# Is there other way to perceive depth beside relying on parallax?

I'm creating a unique alien species which sees the world differently from us or the insects.

Our binocular vision helps us to gauge distance and depth while the array of light sensitive compound eyes rock a little which is sufficient for the critter to register depth. Both rely heavily on parallax, but I need a different mechanism for my alien creature which can only see visible light like us and is unfortunately a deaf... yes it doesn't even feel vibration in the air or ground. No tele-whatever-nosense or magic or augmentation allowed.

• Do you drive a vehicle? At safe driving distances binocular vision is ineffective. Humans use many queues to determine distance. – NomadMaker Jan 22 at 17:42
• @user6760 The pen touching exercise is irrelevant when talking about distance and vision. That exercise doesn't take into account the human body's sense of relative position with itself. A better one would be touching a pen tip in your hand to one sitting on a table, but that's still not the same as judging distance purely visually. – Logarr Jan 22 at 19:45
• Binocular vision is vastly overrated for judging distance. If you close one eye and look around you can judge distance just as well as before. Cues based on one thing in front of another and known size vs perceived size are very important. – Ross Millikan Jan 23 at 5:55
• @RossMillikan - I was born with monocular vision; I was 10 weeks premature and had what was then called retrolental fibroplasia and is now called retinopathy of prematurity (ROP). One eye useless, the other pin-sharp. It sometimes puzzles people that I can't "see" those 3D things with 2 side by side pictures. I have never had much trouble judging distances. As a bonus (although I am a pacifist) I am an excellent shot with a rifle. – Michael Harvey Jan 23 at 10:19
• @MichaelHarvey I lost an eye as a kid, and can juggle with 8 balls, so I can just 3d position pretty well, even though it took a long time to practice. I'm also pretty good with a rifle. Is there any advantage of seeing with one eye, for shooting? – Eric Duminil Jan 23 at 15:09

The following 3 come to mind:

Context Clues: The creature interprets the image and approximates the shape and dimensions of its environment based on lived experience, and knowledge regarding the objects it is viewing. Most actual depth perception (in humans at least) works this way, as parallax is only useful at relatively short range. The downside is that it is computationally intensive (bigger, more calorie hungry brain) and requires a relatively high fidelity eye.

Motion Parallax: As the creature moves it measures the optical flow of the objects in its field of vision, the greater the optical flow the closer the object (assuming the velocity of the creature is large compared to that of the object/creature it is tracking). The same effect can be achieved by moving the eye (or the body part it is in/on). This is another way humans judge distance.

Focal Plane Sweep: The creature adjusts the focal depth of its eyes until the object is in focus, and uses the tension of the muscles adjusting the lens shape to judge distance. Note that all else being equal, the thickness of the focal plane is inversely proportional to the diameter of the aperture (the pupil in a vertebrate eye), so the bigger the aperture, the more accurate this method can be. This also means that if this creature adjusts exposure by adjusting the size of the aperture, this method will be more effective in low light environments. For a multi-faceted eye with a useful resolution the aperture will be too small to make effective use of this method

• Is the act of adding more aperture when focussing on objects at a distance so more light can enter, because the further the distance the less bright objects are? – Trioxidane Jan 22 at 8:59
• Increasing the aperture does increase the amount of light that can enter, but the brightness is not directly related to distance (on a foggy day, an object may appear lighter as it gets further away). There's a good explanation of the phenomena here:en.wikipedia.org/wiki/Depth_of_field – Shawn O'Neil Jan 22 at 9:09
• It's also worth noting that context is how people with only one eye or people with conditions that disrupt coordination of both eyes (e.g. nystagmus) process depth perception. While definitely a disadvantage, it's not crippling; lists of well-known people that had only one eye can be found readily. – GrumpyYoungMan Jan 22 at 12:46
• @GrumpyYoungMan first I came here to post this answer, and went down to post this comment, but they've both already happened! I myself have slight double vision that results in me suppressing one of the two images from my eyes (by concentrating I can switch which is suppressed and, by concentrating even harder can just about bring them into alignment, but I can't sustain either), and I rely primarily on the first and third methods – Tristan Jan 22 at 16:00
• as a result I've always been kinda rubbish at most ball sports (although I found things like hockey, where the ball/puck is mostly confined to the ground easier, because that restriction makes the first method much more viable), but in everyday life I don't experience any difficulty, driving and cycling safely for instance. Sometimes I even manage to catch things! Although I do tend to try to avoid situations where I'll have to – Tristan Jan 22 at 16:02

You can estimate depths using time of flight technique.

There are tools which measure distances using a single photosensor: they shine a laser on an object and by measuring the time to get the signal back they calculate the distance.

Your aliens can use the same approach: they emit a flash of light and by measuring the time for viewing the back reflection they can infer the distance. Echolocation but with photons, not sound waves.

It won't work in dusty/foggy environment and with optically opaque media, but not even our eyes can see the bottom of the sea everywhere, right?

