Why does a person with only one working eye have zero depth perception?
Category: Physics Published: July 28, 2023
By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and Associate Professor of Physics at West Texas A&M University
Having only one working eye does not lead to zero depth perception. Although using two eyes does indeed play a large role in depth perception, there are also many other approaches that the human visual system uses to perceive depth. In general, approaches that enable depth perception are called "depth perception cues" or simply "depth cues". Depth perception is the ability to see a scene as a three-dimensional world containing three-dimensional objects that move according to three-dimensional physics.
All of the various depth cues are described in the sections below, examining first the two-eye depth cues and then the one-eye depth cues. Note that none of my explanations in this article are meant in any way to minimize the suffering and disability that may be experienced by people who have only one functioning eye.
Two-Eye Parallax
One of the most important depth cues is two-eye parallax. Because each eye is at a different location in the head, each eye sees a slightly different view of the world. The difference between what your left eye sees and what your right eye sees depends on the three-dimensional shape of each object and its location in the three-dimensional world. The closer that an object is to you, the greater the difference between what your left eye sees and what your right eye sees. The human brain is therefore able to extract depth information from the difference between what your two eyes see. If the image of a chair seen by your left eye and the image of the same chair seen by your right eye are nearly identical, then the chair must be far away. In contrast, if these two images of the chair are very different, then the object must be very close. This effect is demonstrated in the diagram shown on the right.
The overall geometric effect is called "parallax". When a human is using two eyes in order to take advantage of parallax, it is called "two-eye parallax" or "binocular parallax". The difference between the left-eye image and the right-eye is called "binocular disparity". The ability of the brain to extract depth information from this disparity is called "stereopsis".
Vergence
The other two-eye depth cue is vergence. When your two eyes both look directly at the same object, they must both rotate slightly toward each other to do this. How much your eyes rotate depends on how close the object is. When an object is far away from you, your two eyes only rotate toward each other a small amount in order to be both looking directly at the same object. In contrast, when an object is close to you, your two eyes most rotate toward each other a large amount in order to be both looking directly at the same object. This is demonstrated in the diagram on the right.
The muscles that are involved in the eyeball rotations send signals to the brain about how much the eyes are rotated. The brain can then extract depth from this information. Note that this is an oculomotor depth cue and not a visual depth cue, so images on a flat screen can never convey this depth cue.
One-Eye Depth Cues
Although both two-eye depth cues—two-eye parallax and vergence—play significant roles in depth perception, they are not the only depth cues. The nineteen other depth cues need only one eye to work. These cues are therefore called "one-eye depth cues" or "monocular depth cues". If a person only has one functioning eye, he can still see depth using these nineteen other depth cues, which are described below.
1. Motion Parallax
If you are moving smoothly as you look out at the world, it is equivalent to you remaining motionless and the entire world moving in a corresponding way. As the whole world appears to move, objects that are closer to you will appear to move at a faster speed because of parallax. Your brain understands that all of the moving objects in your view have the same true effective speed (because it's really just one object—your head—that is moving through space). Therefore your brain can determine how far away an object is from you by how fast it appears to be moving. Geometrically, this arises from the same parallax effect that was described in a previous section, but now the different-perspective images result from you moving your eyes to different viewing locations and not from you using two eyes.
The figure below demonstrates the "motion parallax" depth cue. The image on the left shows a random collection of motionless dots in three-dimensional space when no motion parallax is present. In contrast, the animation on the right shows the same collection of motionless dots but now with motion parallax present. The particular type of motion parallax presented in this animation represents you moving your head to the left and to the right repeatedly while gazing continuously forward.
Because the dots in the animation above do not have a meaningful shape or size, are positioned randomly, and are not actually moving, the only depth cue present is motion parallax. As you can see, just this one depth cue by itself creates a convincing sense of depth. Note that this animation was generated from scratch using a complete analysis of the physics involved and therefore may be more accurate than if 3D-rendering software had been used. The next figure also demonstrates motion parallax.
