Monocular depth cues are cues to depth, or to three-dimensional space, which only require the function of one eye.  There are nine monocular depth cues: occlusion, relative size, relative height, texture gradient, familiar size, linear perspective, aerial perspective, shading, and motion parallax. Each of these cues provides some indication of the depth of objects in our visual field.

The following image of my favorite band, The Beatles, clearly has depth.  However, we’re not standing on Abbey Road seeing the scene take place, we’re just looking at an image, so how do we know that there is depth?:  Depth cues!

This image arguably utilizes eight of the nine monocular depth cues listed above and I’ll explain how.

Occlusion, the idea that an object that blocks the view of another object must be in front, is evident in the image.  One obvious example is when looking at the trees and buildings on the right side of the image.  We know that the trees are in front of the buildings because there is a portion of the buildings that we can’t see; that is blocked by the trees.

Relative size, the idea that smaller objects are further away without knowing the actual size of the objects, is also evident in the picture.  For example, we can see several trees in the picture.  Not all trees are the same size, but all of the trees in the image appear much smaller than the large tree in the upper left so we can conclude that the other trees are progressively further away.

Relative height is a depth cue clearly evident in the picture as well. For relative height, objects that appear closer to the horizon line are perceived as being further away and objects that appear further from the horizon line appear closer.  For example, The Beatles are far from the horizon line and appear much closer to the observer than some of the smaller-looking cars that are just below the horizon line.

Texture gradient can arguably be considered a depth cue utilized in the Abbey Road picture.  If you look at the road at the bottom of the picture, you can see the rough texture that the road has.  Now, if you look at the road in the middle of the picture, near the horizon line, the road’s texture is less obvious.  This is because as a texture gets further away, it forms smaller visual angles, or pictures, on the retina, and is less noticeable.

Familiar size, the idea that knowledge of the normal size of certain objects can provide cues to depth, is also evident.  For example, we are familiar with the size of people. In the picture, there is a man standing on the sidewalk beside the black/blue car on the right.  We know that this man is roughly the same size as the members of The Beatles because we have experience with the size of people.  Our knowledge of the size of people clues us in to the fact that the smaller-looking man is probably just further away from the band members and not actually a miniature person.

Linear perspective, the idea that parallel lines seem to converge as they move into the distance to an apparent point called the vanishing point, is clear in the image. This cue can be seen when looking at the way the edges of the road converge in the distance.  We know the road isn’t actually getting narrower; it’s just getting further away.

Aerial perspective is a subtle depth cue in this picture.  With aerial perspective, objects that are further away also appear to be hazier and bluer. This is true for the trees and cars in the middle of the picture near the vanishing point.

Shading, as a depth cue, could arguably be considered a depth cue in the Abbey Road image. With shading, objects that bulge outward (convex) are lighter on the top and darker on the bottom and objects that dip inward (concave) are darker on the top and lighter on the bottom.  This is a more difficult cue to spot in the picture but it can be seen on Paul McCartney’s suit (second from left). Looking at Paul’s shoulder, the fabric is lighter on top of his shoulder (bulging out) than the darker stripe on his arm (dipping in) where his suit is wrinkling.

Motion parallax, the idea that objects that are closer move faster across the field of vision than objects that are further away, is not a depth cue in the Abbey Road image. This depth cue requires motion and, of course, an image does not have motion.

Image Source: “Beatles Vinyl Record Collectors Online Shop.” BEATLES Abbey Road Capitol Records Beat, British Pop Vinyl Record LP. Web. 29. Apr. 2014. <http://www.vinylrecords.ch/B/BE/Beatles/Abbey_Road/abbey_road.htm>