Height in the visual field

Figure 6.5: Height in visual field. (a) Trees closer to the horizon appear to be further away, even though all yield the same retinal image size. (b) Incorrect placement of people (the author and his son, Ethan) in the visual field illustrates size constancy scaling, which is closely coupled with depth cues. (Photo printed by permission of Nadia Inturias, Uyuni, Bolivia.)

Figure 6.5(a) illustrates another important cue, which is the height of the object in the visual field. The Ponzo illusion in Figure 6.2(a) exploits this cue. Suppose that we can see over a long distance without obstructions. Due to perspective projection, the horizon is a line that divides the view in half. The upper half is perceived as the sky, and the lower half is the ground. The distance of objects from the horizon line corresponds directly to their distance due to perspective projection: The closer to the horizon, the further the perceived distance. Size constancy scaling, if available, combines with the height in the visual field, as shown in Figure 6.5(b).