The type of motion parallax presented in this animation represents moving your head forward and backward repeatedly as you gaze continuously forward. Again, because the dots in this animation do not have a meaningful shape or size, are positioned randomly, and are not actually moving, the only depth cue present is motion parallax. Again, the animation creates a convincing sense of depth. The image on the left shows the same collection of dots without motion parallax present. This animation was also generated from scratch using a complete analysis of the physics involved and therefore may be more accurate than if 3D-rendering software had been used.
2. Kinetic Depth Effect
Physical objects tend to move in common ways which your brain understands and can use to extract depth information. For instance, for a rigid object that is rotating, every part of the object travels along a circular path around the same rotational axis in the real world. When viewed, every part will appear to be traveling along an elliptical path around the same rotational axis. Furthermore, the size of a part's elliptical path depends on how far away that part is from the rotational axis. Your brain can detect this and extract depth information. Your brain can also do this type of thing with other common types of motion, such as radial motion, projectile motion, wave motion, and walking motion.
In the figure below, there is a random collection of dots with no meaningful size or shape. In the image on the left, the dots are motionless. In the animation on the right, the dots are rotating in unison about a common rotational axis. In both cases, the observer is stationary. The animation therefore shows the rotational version of the kinetic depth effect. Because your brain subconsciously understands and has experience with the physics of rotating objects, the animation below appears to have depth.
The kinetic depth effect is different from motion parallax. Motion parallax involves a moving observer looking at a motionless world (and therefore always results in the whole world seeming to move in the same direction), while the kinetic depth effect involves a motionless observer looking at moving objects. In some particular cases, the physics of motion parallax and of the kinetic depth effect may be equivalent. However, this is generally not the case. There is no way in which an observer can move his head that will produce a result equivalent to the kinetic depth effect of waving motion, walking motion, explosive motion, and so forth.
The animation above was intentionally created using dots with no meaningful shape, size, or location so that the only depth cue present is the kinetic depth effect of rotational motion. This animation was generated from scratch using a complete analysis of the physics involved and therefore may be more accurate than if 3D-rendering software had been used. The figure below also demonstrates the kinetic depth effect of rotational motion.
In this case, the animation involves the rotation of a single object with a three-dimensional shape rather than several objects with no shape. The creature in the image on the left is not rotating and therefore seems to be flat. In fact, you can't even tell which creature it is. In contrast, the animation on the right is rotating, giving you a sense of the three-dimensional shape of the T-Rex.
Interestingly, because the animation above involves an extended object with a meaningful shape, the rotational kinetic depth effect not only provides you with positional depth information, it also provides you with shape-related depth information.
Note that the two animations above correctly include the size perspective effect that is inherent in the observation of true rotational motion. For instance, the dinosaur's head appears slightly larger when it is closer to you and moving to the right, and then slightly smaller when it is farther away from you and moving to the left. Because of this perspective effect, your brain can only properly see the dinosaur above rotating in one direction (with its head closest to you when it's moving to the right). This is different from bistable rotation animations that intentionally omit size perspective effects in order to create bistable perception illusions. The figure below also demonstrates the kinetic depth effect.
In this case, the animation involves three-dimensional wave motion. Because your brain subconsciously understands and has experience with true wave motion, the animation above appears to have depth. Note that the animation above contains only a random collection of dots that have no meaningful shape or size. Therefore, the only depth cue present is the kinetic depth effect of wave motion. The animation above was generated from scratch using a complete analysis of the physics involved and therefore may be more accurate than if 3D-rendering software had been used.
The figure below demonstrates a different type of kinetic depth effect. In this case, the animation presents the familiar motion of a person walking past a wall. Because your brain subconsciously understands and has experience with walking motion, the animation below appears to have depth (i.e. the walking person seems to be in front of the wall). Amazingly, the animation below contains no direct shape information of any kind, and yet you can still see a human walking. This animation was specifically constructed so that it contains only one depth cue: the kinetic depth effect of a person walking. The image on the left shows the exact same collection of dots but without any kinetic depth effect. As a result, you don't see on the left a human in front of a wall. Note that for accuracy, this animation was generated using motion capture data.
