I've acquired a new superpower (danielwirtz.com)

While intuitive, I’m not so sure. I look at the center line and slowly cross my eyes until the 3rd image slides into place and then I get focus lock. At no time do I feel my eyes uncross and go the other way. Hmm!

See my other comment about cross view vs parallel view, looks like you can do one vs the other and the author can do the opposite.

Both work equally well if you just want to spot differences. Cross-eyed view is somewhat easier to do, since people naturally cross their eyes when looking at something close to their face, but there is no natural reason for one’s eyes to diverge. But cross-eyed view also gives a subjectively smaller image, and is also not the usual way autostereograms are made to be seen.

For spot-the-difference, crossing your eyes is more effective and easier to "dial in" than uncrossing them. You're essentially making each eye look at the opposite image. If you try uncrossing, then you need to make sure the images are at the exact correct distance to cause them to overlap with that technique, because you can only uncross your eyes enough to look straight ahead.

I find that there are different techniques to seeing both.

If I stare at the image and cross my eyes until focus lock I get crossview where the image goes back into the page.

If I bring the image right up to my eyes and stare through it into the distance, then slowly move the image backwards into my gaze until I get focus lock, then I get parallel view where the image pops out of the page.

I have always wondered the difference between the two and why it happens. Thanks for shedding some light on it :)

EDIT: I have just managed to achieve both without moving my head or the image for the first time in my life! Just by trying to look further 'past' the picture into the distance, and then by slightly crossing my eyes and focussing at a point in front of the picture.

I have been trying to do this for 30 years, and it is only your explanation which helped me to do it. Thanks so much!

Note that for parallel viewing the left edges (or centers) of the images should not be farther apart than your own eye spacing aka interpupillary distance (IPD) sometimes just called PD.

That imgur may need to be shrunk depending on your screen for parallel to work.

I can do both pretty comfortably, but there’s a definite bias to parallel, way easier for me.

Cannot figure out what is the difference. I can focus on both seemingly without changing anything, even though they aren't both in focus at the same time. But I don't know what I'm doing differently, I just move my eyes up or down, adjust a bit, and that's it.

My thoughts exactly. Could be done with a git diff in a VR headset

I sure didn't expect there to be a plausibly work-related angle when I came to this thread

New business idea: git diff, but it uses Mechanical Turk to hire an army of cross-eyed diff spotters.

Somewhat related, but I've been using the Semantic Diff extension in VSCode, works better than standard git diff.

It sounds like you mock, but: you can do this, or near enough, and you can do it already, and it works really well!

0. Pick suitable tool. Firefox works. Notepad++ works. GIMP works. Emacs, not so much, but if you use Emacs, you know that you can fix this

1. Load file A into a tab

2. Load file B into a tab

3. Close all other tabs

4. Hold down the tab switch shortcut key and note the result

For images this is actually pretty decent and I've used it a lot. Good for figuring out what the differences actually are when your image-based tests fail, and similarly after making a speculative change. Let your eyes do the difference operation for you. That's what they're there for.

For text: you'd be better served by some other kind of tool. But text is just an image with letters in it, so the same principle applies. It does work!

(I've previously read a blog post, link to which I of course now can't find, about how old-style Photoshop undo was designed with this sort of thing in mind. Instead of working through the operation queue like normal people, it simply switched repeatedly between previous state and current state - the idea being that you'd make a change that you weren't certain of, then press the key repeatedly to see before and after. No need to think. Thinking isn't appropriate here anyway. Just let your eyes look at what they're seeing.)

I think thats the best part of this skill. Everyone did 'Magic Eye' images as a kid, but to be able to take that useless skill and apply it to something more useful and interesting is really cool.

I did until it was corrected with LASIK a year ago, and I could still do magic eyes, both with and and without my glasses on.

I have a small astigmatism (and wear glasses) and I was able to do it, but I feel like if I did any more today I'd have a headache.

