I've acquired a new superpower (opens in new tab)

At a local bar they had a game machine, and if you got a high score on any of the games, your tab for the evening was free.

One of the games was a "spot the differences" between two pictures with an ever decreasing timer for each round. Using this trick I was able to easily surpass the high score, and garner a crowd watching me perform this mind numbing feat.

Probably my peak fame right there.

We got a magic eye book when I was maybe 6 - some time early elementary school. After learning how to do it, and also trying it by crossing my eyes to see an "inverted" image, I started doing it whenever I saw some repeating pattern IRL. It was most interesting when it was slightly uneven, for example a fence with sloppily applied vertical planks. Doing the magic eye would make it seem like some of them are closer to you than others. Eventually I tried the same on those "spot the difference" games since well it seemed kinda obvious to try, and I was blown away that it accidentally gave me that "superpower". I think that was pretty smart for a 6yo. Has only gone downhill ever since. ;-)

I discovered this trick independently about a decade ago, to use cross-eyed viewing to easily spot differences between two similar images. Like you said, the parts that mismatch appear to shimmer and be unstable, making them obvious.

However, I feel eye strain from doing it, so I prefer other methods. 99% of the time, I do https://en.wikipedia.org/wiki/Blink_comparator instead, just switching between two images with zero flicker and zero displacement offset. Also with both eyes, it's easier to spot certain kinds of subtle differences like color shifts, JPEG-like compression artifacts, tiny differences in antialiased renderings, etc.

One benefit of the cross-eyed method, though, is that you can difference videos. But the use case for that is rarer than differencing images.

If you've done Magic Eyes, this is straightforward. Was able to get all 3 of the test images quickly.

This is with focusing beyond the screen. Focusing in front of the screen is something I am unable to do, and not for want of effort.

Also, your eyes might accidentally do this if looking at tiled patterns, e.g. wallpaper.

Relative image size (e.g. view distance) is important.

When auto stereograms were all the rage in the late 80's I had a program on my Mac Plus that let me make/edit them and I used to edit for hours WHILE looking at them in 3D. Then one time I was walking down a hallway with a repetitive wallpaper pattern, my eyes did the thing, the entire hallway appeared to shift in front of me, and I stumbled and fell. Still to this day my eyes will sometimes automatically snap into 'alternate' focus when viewing a repetitive pattern.

Note that there is a difference between crossview and parallel view. See this image [0] and try to overlap them. Depending on what you see in the foreground, that is the type of view you're able to see.

Basically, it determines whether the 3D view you're seeing from the stereoscopic pair is convex (pops out of the page) or concave (goes into the page). It is of course possible to learn both views but most people naturally see one or the other. You can go to r/crossview or r/parallelview depending on which one you see.

[0] https://i.redd.it/g5ilwgk99r781.jpg

This whole "just cross your eyes" thing has never worked for me, not once. I've seen these strange patterns printed on the backs of notebooks that supposedly make some sort of 3D effect when you "just cross your eyes". Later, when I saw similar images online, I was able to at least visualize these hidden shapes by opening the image in photoshop, duplicating the layer, setting the copy to "difference" and moving it left or right. The regular texture would eventually disappear and the shape would emerge. It's still a mystery to me what it feels like to view these the intended way though.

This I think is an underrated tool for scientific visualization. I made a matplotlib add-on that will let you make stereogram plots of data, and use this cross eyed or parallel eyed techniques to see it in 3D. https://github.com/scottshambaugh/mpl_stereo

I'm frequently surprised by the amount of seemingly ordinary skills I picked up as a bored child that other people didn't. This was an obvious way to solve those "spot the difference" pictures in magazines.

I wonder what skills other people picked up that I didn't.

Some recent example of things I shared:

+ When your belt buckle hangs a little loosely on the front of your pants. You can hook the buckle's prong onto the front button of your pants and it'll stay put. So many people are excited to learn this.

+ Putting a jacket or any open-front garment on quickly. I saw someone struggling to maneuver their second arm in a tight jacket behind their back. I explained that if they hold their jacket out in front of them, put their hands in the arm holds, and slide their arm in further as they swing it around their body they'll get it on in a moment. It's also more stylish. They were so surprised.

Good old vdiff: https://catb.org/jargon/html/V/vdiff.html

Visual diff. The operation of finding differences between two files by eyeball search. The term optical diff has also been reported, and is sometimes more specifically used for the act of superimposing two nearly identical printouts on one another and holding them up to a light to spot differences. Though this method is poor for detecting omissions in the ‘rear’ file, it can also be used with printouts of graphics, a claim few if any diff programs can make. See diff.