The animation below also demonstrates the kinetic depth effect of a human walking. In this animation, you not only perceive a three-dimensional human figure walking. You also perceive that the human is walking obliquely toward you in three-dimensional space, which gives an added sense of depth. The image on the left shows the same collection of dots without any kinetic depth effect present.
Incredibly, the animation above literally shows just twelve black dots with meaningless shapes and sizes moving on a flat white screen. And yet, your brain can perceive a stunningly realistic three-dimensional human figure moving in a stunningly realistic three-dimensional way. This is because your brain is so finely attuned to the detailed three-dimensional physical motion involved when a human is walking and how it appears to the human eye. Note that this animation includes a two-second frozen image at the beginning so that you can effectively recognize that it is motion that gives the sense of depth. As before, the animation above was generated using motion-capture data for the sake of accuracy.
3. Depth from Optical Expansion
When an object is moving steadily toward you, its apparent size increases in a specific way. The rate at which it appears to get bigger depends on how far away it is and how fast it is moving toward you. When the object is far away, it will appear to get bigger very slowly. When the object is very close, it will appear to get bigger quickly. This effect is called optical expansion. Your brain can deduce not only the object's motion but also the object's distance. Note that the reverse is also true: an object moving steadily away from you appears to get smaller at a rate that proportional to its distance.
When a baseball is thrown toward you, your brain uses optical expansion to keep track of its distance. This helps you properly catch the ball at the right time. The optical expansion depth cue is similar to the kinetic depth effect cue, except that in the kinetic depth effect cue, your brain is analyzing the apparent speed at which the object changes location in space. In contrast, in the optical expansion depth cue, your brain is analyzing the rate at which the apparent size of the object changes. A train that is traveling directly toward you would have zero kinetic depth effect but would have significant optical expansion.
The figure below demonstrates the "optical expansion" depth cue. The animation on the right shows a baseball that is correctly experiencing the optical expansion that results from being thrown toward you. For comparison, the image on the left shows the same baseball without optical expansion. (Also included in the animation is a little bit of spin and kinetic depth effect because the animation would look strangely unnatural without these.)
To be completely clear, optical expansion does not only involve an object appearing to get bigger as it moves toward you. It also involves the object getting bigger in a specific, non-linear way that is dictated by the physics. Your brain subconsciously understands and has experience with this physics and can therefore extract depth information. Note that this animation was generated using physically accurate calculations of the baseball's location, speed, and apparent size as a function of time for a baseball undergoing curveball projectile motion.
4. Familiar Shape
If an object has a familiar shape, your brain can recall from memory the apparent shape of that object that corresponds to viewing the real object, and then extrapolate from there. In this way, the three-dimensional shape of the object can be perceived without needing any other depth cues. The figure below demonstrates the "familiar shape" depth cue.
The image on the right contains in reality a collection of straight black lines and gray areas on a flat white screen. However, the lines are arranged in the familiar shape of what you see when you look at a real table. You therefore perceive depth. The image on the left shows the exact same number of straight lines attached at the same places as the right image, but it does not seem to have depth because the angles of the lines are all "wrong". In other words, the object on the left does not have the correct familiar shape that occurs when viewing a real table. Note that for the image on the right, I have intentionally drawn the table without perspective effects so that the only depth cue is the familiar shape cue.
5. Relative Size
If two objects in your field of view are the same type of object, then your brain assumes that their true physical sizes must be the same. Therefore, your brain assumes that the difference in their perceived sizes must be solely caused by perspective effects. Your brain can therefore extract depth information based on how much the perceived sizes of the two objects differ. For instance, if two single-story houses are in view, then the house that appears to be five times taller than the other house must be about five times closer to you. The figure below demonstrates the "relative size" depth cue.
For the image on the right, your brain notices that each of the four objects has the same shape and therefore assumes that they all have the same true size. Therefore, your brain perceives that the smaller objects must be farther away. In contrast, the objects in the image on the left all have the same size and therefore appear to be at the same distance. I have intentionally chosen here an object with an unfamiliar size and shape so that only the depth cue present is the relative size depth cue.
