Monocular cues

There are various monocular cues to depth. They are sometimes called pictorial cues, because they have been used by artists trying to create the impression of three-dimensional scenes while painting on two-dimensional canvases. One such cue is linear perspective. Parallel lines pointing directly away from us seem progressively closer together as they recede into the distance (e.g., railway tracks or the edges of a motorway). This convergence of lines can create a powerful impression of depth in a two-dimensional drawing.

Another aspect of perspective is known as aerial perspective. Light is scattered as it travels through the atmosphere, especially if the atmosphere is dusty. As a result, more distant objects lose contrast and seem somewhat hazy. There is evidence (e.g., Fry, Bridgman, & Ellerbrock, 1949) that reducing the contrast of objects makes them appear to be more distant.

Another cue related to perspective is texture. Most objects (e.g., cobble-stoned roads; carpets) possess texture, and textured objects slanting away from us have what Gibson (e.g., 1979) described as a texture gradient. This can be defined as an gradient (rate of change) of texture density as you look from the front to the back of a slanting object. If you were unwise enough to stand between the rails of a railway track and look along it, the details would become less clear as you looked into the distance. In addition, the distance between the connections would appear to reduce (see Figure 2.6). Evidence that texture gradient can be a useful cue to depth in the absence of other depth cues was provided by Todd and Akerstrom (1987).

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