An interesting variant of the vdiff technique usable by anyone who has sufficient control over the parallax of their eyeballs (e.g. those who can easily view random-dot stereograms), is to hold up two paper printouts and go cross-eyed to superimpose them. This invokes deep, fast, built-in image comparison wetware (the same machinery responsible for depth perception) and differences stand out almost immediately. This technique is good for finding edits in graphical images, or for comparing an image with a compressed version to spot artifacts.

For anyone who wants to learn this, try this way using your finger as a helper.

Put the images in front of your eyes.

Bring your finger between your face and the image at almost middle of the distance.

Now look at your finger.

Move your finger back and forth and notice the background (where your picture is)

While doing this, notice that at a particular distance, the images in background will perfectly overlap each other.

That's your moment.

Pull out your finger and look at that image.

--

It worked on everyone I have tried to teach. You may always need help of your finger or a tip of a pencil or whatever. But it's lot easier to get those images to merge this way.

Wait, that's not crossing your eyes, it's uncrossing them. Ordinarily if you look at something nearby, your eyes aim at a common spot. But when viewing a stereogram, you need to convince your eyes to aim at a spot more distant than the subject.

This is easy with practice, however IMHO it helps to be significantly nearsighted. Then you simply take off your glasses, and can look at something nearby with infinity focus, which is naturally associated with uncrossed eyes.

I don't know whether it's possible to train yourself to diverge your gaze, i.e. stereoscopically see images that are separated more than your pupil distance. Certainly I can't do that.

Front page of HN. Funny to imagine thousands of people sitting in the office crossing their eyes at their computer screen right now.

This reminds me of the 4D maze. Let's see if it's still around... yep: https://www.urticator.net/maze/

I tried using it, like the author, to visualize and investigate 4D data. But my stone-age brain, regrettably, seems limited to intuiting only 3 dimensions.

Weird timing. I dunno why this works but I've been using it to see mice.

You see, I noticed that I have a mouse problem in my garage. I figure if I've seen one mouse, there are probably more. So, I stood on some stairs in my garage and crossed my eyes to sort of blur the scene. It allowed me to catch movement more quickly and I was quickly watching multiple mice run around the edges of the area.

I have a true vision-based super power.

my vision is so bad with nearsightedness that when I take corrective lenses off, I can focus on an ipad mini screen within 10" of my face and perceptually it is the same as focusing on a distant movie theater screen. No straining, eyes totally relaxed.

With the lights off, it's better than being in a theater. I tried an ipad pro in the Apple store and it felt like I had my own personal unfairly huge IMAX screen.

An alternative technique called “divergence” [1] (pulling your eyes apart) is significantly less straining on your eyes than crossing them (“convergence” [2]) while being equally as effective spotting differences, even on video. It’s also what your eyes naturally do when you watch stereoscopic 3D with tinted glasses—the stereoscopic images are pulled out (divergence) not pushed in (convergence/cross-eyed). I’ve been doing this since I childhood. If you get good at it, you can watch side-by-side 3D videos in 3D with just your naked eye (e.g. VR)! I believe there’s a reddit covering the more prurient variety of that.

[1] https://en.wikipedia.org/wiki/Vergence#Divergence

[2] https://en.wikipedia.org/wiki/Vergence#Convergence

I didn't know this had a name or was considered a skill. I've done this since the 90's when those magic eye books became popular.

I even managed "impossible mode" in 2 or 3 seconds.

Who's been doing this since they we're maybe 7 years old :)

If you can easily cross / unfocus your eyes, it also allows you to see 3D images easily. Try:

https://www.reddit.com/r/ParallelView/

https://www.reddit.com/r/CrossView/

I've heard about this before, but I've never actually managed to do it until just now. I needed to sort of "tune the parameters" a lot so that my eyes were crossed but also focused, since I've had a lot of trouble actually getting them in focus when I'm doing it, and the effect isn't as pronounced as I expected it to be

Every Asshole knows this trick for targeting spacecraft

https://youtu.be/XGdjKvivJA8?si=lRrfl6rHzAsE7nEO

I've used this to quickly read through a few hundred page documents given to us only as a scanned pdf which was too low quality to run ocr (at the time) on. The sleazy counter party was very upset when I came back with notes on them not adding the changes we asked for on the drafts they sent back within minutes of them sending them back.

On Amiga, there was a 3D demo with this trick. You can "jump in" your eyes to the proper position if you hold your finger at the place where the 3d image should appear, ca. 10 cm close to your eyes, and focus on it. You'll be cross-eyed without knowing it.

