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I think most of our ability to judge relative distance is based on our brains judgement of lighting, texture, inference, and sound. While having two eyes helps a lot, you can still navigate a complex office environment with one eye closed. It just takes a bit more care.

When I was younger I remember hearing about how we can do all these things because we have 2 eyes. And that depth perception is what gives us the ability to not walk into walls, and do other things including driving.

I have thought about this many times and often wondered why when closing one eye I am still able to function.

Sense then I have thought strongly that having depth perception is used for training some other part of our brain, and then only used to increase accuracy of our perception of reality.

Further proof of this is TV. Even on varying sized screens humans tend to do well figuring out the actual size of things displayed.

Take one class on perception, read one textbook, you'll immediately find that stereo perception isn't very important. Your brain uses a host of depth queues, and stereo vision is just one of them.

Some of them translate trivially to photos/TV/etc, like convergent lines or texture gradient. Some of them are surprisingly physical, like feedback from your eyes about vergence or focal distance.

Stereo is highly effective up close, say within 10 meters (yards). And it works faster than many modes. It's absolutely fantastic for catching things out of the air. Given our intraocular distance, it's basically garbage past, I dunno, 30m or something? (obviously it degrades smoothly across distance)

I've heard more than one academic (evolutionary cognitive psychologists, etc) speculate that the single biggest evolutionary advantage of having two eyes is to have a spare in the event of damage. That might well be just whimsy and exaggeration, but I think it puts a helpful alternate perspective on it (pun!).

I'm skeptical of the claim that a major reason for having two eyes is depth perception.

One reason why you're still able to function is that you don't rely on your sense of depth that much these days. i.e. You don't need to gage where a spear or arrow will land. Even in a car, you are effectively on a one dimensional track and only have to decided to go left or right.

If you only had one eye, then in situations where there is lots of pressure to perceive depth I think you'd have to move your head around a lot.

Which makes me wonder, which human activities demand the best depth perception?

See my sibling comment: with respect to stereo vision, its greatest strength is nearby fast-moving things, great for stuff like dodging or catching or punching.

If you wanna launch spears or arrows, depth perception is incredibly important, but stereo vision will not help. Not with this intraocular distance, anyway.

Humans can determine the size of objects because we look for references in the scene and we understand the context.

If a person is standing next to a bush then we roughly know their height since we know the range of sizes that a bush could grow to. Likewise the size of someone like Thanos from Avengers would look odd in a documentary but because its a superhero movie we assume that's normal.

Self driving cars to my knowledge do none of this.

Stereo depth perception is not that important. People born without it end up being able to navigate pretty well, dodge walls, climb stairs, etc. It just takes practice.

Fun trick: Look at a photograph with one eye closed. Your brain will do ... something and the picture will look 3d

That would be the same trick it does when you look at it with both eyes open...

About 10 years ago I went to an eye doctor with a small object in my eye, and she had to cover it after removing the small object.

Driving back home with 1 eye was scary even though I was going much slower. It is possible to drive with 1 eye, but much much harder than with 2 eyes.

Did you drive any further than just the way home? I would bet most people would adapt quite quickly.

No, I wasn’t experimenting, but I haven’t had any car accidents in my life, and I find that more valuable

In these modern times yes, there's little selective pressure keeping depth perception sharp. That doesn't mean most of our ability to judge depth is from monocular clues (though that could be true).


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