Moreover it would require extremely fast elaboration time to be effective at close range, considering that at 300 m the TOF would be around 20 $$\mu s$$.

• If you can generate coherent photons (all in phase), you can use interference to get very high precision I believe. renishaw.com/en/interferometry-explained--7854 – Yakk Jan 22 at 21:50
• @Yakk at a very stable frequency that doesn't drift over time or with with temperature... while also being able to sweep that frequency, because measuring the phase of optical photons will give you the distance modulo 700nm or so. In vivo. It might, however, be useful when estimating relative velocity to ludicrous accuracy? – John Dvorak Jan 23 at 21:45

### They bob their head slightly as they walk.

You do not need two eyes to perceive depth - you can have one eye move over time with an inertial sensor like we have in our inner ear and the brain can compute depth from that.

Try it yourself. Shut one eye, move your head a bit, either side to side or up and down, and notice the relative movement of objects depending on how far away they are? You can tell depth this way.

They don't even need to actually move, a 2D picture updated over time is enough to calculate depth 99% of the time (proof: Go play a computer game or watch a movie). The moving head gives depth information that other 1% of the time.

• Why pigeon bob their head?youtube.com/watch?v=Al6OCJYO2cA&ab_channel=Narek – user6760 Jan 22 at 12:50
• What you're describing is parallax. Instead of parallax between two eyes simultaneously, this is just parallax between two views from one eye at different times, but the mechanism is basically identical. – Nuclear Hoagie Jan 22 at 15:49
• @user6760 I believe pidgeons (and other birds) bod their heads when they walk for balance reasons, like how humans like to swing our arms. I heard that somewhere, not sure if it's true or not. – Dark Malthorp Jan 22 at 17:09
• Lizards and lagomorphs (rabbits, hares, etc) have side-facing eyes, so bob their heads before making an unfamiliar jump. Macropods (family that includes kangaroos) have forward-facing eyes, and tellingly, do not exhibit head bobbing. (And watch a few parkour videos, why don't you?) As @NuclearHoagie says, head bobbing is indeed parallax vision. Otherwise, simply moving through an environment conveys sufficient depth information to avoid obstacles. Lagomorphs are also burrowning creates, and have substantial whiskers for sensing obstacles (aside from large ears and sensitive noses). – Rich Jan 22 at 18:38
• @DarkMalthorp I don't think it is for balance. It allows their vision to remain precisely fixed on something as they move. There is a demo on youtube where a guy is holding a chicken and he moves its body around in circle but its head remains at the exact same point in space. – DKNguyen Jan 22 at 21:06

## Echolocation

Various animals (notably bats, dolphins) are able to map a 3D mental image of the world around them by emitting sounds and hearing them echo off of nearby surfaces. While it is not true that bats are "blind" - they do have functional eyes - they are still able to navigate while flying in very low light or even no-light environments using this. Dolphins seldom get deep enough to be in total darkness under water, but they still use a form of echolocation in order to detect nearby fish and other edibles in the water, which might not be in direct visual range of their eyes.

• That was my first thought too, until I got to the sentence where he says they're deaf. – Jay Jan 22 at 16:44
• @Jay Hmm, guess I missed that. I would honestly find it hard to believe that any living creature would be unable to detect vibrations even through touch. (Echolocation does not necessarily require ears, but does need some sense of touch.) Even plants are often able to detect vibration, no brain required. – Darrel Hoffman Jan 22 at 17:00
• Sure. In a lot of these worldbuilding questions, and other similar hypotheticals, the person asking the question throws in some assumptions to rule out the obvious answers. Like I was just watching a mystery story on TV the other day where the big mystery was how the criminal could have pulled off this crime, and throughout the show they introduced clues to rule out all the easy answers. There was someone standing in the only doorway who swore that no one went past him, etc. – Jay Jan 22 at 17:20

If your alien can emit laser rays, it can measure the distance like described here or here. That's one technique machines use to measure distance with a laser. How the mechanism works: The laser is positioned on the right side* of the eye and points a little bit to the left side. If the object is near, the dot of the laser is more to the right. The farther the object is away, the more the dot will move to the left side. The alien can find out the distance based on the position of the dot.

*all other sides work too, but like this it's easier to explain

• Hello Sadap! Welcome to World Building :) We would appreciate if you could also add a basic description of the mechanisms in the links into your answer. – The Square-Cube Law Jan 22 at 18:03
• @TheSquare-CubeLaw I added an explanation – bb1950328 Jan 22 at 18:39
• This measurement technique makes a lot of sense, +1. – The Square-Cube Law Jan 22 at 18:43
• The Pring race from David Brin's "Uplift" books use this technique. They are one-eyed arboreal creatures. – Greenstone Walker Jan 25 at 0:41

What senses can we postulate that these creatures have?

You said they're deaf so I guess that rules out echolocation.

But do they have some organ that can send some other sort of signal that bounces off the target object and returns, and which their brains can then time the trip to determine the distance? If not sound, maybe radio waves. Or less exotic, a burst of air. Or whatever.