Also, I was using the trick as a "cheat" when I played Tiny Lands on Nintendo Switch, it's a "spot the difference" game, but with 3d landscapes (you can rotate and zoom): https://www.dekudeals.com/items/tiny-lands

I've got -7.5 myopia/nearsightedness in both eyes, with astigmatism. As a result, my eyes can easily go out of focus to do Magic Eye or this type of thing. The bonus superpower is, if I take my contacts out and get really close up on something, it's like I'm looking through a microscope; if I happen to have glasses on, sometimes I can also catch the light and focus in just the right way to further magnify what I'm seeing already zoomed in. In those instances I see whatever's reflecting through the glasses, so mostly eyelashes and skin/pores, but it's fascinating nonetheless. Can't see a damn thing beyond the tip of my nose without corrective lenses though LOL

You can use this same technique to view glasses-less 3D (stereoscopic) images. It's also fairly easy to create your own. Take two photos but offset the camera lens by approximately eye-width. Open the image editor of your choice and place the images side-by-side. View the composite image cross-eyed and you are now viewing a 3D scene.

Also worth noting there are 2 versions of this kind of cross-eyed focus depending on whether your eyes are focusing on a point in front of or behind the actual image. This determines which side the left and right eye images should go on in the composite. I find it easier to focus on a point in front of the images but IME most examples online are for focusing on a point behind the image.

that's why they mirror one of the images in "find the differences" in puzzle competitions, e.g. USPC https://wpc.puzzles.com/uspc2024/

The stars puzzle helped me find a speck of dirt on my phone screen.

It blows my mind that every human doesn't know this. I figured it out as a child. The easiest way to prevent this is to have one of the two images be slightly tilted. I can rotate my eyes but it's much harder than crossing them.

As I understand it, Magic Eye stereoscopic images were originally developed by brain scientists studying how we see in 3D (i.e., how the brain processes 2 eye inputs into a 3D image).

There were two competing theories:

1. The brain first does a recognition pass (that's a house, that's a person, etc.) and compares the two eyes to see which objects have moved.

2. The brain compares the two eye inputs first, at the "pixel" level and figures out which pattern of pixels has moved, then afterwards, applies recognition to the resulting 3D image.

Magic Eye would only work if #2 is the correct theory (because in Magic Eye, there is nothing to recognize until AFTER you convert to 3D).

Add some trigonometry, and this superpower has historical military applications:

  https://en.wikipedia.org/wiki/Stereoscopic_rangefinder
  https://www.youtube.com/watch?v=cbXyAzGtIX8

I didn't spend the time reading the comments to see if this has been said already, but crossing your eyes works but learning to diverge your eyes is better in some situations. It's easier to learn to cross because you can just look at your nose, and you can bring together two images that are further away by crossing than by diverging. But diverging is the way we are normally (when looking across the room say instead of at a book in front of us) so it's more comfortable.

For whatever reason, it was far easier to do this on my phone than on my laptop, even after trying to fiddle with various combinations of zooming in/out and adjusting my distance to the screen. In some cases, it can also be easier to do without glasses (if you customarily wear them) as it makes the defocusing easier.

The effect itself is basically similar to the Magic Eye stereograms (https://en.wikipedia.org/wiki/Magic_Eye) which by themselves are pretty neat; your brain can rapidly detect subtle offsets in random patterns to reconstruct depth cues. In the case of “spot the differences”, the shimmering is due to the irreconcilable disparity between left and right images, which manifests visually as a glaringly obvious “unphysical” apparition - e.g. flickering between the left/right images, or appearing to be out-of-plane with the rest of the picture.

Yes that top reddit comment is right. Years ago those magic eye pictures came out and at first I struggled to see the 3d pictures. Eventually it became trivial. Then somewhere I noticed it worked really well for those spot the difference puzzles. They often come on kids menus at some restaurants. I could spot all ten differences in seconds and my kids would be amazed. I finally tried to teach them but they found it hard to do. But ya in reality what this girl did was super easy and not challenging in the slightest. When you do it and cross your eyes inwards you cross your eyes until the pictures line up in the middle. The majority of the picture will be clear but any spots that are different will seem blurry. The trick is just not focus on anything in particular and just cross images until they clear up. Then make note of where it seems blurry. Cool trick but definitely ruined spot the difference puzzles for me.

What's astounding about this is simply the staggering amount of visual processing your brain is capable of doing in parallel.

To really blow peoples' minds use both hands to tap the differences, keeping the left hand for the left image and the right hand for the right image!

From your POV the images are merged so your hands will look like they're tapping a single image, but from the audience's point of view you look like a savant with multi-attention!

That's amazing! Within 40 seconds I overlapped the two hard mode images and saw 8 shimmering objects set within a clear picture. It took me a little longer (maybe a minute), but the two impossible mode pictures snapped together and I saw a single shimmering star within an otherwise crystal clear photograph!