Having similar acuity in both eyes made a huge difference for me. I'm somewhat astigmatic but was able to do this eye "uncrossing" trick just fine. I had a significant loss of acuity in one eye and that's what left me unable to do this (or to watch 3D movies).

Checking in!

I’m frequently baffled by how unaware most people seem to be about the absolute basics of how their eyes work. Like, people don’t even seem to be aware of how their stereo perception is largely made from two images, or any of the implications that has. I actively think about the two images maybe dozens of times per day.

I figured everyone because I had a puzzle book that instructed readers to do this.

Yup I used to do this with tile floors as a child.

This helped, thanks.

You can also just back away from the screen. For the 3rd one I had to back up about 6 feet from my monitor before it clicked into place, once it is in focus you can move closer to see the difference better.

reminds me of convolution operation.

I think you're still not doing it quite right. The effect is really quite obvious and pronounced.

To do it with crossed-eye view, try looking at your finger and slowly moving the finger closer to your eyes until you see a third image come into view in between the two on the screen. At some point your brain will/might let you focus on that image.

You dont 'tell' your eyes to cross, you just look closer or further away. Try looking at the image at normal distance, whilst imaginging that you are looking into the distance at a beautiful view or at the horizon on an ocean. It is this difference in distance focusing which causes the illusion.

The never work for me because my eyes don't work well together. Just not team players. I'm almost always looking through just one or the other, annoyingly usually the one that would least be preferable.

I have a slightly lazy right eye, so this has always come naturally to me, but I will say it's considerably easier to achieve the false focal lock on printed material— something about screens, even quality ones with high refresh rates, just isn't the same.

As a kid, I got a Magic Eye book and learned to see it by crossing my eyes (ie, focusing in front of the screen). I thought it was pretty interesting when I realized that I was seeing all the images inverted ("peaks" were "valleys" and vice versa) due to the way I was focusing. Alas, I never was able to see the images "correctly".

I can get the images to merge but the differences don't stand out.

I’ve done Magic Eyes a lot, but I’m failing on this. (However, I found the difference in the coffee beans picture reasonably fast without the eye-crossing trick, and before reading what the difference is.)

Maybe we are the opposite. As a kid, I could only do cross-eyed-focus-in-front-of-screen, but not "focus beyond the screen". Or a book at the time.

So I was able to see the 3D in Magic Eyes, but the 3D effect was inverted.

Today as an adult I am able to focus beyond the screen, but it's still much easier for me to do it cross-eyed.

I also got all the images in the post almost right away. But my eyeballs focused in front instead > _ <

I'm great at magic eyes / stereograms and have a ton of posters around my house with them, but I still had trouble with seeing the differences in the test images. I easily locked in my focus on the overlapping cat images but only one difference stood out to me. I eventually got them all but it wasn't that easy (maybe with practice I could get there). The differences are noticeable when I focus right on it, but when I'm looking at the whole image it's harder to tell what is missing from one eye.

Yup, I loved Magic Eyes as a kid. This was easy.

Nevertheless, I was astonished that "impossible mode" literally took me only 1-2 seconds to find the missing star.

Like, I knew our vision is good at interpreting depth from images. I figured it would be all right at finding large areas of differences. I had no idea a single freaking pixel could stand out like a sore thumb.

How do you do this focusing beyond the screen!?

I'm trying so hard to make this happen. Stare really far in the distance and then move the image in front of my face on my phone. No matter the distance between my face and my phone i can't overlap the images.

Focus in front of the screen is the easy one. How do you get beyond....

I can do magic eyes, but this technique doesn't work for me. My left eye is dominant enough that the whole image just looks like what my left eye is seeing.

I challenge you to use this refocusing ability to pass the Stroop Test: https://faculty.washington.edu/chudler/words.html

You are given a list of color names, written in font colors that are incongruous with the written word. E.g. "Red" will be written in a blue font, and "Green" will be written in a yellow font.

You have to say the font color and not the written word.

It's challenging, but it's yet another application where going cross-eyed confers an advantage.