6. Familiar Size
If a certain object has a known size, then its perceived size corresponds to how far away it is, even if there are no other objects in the field of view to compare it to. Your brain can therefore extract depth information from the perceived size of the object. For instance, an apple is usually a few inches tall. An apple that appears to be much smaller than this must therefore be far away. The figure below demonstrates the "familiar size" depth cue.
The image on left includes two non-specific, unfamiliar objects so that no depth cues are present. As a result the two objects in the left image appear to be the same distance away. In contrast, the image on the right includes two familiar objects. Because you are familiar with baseballs and soccer balls, and you know that the true size of a baseball is smaller than the true size of a soccer ball, your brain perceives that the soccer ball must be farther away. In order to get this effect to work well while looking at this figure, try to visualize the balls as real objects in a real scene.
7. Estimated Size
Amazingly, even if you see an object that has nothing to compare it to and has an unfamiliar shape and size, your brain can still extract depth information from its perceived size by estimating its true size. In other words, your brain estimates the most probable true size of the object and then uses this as if it were a familiar size depth cue. The estimated size depth cue is not particularly effective because the estimated size will typically not be very accurate. The figure below demonstrates the "estimated size" depth cue.
Although the objects in the image on the right are unfamiliar, your brain may assume that cylindrical objects in everyday life (like soup cans) tend to have a small true size while conical objects in everyday life (like Christmas trees) tend to have a large true size. Therefore, your brain may assume that the conical object in the right image is much bigger in true size and therefore must be farther away from you than the cylindrical object because it does not look that much bigger. If you have a hard time seeing depth in the placement of the two objects in the right image, don't worry because this depth cue is not particularly effective.
8. Uniform Size
For a single, extended object that is known to be roughly constant in size along its length, the parts of the object that appear to be smaller must be farther away because of perspective effects. For instance, a baseball bat is roughly constant in width along its length. Therefore, the end of the baseball bat that appears to be much smaller than the other end must be much farther away. In art, this effect is called "foreshortening". The figure below demonstrates the "uniform size" depth cue.
A cylindrical rod in the real world has a uniform size along its length. Therefore, when one end of the rod appears larger than the other end, your brain correctly sees the larger end (the red end in this case) as the closer end. When looking at the image on the right, notice how the red end of the rod seems to be sticking out of the screen. In contrast, the image on the left shows the same rod but without the "uniform size" depth cue present.
9. Parallel Lines
This cue can be thought of as a general case of the uniform size depth cue. This is because when two lines are parallel to each other in the real world, this is equivalent to a single overall object having a uniform size along its length. For instance, a straight road extending away from you has a uniform size along its length, but can be thought of as two parallel lines (i.e. the two sides of the road).
Two lines that are parallel to each other in the real world will be perceived as converging toward each other as they stretch farther away from you. If your brain knows that the two lines are parallel in the real world then it can extract depth information based on how close the lines appear to be. The places where the lines appear closer to each other must be farther away from you. The figure below demonstrates the "parallel lines" depth cue.
The image on the right shows a scene involving two roads on a flat ground plane with this depth cue at work. Therefore, these roads appear to be stretching away from you into the distance. In contrast, the image on the left shows the same scene but without this depth cue, leading it to look flat.
For a set of parallel lines that all extend exactly away from you, they will all appear to meet at one vanishing point at the center of your field of view, as shown in the figure above. In contrast, if a set of parallel lines extends away from you at an oblique angle, then these lines will all appear to meet at one vanishing point that is not at the center of your field of view. This effect is shown in the figure below.
The image on the right shows two sets of parallel lines on the ground that each has its own non-central vanishing point. The image on the left shows the same scene but without any depth cues. In general, every set of parallel lines has its own vanishing point. Therefore, there could be an infinite number of independent vanishing points in an image. Interestingly, if all the sets of parallel lines in a scene are all parallel to the ground plane, all of their vanishing points will lie on the horizon line (which is where the sky appears to meet the ground). This may seem like a rare case, but humans love to build things with surfaces parallel to the ground, so it is actually quite common. It is so common, in fact, that some people mistakenly think that vanishing points must always lie on the horizon.
