
VR optics and why inter-pupillary distance (IPD) means too many things - walterbell
http://tomforsyth1000.github.io/blog.wiki.html#%5B%5BVR%20optics%20and%20why%20IPD%20means%20too%20many%20things%5D%5D
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NoodleIncident
> It's actually astonishing how often developers accidentally switch left and
> right eyes in code – effectively having the ICD be negative rather than
> positive – and it takes ages to consciously notice, if ever. But your
> subconscious notices, and you will end up being sick, and you won't know
> why!

This is the most amazing anecdote in the article to me. Is the brain able to
use this reversed-perspective data to determine distance and scale, or does it
give up and pick a single eye? Is it like those glasses that flip your vision
upside down, in that you'll get used to it after a week, and have to readjust
to the way things normally are afterwards?

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scotty79
The effect of swapped cameras is sort of 3d-ish, but doesn't look quite right.
When you switch it to correct it pretty much instantly looks way better.

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tomjakubowski
Is it like how a Magic Eye picture looks when you cross your eyes to "match"
the sides rather than relax and focus in the distance?

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MrEldritch
Yes, exactly analogous!

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ebg13
The Valve Index has an IPD adjustment slider on the bottom. I can tell that it
does...something...but, with very wide margins, I can't tell what the right
setting actually is for my eyes. Anything +/\- a hell of a lot all just looks
fine. So maybe it doesn't actually matter that much.

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NoodleIncident
In this article's terminology, I think that slider controls "IAD", and the
reason why it doesn't change much is explained under the subheading
"Collimated images"

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ebg13
> _In this article 's terminology, I think that slider controls "IAD"_

Of course it does; obviously the slider doesn't move your pupils. But it's
meant to compensate for your personal average IPD. I think you're right that
the collimated images section probably explains the result, though.

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killjoywashere
As a pathologist, I spend a fair chunk of every day using microscopy optics
which are in every way equivalent to AR's IPD problem. The honest answer is
your brain has to learn to integrate those two images, and it will take months
of daily use for multiple hours, with associated headaches, neck aches, back
pain, etc to figure it out exactly right, but the trick is that by then, you
won't have figured it out exactly right. By the time everything snaps into
place, you brain will have also learned to snap it into place. My eye-brain
system gathers information through oculars in a way that others do not. It's a
matter of physical training, like so many things.

That's not to say quality doesn't matter. My undergrad is in physics, and the
quality of the oculars in a clinical microscope puts to shame pretty much
everything else I've seen. Even telescopes at the tops of mountains aren't as
nice, ergonomically.

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Fr0styMatt88
Somewhat related - I made the mistake of putting effort into learning how to
stereo free-view Google Cardboard / Daydream VR apps. At the time it was kind
of interesting what it felt like.

Except I must have made something super-sensitive in my visual system, to the
point where now it’s almost TOO easy to stereo-fuse repeated patterns — though
having said that, my visual system is kind of different I think anyway. Now if
I see repeated patterns I’ll start getting phantom stereo fusion if I keep my
eyes looking at one spot and relaxed too long. It’s not distressing or
anything, it’s just really unusual.

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RandallBrown
The article was interesting an informative, but it's really surprising to me
that all humans have the same sized eyeballs. I would never have guessed that!

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shadowgovt
What I'm most curious about is what mechanisms control that, given the most
obvious one to me---shining light through the developing lens and determining
if the retina is getting good image resolution---is obviously impossible
inside a body.

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throwaway34241
The actual mechanism might be something like that!

Nearsightedness is often caused by the eyeball developing to be too long. It’s
actually dramatically increased in some places in Asia, where 90% of
schoolkids can be nearsighted. [1]

There’s a lot of research pointing towards biological processes involving
contrast (which is probably a decent proxy for focus) being involved in eye
development, with maybe different visual stimuli (due to more time spent
indoors) being a potential cause of the increased myopia rates because it
doesn’t activate the specific mechanism in the right way. [2] [3]

[1] [http://healthland.time.com/2012/05/07/why-up-to-90-of-
asian-...](http://healthland.time.com/2012/05/07/why-up-to-90-of-asian-
schoolchildren-are-nearsighted/)

[2]
[https://www.sciencedirect.com/science/article/pii/S004269891...](https://www.sciencedirect.com/science/article/pii/S0042698917301372)

[3]
[https://www.nature.com/articles/s41598-018-28904-x](https://www.nature.com/articles/s41598-018-28904-x)

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scotty79
Does shortsightedness and astigmatism matter much for VR or are the hardware
lenses so strong that your eye incorrectness becomes just rounding error?

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navjack27
I have horrible far vision and can't see without my glasses in VR. I also have
a very pesky lazy eye and need a prism in my glasses lenses.

Whatever your eyes have you will bring with you in VR pretty much.