If the creatures exist in an environment with air, they could judge distance by how "foggy" the image is, i.e. the more air an image has to pass through, the more distorted it will become. Humans do this in a very vague way: you know an object is far away because it's dim and hazy. This gets complicated because air density and composition varies, but perhaps these creatures have another sense that determines the composition of the air, how much humidity, etc, and then adjusts the computation in their brains.

They could have a very fine sense of scale. They know that an object is a certain size. When it's close up it looks big, when it's far away it looks small. Some objects, of course, have very variable sizes, but one could imagine a creature that can detect scale from detail. Again, humans do this to some extent, and it's always good for an optical illusion or a joke to have an object that looks like an object that you think you know the size, but really it's a very different size.

For any such alternative sense, you could say, "But the calculations that would have to be done in their brains for that to work would be very complicated." Well, sure. But the calculations that have to be done in our brains for us to determine distance by parallax are pretty complicated. What bats do to determine distance by echolocation is pretty complicated. Etc. The complexity and sophistication of living things is amazing. That's one of the classic arguments for intelligent design.

• Fog was my first thought, probably influenced by Flatland – trentcl Jan 22 at 18:29

Expanding on the laser solution that Sadap suggested, there's an approach called structured light where one illuminates the scene in a specific way from one point and observes it from another.

If one can control the lighting patterns emitted, one can actually distill a great deal of depth information without moving either the source or emitter. You just keep changing the light patterns to ones which provide the information you are lacking.

However, practically speaking, the most important way to determine depth is high quality cognitive processing. To demonstrate this, close one eye. Keep it closed. Now get out of your chair, walk to the fridge, get a glass, and pour yourself a drink. Drink it. Now go out your front door, check your mail, and come back.

If your brain is functioning normally, that should have been so easy for you that you might as well have had both eyes open. The vast majority of the depth information you work with is inferred from what you know about the scene. Tools like parallax only play a major factor in situations where your other visual cues are not providing you enough information. Then you become more dependent on parallax. This is why 3d glasses are a novelty, rather than a fundamental part of our movie theater experience over the last 50 years.

Relying on normal visual cues and relying on structured light only when the situation calls for it could lead to interesting alien behaviors. If they are showing interest in something, it will be immediately apparent because they'd give the object a flash of structure light patterns to get a better sense of what it is. It would be the alien equivalent of a dog tilting its head so that it can better place your voice.

• +1 for the final paragraph, as the most substantive addition not covered by other answers. – Dan Henderson Jan 24 at 7:22
• interesting, I didn't know about this usually for monocular vision to work the brain need to already know the size of the object for comparison but I'm going to explore this deeply ;D – user6760 Jan 24 at 12:15
• @user6760 It needs to "know" in a very loose reading of the word "know." There are plenty of examples where this fails, such as the famous Ames Room. But the brain is astonishingly good at doing such guesswork after not falling on its face too many times over the course of a decade. It is truly a master of sensor fusion. – Cort Ammon Jan 24 at 17:28

Another way (with a telescope or very high resolution eye) is to use the optical quality of the view.

This may be no use below 1/4 mile, but haze in the air makes it easy to distinguish between something one mile off and 5 or 10 (or 50) miles off, due to the reduced contrast at greater distances.

Or on a hot day, or over water, close objects remain steady while distant ones shimmer in the haze.

And that's just on this planet; who knows what tricks the atmosphere elsewhere might play?

This allows approximate distance detection over ranges where parallax or plane of focus become difficult or impossible.

Thick Retinas

Jumping spiders have multi-layered retinas which allow them to use focus to determine distance without having to do any focus sweep.

Their eyes have fixed lenses, so they can't change focus. But their retinas are very thick, comparable to the focal length of the lenses. So every object is in focus in some plane within the retina. Where in the retina the object is in focus tells the spider how far away it is. Or possibly, it's how out-of-focus the object is within a specific layer of the retina. This is what the scientists who analyzed the retinas of jumping spiders concluded, but I'm skeptical. (See https://ir.soken.ac.jp/?action=repository_action_common_download&item_id=4203&item_no=1&attribute_id=22&file_no=1)

Either way, you need retinas that are a significant fraction of the length of the whole eye. And you need light to be able to penetrate a long way through the tissue of the retina. So either you need tiny eyes, or weirdly transparent retinas (except the pigment cells themselves). Maybe a retinal system composed of separate veils with transparent material between them?

Focus

If your creatures' eye(s) have a very shallow focal depth, then they can sweep their focus point from a few cm away to a few km. When the object comes into focus, they will know its distance from them.,

I come from the field of Computer Vision. Recently people have developed algorithms that can accurately estimate depth from a single image. The idea is to learn cues from objects in the image. Here is an example paper:

https://www.cs.cornell.edu/~asaxena/learningdepth/ijcv_monocular3dreconstruction.pdf