In every day life, humans tend to build objects that are a box shape or a collection of box shapes, such as buildings, desks, shelves, cabinets, books, tables, beds, and so forth. The edges of a box form three sets of parallel lines. Therefore, a collection of boxy objects that have their faces aligned will have only three vanishing points. First instance, a row of houses has most of its edges appear as lines converging at one of the three vanishing points. For such cases, artists speak of drawing in "three-point perspective".
Sometimes in art, the vertical vanishing point is ignored (so that all lines that are vertical in real life are drawn as vertical on the paper). For a collection of aligned boxy objects, this reduces the situation down to two vanishing points, which artists call "two-point perspective". If there is a collection of aligned boxy objects and two of the dimensions are drawn without perspective, then there is only one vanishing point, which artists call "one-point perspective". These concepts are shown in the figure above.
Note that the parallel lines depth cue is not a special case of the horizon effect depth cue. The perception of depth established by parallel lines arises from the lines converging at a vanishing point and not from objects being close to the horizon. In fact, the parallel lines depth cue works even if there is no horizon at all. The figure below shows a situation where there is no horizon.
In the image on the right, all of the lines that are running along the length of the tunnel meet at the central vanishing point. In contrast, the image on the left shows the same tunnel without the parallel lines depth cue present, insofar as that is possible. Note that if there is not a horizon but there are vanishing points, the horizon effect still occurs in the sense that the closer an object appears to be to a vanishing point, the farther away it seems to be. However, the vanishing point horizon effect still has nothing to do with parallel lines directly.
10. Texture Gradient
Similar to how objects that are closer to you appear larger, parts of the pattern in a texture that are closer to you will appear larger. Your brain can therefore extract depth information from how the different parts of a texture compare to each other in perceived size. Additionally, the texture of a surface can indicate the tilt of the surface, which can help portray the three-dimensional shape of objects. The figure below demonstrates the texture gradient depth cue.
In the image on the left, all of the spots of the textured surface are perceived as being the same size, the same shape, and at about the same spacing, making this image appear flat. In contrast, the image on the right shows that the dots near the top of the image are smaller, closer together, and more distorted than the other dots, giving the impression that they are farther away. Note that the left image and the right image in the figure above show the exact same textured surface with the dots in the same places.
The texture gradient effect works not only on flat ground planes. It can also portray the three-dimensional shape of complicated objects and scenes. For instance, the figure below is the same as the figure above, except that a canyon has been cut in the ground.
The three-dimensional shape of the canyon is made apparent in the image on the right by the texture gradient depth cue. Note that there are no other depth cues present in this image (except for a small amount of recess shading). The image on the left shows the same texture and the same canyon but without the texture gradient depth cue. Another example of the texture gradient depth cue is shown in the figure below.
As this figure demonstrates, a texture gradient does not have to consist of a pattern that has been painted on a flat surface. It can also consist of a large collection of three-dimensional objects that are situated so that they approximately form a flat surface. An additional example of the texture gradient depth cue is shown in the figure below.
As this figure demonstrates, a texture gradient does not have to consist solely of independent features or objects. Rather, it can consist of an interconnected pattern. The image on the right includes the texture gradient effect. As a result, the top of the image appears to be farther away from you than the bottom of the image. In contrast, the image on the left shows a texture but without the texture gradient effect, making it look flat.
11. Horizon Effect
For an object sitting on the ground, the physics dictates that the closer the object's center appears to be to the horizon, the farther away the object is from you. Your brain can therefore estimate how far away an object is from how close its center appears to be to the horizon line. The figure below demonstrates the "horizon effect" depth cue.
In the image on the left, all three objects are at the same height in the image. In contrast, the image on the right shows the same three objects but at different perceived heights. Your brain sees the blue cone as visually closer to the horizon and therefore perceives that it is farther away from you than the other objects.
