system identifies which angles in the picture represent right angles in the scene, which is (as we have seen earlier in this chapter) a precondition for locating the center of projection. Because the image of a right angle can run anywhere from 0° to 180°, drawings of right angles have no particular signature, and therefore they can be identified only by some more elaborate procedure. There are two views on the nature of this procedure. According to the first view, right angles are identified by first recognizing the objects in which they are embedded. For instance, with respect to Figure 5.1, such an approach would assume that the visual system first recognizes that the picture represents a building and then identifies the features likely to represent right angles. According to the second view, right angles are recognized by first recognizing rectangular corners (i.e., the concurrence of three lines at a point so that all the angles formed are right angles) in which they are embedded. This is possible because, as we will presently see, rectangular corners do have a signature.

The first view, the perception of right angles by an appeal to the semantics of the represented scene, is exemplified by the trapezoidal room created by Adalbert Ames, Jr. This is a room whose plan is shown in Figure 7.7, which looks like a rectangular room to those looking at it through the peephole. Here there is no dilemma. There is ambiguity, however: For an immobile viewer, the visible features of the room are compatible with many possible rooms, including the one the typical viewer reports seeing, which is rectangular, and illusory. But now bring two people into the room; they are at different distances from people into the room; they are at different distances from the observer looking through the peephole and so subtend different visual angles. Now we have a dilemma: If the people are seen equal in height, they must be at different distances, and because their backs are against the rear wall, the rear wall cannot be perpendicular to the side walls. On the other horn of the dilemma, if the room is still mistakenly seen as a normal rectangular room, then - so goes the unconscious inference - the people must be at equal distances from the observer; but because they subtend different visual angles, they must differ in height.

		Fig.7.8 Distorted room as seen by subject.

As may be seen in Figure 7.8, when the viewer is faced with a choice between seeing an oddly shaped room and seeing two adults differ dramatically in height, the latter is chosen. We choose to see grotesque differences in height rather than a distorted room possibly because sizes of human beings vary more in our experience than the angles of room corners. Such an explanation tacitly assumes that the viewer first unconsciously recognizes that the scene represents a room; and because a room implies right angles, the viewer then unconsciously resolves the dilemma of the Ames room by choosing rectangularity over equal heads, which assumes that the semantic interpretation of the scene as a room precedes and determines the interpretation of its features. In other words, our familiarity with an object depicted in a picture may be sufficient to determine its perceived shape. We do not know whether we perceive the Ames room as we do because of our familiarity with rectangular rooms, but Perkins and Cooper (1980) have provided us with an elegant demonstration that leads us to conclude that familiarity with the object is probably not critical in perceiving rectangularity in real objects. In Figure 7.9, we see two views of the John Hancock Tower in Boston, one of which appears to have a rectangular cross section, the other of which appears strangely distorted. This impression is not confined to looking at pictures of the tower: One gets the same impression by looking at it from the vantage points of these pictures. The cross section of the building is actually a parallelogram, and so the view that appears distorted (because it does not fit our preconceptions about the shapes of buildings) is in fact the more veridical one. So we conclude that our knowledge of architecture does riot override the effect of purely optical changes in the projection of an object.

Left: View satisfies Perkin's Law Right: View does not satisfies Perkin's Law

Fig.7.9 Views of John Hancock Tower, Boston