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Why does a person with only one working eye have zero depth perception? (Part 3)
Category: Physics
Published: July 28, 2023
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.