Interestingly, the horizon can also take the form of a vanishing point. For instance, for objects sitting in a tunnel, the closer that an object appears to be to the tunnel's vanishing point, the farther away it seems to be. The figure below demonstrates the "horizon effect" depth cue when there is a vanishing point instead of a horizon line.
In the image on the right, the blue cone appears to be closer to the vanishing point and therefore is perceived to be farther away from you. The image on the left shows the same scene, insofar as it is possible, without the horizon effect depth cue or the parallel lines depth cue.
12. Occlusion
When a near object is roughly in the same line of sight as a distant object, the near object will partially or completely block the view of the distant object (assuming it is not transparent). Therefore, the object that is being blocked from view must be farther away from you. This effect can be called occlusion, interposition, eclipsing, or overlapping. Your brain understands this effect and can use it to determine the relative distances of objects. The figure below demonstrates the "occlusion" depth cue.
In the image on the left, the three objects are all clearly visible with no occlusion and therefore you cannot tell which object is closer. In contrast, the image on the right shows the same objects but includes occlusion. You are therefore able to perceive the red cylinder as being closer to you and the blue cone as being farther away from you. (A small amount of horizon effect had to be included in order to prevent the objects from unnaturally penetrating each other.) Note that the occlusion depth cue can only tell you which object is closer to you. It cannot tell you the absolute distance of an object.
The occlusion effect does not have to involve three-dimensional shapes. Even with flat pieces of paper, you can tell which piece of paper is farther away because it is the one being occluded. This is shown in the figure below.
The image on the right shows one paper occluding another paper, in two different configurations. In both configurations, the paper that is being partially blocked appears to be farther away. The same two papers are shown in the image on the left but without the occlusion depth cue, making it impossible to tell which one is farther away. The figure below also shows occlusion.
However, in this case, there is a single object with its front face occluding its back face, rather than one object occluding a separate object. The figure on the right shows a box that is defined by its edges, presented in two different configurations. The occlusion effect gives you a sense of which face of the box is closest to you. In this way, occlusion can help give a sense of depth to an object. In contrast, the figure on the left shows the same box without occlusion information. As a result, you can't tell which configuration the box is in or which face is closest to you. The figure below shows another example of occlusion.
In this case, for dramatic effect, the occlusion depth cue has been combined with the optical expansion depth cue and the kinetic depth effect. When the baseball is partially hidden by the bars, you perceive it to be moving behind the bars. When the baseball partially hides the bars, your perceive it as moving in front of the bars. Because the white bars are visually part of the frame, the baseball seems to fly out of the image at the end.
13. Surface Shading
The way that light falls on an object depends on the three-dimensional shape of the object. Therefore, your brain can extract depth information from the shading on an object. The parts of an object that are darker tend to be the parts that are titled away from the light source. Therefore, if the position of the light source is known (or can be estimated), the tilt in three-dimensional space of each part of an object's surface can be deduced from its level of shading. The figure below demonstrates the "surface shading" depth cue.
Note that in this case, we are not focusing on the depth perception related to the position of each object but on the depth perception related to each object's three-dimensional shape. In the image on the right, the surface shading enables you to see the circular object as a three-dimensional sphere and the other object as a three-dimensional cylinder. The fact that the shading varies smoothly along the surfaces enables you to perceive the sides of the cylinder and the entire sphere as smoothly round. In contrast, the image on the left shows the exact same objects but without surface shading. As a result, the two objects look like flat paper cutouts.
14. Recess Shading
The points on an object or landscape that are recessed will appear darker because light has a harder time reaching down into the recess. The recess shading therefore conveys the depth and shape of the recesses. Through this depth cue, your brain is able to perceive the presence, the shapes, and the depths of holes, recesses, cracks, corners, inlets, and narrow spaces. The figure below demonstrates the "recess shading" depth cue.
