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You’re one year older and you wake up to a big surprise– your friends are blindfolding you and taking you out to a birthday treat! You drive for a while, walk a couple of blocks, take off your blindfold and find yourself in a grassy park with a cake in front of you. Lucky you! However, you’ve never been here before, so how do you know if you’re actually at a new place or just in front of a huge, detailed picture?

Well, it’s not that likely that you’d be brought to a place where there is such a large image, but being able to tell the difference between a complicated 2-dimensional picture and a 3-dimensional place with objects is something we often take for granted. Thanks to what are called depth cues, we can use these hints to tell that a tree is farther back behind another tree, and not equally far from you as they would be in a flat photograph.

Here we’ll go over monocular cues– that is, you only need one eye to be able to see these depth cues. (Binocular cues, or cues that require two eyes, get a bit more complicated.)

Accommodation: The ciliary muscles in your eyes stretch the lens within your eye and make it thinner when you are looking at something far away (and the opposite when looking at something closer). The process of your eye adjusting to keep an object in clear focus is called accommodation. Your visual cortex (the vision center of your brain) interprets the sensations of your eye muscles contracting and relaxing and uses that information to understand the depth and distance of the object.

Aerial Perspective: Objects that are farther away will contrast less against each other, in terms of how light or dark they are, and will overall look less vibrantly-colored. The colors of objects at a great distance away also appear bluer. This is due to how light hits and reflects off of the atmosphere. Next time you’re in a large, open space, see if objects in the far distance appear this way.

Familiar Size: Your brain uses previous knowledge of the size of an object (such as the typical size of a cat) and combines that with information about the angle at which you are looking at the object to determine how far away from you an object is located.

Depth from Motion: With this cue, our brain recognizes that the size of an object changes when it is in motion– it appears larger the closer it gets to you, and smaller the farther away it moves.

Motion Parallax: When you move forward, the way the objects around you move past you can inform you as to how far they are from you. Objects closer to you will appear to move faster than objects farther away. Next time you’re in a car, check this out— the trees closer to you should look like they’re moving faster than the trees and objects in the distance.

Occlusion: The fact that an object is occluded, or out of sight, by another object lets you know that one object is closer to you than the other. If you see 3 houses where the first house blocks half of the second house, and the second house blocks part of the third house, you know that the first house is the closest to you, the third house is furthest from you, and the second house is somewhere in between.

Peripheral Vision: Due to the fact that your eye is curved, what you see in your peripheral vision (the very outer edges of your vision) appears curved. However normal it may seem to us, these curves provide our visual systems with information about what’s around us. The city photograph below (taken with a curved lens) shows an example of this.

Perspective: Lines that are parallel in real life appear to get closer together as they recede in the distance. The fact that this happens can help us figure out the distance between two objects. Consider the image below: it would be tougher to tell how far the two balls are from each other if there were no train track (would the blue ball just be smaller and floating above the yellow ball?), but with the lines of the train track (do you see how the outer lines of it converge?), it’s easy to tell that the blue ball is just much farther away than the yellow ball below.

Relative Size: With this depth cue, our visual systems recognize that if there are two objects that you know are the same size (think of two of your Chevron car toys), but you see one as smaller than the other, then the smaller one must be farther away. How much smaller that car is, as compared to the other car, helps us figure out how far it is— if it’s not much smaller than the first car, we know it’s not too far away, and if it’s a lot smaller, we then know that it’s pretty far away.

Texture gradient: Imagine you’re sitting on the beach. Can you hear the waves? Now take a look at the sand— it’s easy to see the individual granules of sand near you (including their shape, size and color). However, as you look farther away, the texture of the sand becomes tougher to see. The fact that textures change as they recede informs our visual systems with depth information.

Now that you know a little more about depth perception, take a look at these mountains. How many depth cues can you identify? (Answers are below)

Answers: 1. aerial perspective, 2.occlusion, 3. texture gradient (just barely, but you can see the texture better on the closer mountains than the farther away ones)