I recently had to drive about an hour away from campus for a social with our organization. One of my friends drove there so I sat in the back of the car. As I was looking out the window, I had noticed that the objects that were much closer to the road were moving a lot faster than the objects that were farther away from the road. This was a road in the middle of Pennsylvania, so there were a lot of cows, as well as some houses, that were closer to the road. Farther away, there were trees and still farther away than that, there were mountains.
As we were driving, I had noticed that the cows that were closer to the road were moving faster across my field of vision than the trees that were farther away from the road. It immediately made me think of the monocular depth cues in class. It also reminded me that we perceive depth from these objects and how quickly they move across the retina. This particular depth cue is called motion parallax. It is one of the only depth cues that requires movement and can only be seen in moving object. Many other depth cues can be seen in paintings. However, motion parallax can only be used when viewing moving objects.
Motion parallax is a depth cue based upon our motion and is based on the fact that objects closer to us move faster than objects farther away. Therefore, we know that the objects that are moving faster are closer to us in our field of vision. Motion parallax can also be done without the movement of a car. For example, pigeons use motion parallax by bobbing their heads up and down. For motion parallax to be used as a depth cue, it simply requires some sort of movement from the observer.