The image on the left contains three holes that have no recess shading. As a result, they don't even look like holes. In contrast, the image on the right shows the same holes but now with recess shading included. As you can see, the shading enables you to see the holes and to see their three-dimensional shapes. The figure below also demonstrates the "recess shading" depth cue, this time combined with the "parallel lines" depth cue.
As a result of the depth cues, the image on the right appears to show an arched tunnel that stretches away from you into the distance. As you can see, including two depth cues instead of one makes the image's sense of depth even more convincing. For comparison, the image on the left shows the same tunnel without any depth cues, insofar as it is possible.
The figure above shows the same tunnel as in the previous figure, but now including only the "texture gradient" and "recess shading" depth cues, instead of the "parallel lines" and "recess shading" depth cues. For comparison, the image on the left shows the same tunnel without any depth cues, insofar as it is possible.
15. Shadow Shape
The shape of a shadow depends on the three-dimensional shape of the object that is casting the shadow. Therefore, your brain can partially deduce three-dimensional shape information from an object's shadow. The figure below demonstrates the "shadow shape" depth cue.
In the image on the left, you see the outline of some creature, but it is hard to see the three-dimensional shape of the creature or even what kind of creature it is. In contrast, the image on the right shows the same creature but now being illuminated from the side so that its shadow falls on the left wall. This shadow reveals the creature to be a T-Rex and partially reveals the three-dimensional shape of this T-Rex. In general, this depth cue works even if the illumination is not aimed directly toward a wall, as demonstrated in the figure below.
The image on the right involves a shadow that is cast obliquely on the ground. This shadow reveals that this structure is townhouses. This shadow also enables your brain to more effectively see the townhouses as three-dimensional objects. In contrast, the image on the left shows the same structure without a shadow, which causes it to appear as a non-descript blob of black.
16. Shadow Size, Location, and Blurriness
The size, location, and blurriness of an object's shadow all depend on how far away the object is from the shadowed surface. In general, the farther away an object is from the shadowed surface, the larger, the blurrier, and the more shifted its shadow will be. Your brain can therefore deduce distance information from the size, location, and blurriness of shadows. The figure below demonstrates this depth cue.
The image on the right shows three balls and their shadows. The shadow of the rightmost ball is larger, blurrier, and more shifted, indicating that the rightmost ball is farther away from the ground and closer to you. In contrast, the image on the left contains the same three balls but without shadows so that there is no depth to the scene beyond the roundness of the balls. The figure below also demonstrates these shadow effects.
The image on the right shows the shadow location depth cue at work but does not include differences in shadow blurriness or shadow size. Even with just this one type of shadow depth cue at work, your brain can still perceive that the rightmost paper is farther away from the checkered surface and closer to you.
17. Atmospheric Effects
When an object is very far away, the air between you and the object changes its appearance. Air is not perfectly transparent. The nitrogen and oxygen molecules that make up 99% of atmospheric air give a distant object a slight white-blue tint under daytime lighting conditions. As an additional effect, the water droplets in the air give the air a slight white or murky grey appearance. These effects also cause the final image to diminish in contrast, color saturation, and sharpness.
The end result is that the farther away an object is, the more it will have a muted blue-white color and a softer, blurrier appearance. Your brain can therefore deduce the distance of an object based on how much its image is degraded by atmospheric effects. Note that atmospheric effects only become significant when the light from an observed object travels through large amounts of air. Therefore, this visual cue only works for objects that are very far away (unless it's an extremely foggy day).
You probably use this visual cue more than you realize. Astronauts who have walked on the moon reported that because the moon lacked an atmosphere, all of the distant hills looked much closer than they actually were, which was disorienting. They reported that as they walked toward a hill, it seemed to recede at the same rate. The figure below demonstrates the "atmospheric effects" depth cue.
In the image on the right, a series of mountains at different distances are observed to have different shades and colors because of the intervening air. In contrast, the image on the left shows the exact same mountains but without any atmospheric effects. As a result, all the mountains visually merge together and look flat. Note that the figure above was intentionally drawn as simple as possible in order to clearly demonstrate the effects. The figure below also shows atmospheric effects, but now using an actual photograph of the real world.
The image on the right is a raw photograph of a mountain landscape, without any photo editing. The blue-white tints in this photograph are completely natural. This photo demonstrates that the farther away a mountain is, the more it appears blue-white, unsaturated, and contrast deficient. Note that the sky is blue-white for the same reason that the distant mountains are blue-white, because of the effect of the atmosphere on the light passing through it.
The image on the left shows the exact same photo but without any atmospheric effects. To create the image on the left, I took the raw photograph and removed the atmospheric effects using photo editing software. (This involved removing the blue tint and increasing the saturation and contrast one layer of mountains at a time.) Notice how all of the mountains in the left image seem to merge together into one indistinct mass without much depth. Interestingly, the image on the left looks like it came from a video game that failed to properly include atmospheric effects.
18. Accommodation and Pupil Response
In order for the human eye to properly focus on objects that are at different distances from it, the ciliary muscles in the eye must change the shape of the eye lens by changing the amount of muscle contraction. To bring a distant object into focus, the ciliary muscles relax, which allows the lens to flatten. To bring a near object into focus, the ciliary muscles contract, which pushes the lens into a rounder shape.
The human eye has sensory mechanics to detect how much the ciliary muscles are contracted. In this way, your brain can deduce the distance of an object by focusing on it and then sensing the contraction level of the ciliary muscles. Interestingly, this depth cue depends on muscle contraction information rather than image information, so I can't demonstrate how it works using images.
Pupil response also helps accommodation. The size of the pupil slightly effects how much an object appears to be in focus. The shape of the lens in your eye gives rise to optical aberrations. As a result, the more of the lens that is used, the blurrier the image. Therefore, your pupil works along with the ciliary muscles to bring objects into focus. Your brain uses pupil constriction information along with ciliary muscle contraction information to determine the object's distance.
19. Depth from Defocusing
When the human eye brings a certain object into focus, objects that are at a different distance will appear blurrier. The amount of observed blur depends on how far away in the forward direction the other objects are from the object that is in focus. Specifically, the farther away an object is in the forward direction from the object in focus, the blurrier it will appear. Your brain can therefore deduce distance from the amount of defocusing blur. The figure below demonstrates the "depth from defocusing" depth cue.
The image on the left shows three strawberries without defocusing blur. As a result, they all appear to be the same distance away. In contrast, the animation on the right shows the same strawberries with defocusing blur included. (A small amount of the relative size depth cue has also been included to prevent the image from looking unnatural). In the animation, the point of focus repeatedly shifts between the left strawberry and just in front of the middle strawberry. As a result, the left strawberry appears to be closer to you.
Summary
As you can see, the human visual system uses about twenty-one different depth cues! The exact number will depend on how you decide to group special cases into categories. Of the twenty-one depth cues, only two require using both eyes. The other nineteen depth cues work perfectly with only one eye.
Movie theaters that present two-eye parallax depth cue information in addition to the traditional one-eye depth cues require wearing special glasses to properly see the images. These movies are commonly, but incorrectly, called "3D movies". They do not include all of the depth cues and are therefore not fully 3D. These movies are more accurately called "stereoscopic movies". Also, traditional movies that don't require wearing the special glasses contain most of the one-eye depth cues and are therefore already very close to being 3D-realistic. Because of this, "3D movies" are not that much more 3D-realistic than traditional movies. This is perhaps why "3D movies" have not replaced traditional movies, despite having existed for over a hundred years. Neither traditional movies nor "3D movies" contain accommodation cues, pupil response cues, true depth-of-focus cues, vergence cues, or true motion parallax cues. Despite all of this, traditional movies and "3D movies" can still appear convincingly three-dimensional because they include many of the other depth cues.
In summary, the human visual system is quite capable of seeing depth even if one eye is not functioning. Thankfully, this also means that humans can see depth for objects and scenes displayed on a flat computer screen or movie screen. It also means that artists who understand the one-eye depth cues can create a convincing sense of depth when painting or drawing on paper, canvas, or another flat surface.