Cool site that I often use with this trick to impress some friends: https://www.spotthedifference.com/practice.asp?m=2&l=4

I cannot do this by crossing my eyes (focusing on a point between you and the image), I have a hard time getting the cross to stay consistent and it never really "locks in" for me. Instead of crossing my eyes, I unfocus them, effectively look through the image. Once I get the repeating part to overlap cleanly, after a second or two, my pupils adjust their focus and the image fades from blurry to clear in a really satisfying way and kind of "locks in" in a way that takes little to no effort to maintain. With a bit of practice, I can even move my eyes around and look at different parts of the two overlayed images without distrupting the effect at all.

I don't know if it's just my brain working differently or if a there is some confusion in the discussion between crossing your eyes and focusing through an item.

I've been doing this since I was a kid as well. When I was younger some restaurants would have video monitors with games on them, and one of them was spot the difference. I essentially maxed out the score and still held the highest score when I came back in town years later. I wonder if its still around...

I use this technique to get web layouts pixel-perfect with the mockup, just put both windows next to one another and superimpose them with your eyes. Works great. There are tools that do this by overlaying an image with 50% alpha but it doesn't work as well.

Last year when there was a bunch of fuss about Kate Middleton not having made any public appearances there was a minor flap where people claimed that a photo she'd released was just an edit of an earlier photo.

There was a tweet presenting two photos, one old and one purporting to be new, where she was holding strikingly similar poses. The claim was that the new one was just an edit of the older one. I used this technique and immediately the minor differences stuck out like a sore thumb- her hand was rotated more in one, her hair was laid differently, etc.

Well this is interesting. I was able to find the impossible one within 2 seconds without crossing my eyes. The easy one took about 5 seconds, 3 seconds for hard. I've always been hyper-vigilant with patterns (to a fault), so not sure if that's playing into this.

Is that what other folks are experiencing also? I see most comments are trying it with their eyes crossed, but what about without?

EDIT: ok I just watched the video. No eyes crossed. For the balloons one I beat out the girl in the video by 2-3 seconds. For the birds about the same. The skittles one tripped me up, couldn't find it. The other few I found around the same time, the lights at the end I didn't find in time either. It seems I'm quicker when there's not too many colors involved. Still that's spooky.

I use this for quick comparison of minor differences in code / data (e.g. tables, DNA sequences). Variations in indentation are highlighted also, by standing out of the plane.

It's good to hear reports of successful viewing. I've got a 3d / stereogram photo gallery app on the back burner; sounds like a reasonable number of people would be able to view it. There are plenty of guides on how to learn this; some are linked here https://www.reddit.com/r/CrossView/wiki/index/. You-tube used to have support for this; there are still videos tagged yt3d - just regular videos now, not interlaced.

Many decades ago, during my time as a NASA Space Shuttle engineer, a co-worker couldn't locate a rogue unbalanced parenthesis in a complex program listing, in the days when computers, instead of identifying a particular syntax error, would print lame error messages like "Something went wrong." Worse, we wrote programs by punching 80-column cards -- no syntax highlighting, no color monitors.

My co-worker printed two paper listings, one with the error, one without, and asked me to count parentheses as he was doing, over a dozen pages. But because I knew this "superpower" trick, I laid out pairs of pages and crossed my eyes. A few seconds later I found and circled the error.

"Ta-daa!" I said. He never forgave me.

I find it very easy to overlap images, I think because of the eye exercises I had to do as kid.

I don't recall what these exercises were for, but there were two:

1. Stare at this image of two incomplete cats, and merge them together into a single complete cat: https://www.google.com/search?q=eye+muscle+cat+card

2. This strip of cardboard has a number line on it. Put one end half way down your nose, perpendicular to your face. You will see two lines. Merge them at their furthest point, then merge the next nearest point, repeat. (I think this is called the 'Brock String Exercise', but can't find an image similar to the one I recall.)

This did not work for me. I was able to invoke a middle image, but there was no shimmering. After I found the difference the old-fashioned way, I realized that the middle image showed the distinguishing feature as it is on my non-dominant side.

When I was little I was doing this on carpet patterns because they suddenly become 3D. It was very fun. But I never realized it could be used for this purpose.

There is also one other similar funny ability I have: Vibrating my eyes. I can willingly vibrate my eyes. I don't think it's any useful ability as I just see shaky when I do it but it's fun to do in front of people. It scares some of them. There is actually a community about it in Reddit: https://www.reddit.com/r/Eyeshakers/

Nice to see someone discover this! I've always been partial to spot the differences and crossview images - I am able to cross each eye independently of one another, which makes overlapping these sorts of images very easy. For example, I could cross my right eye, while my left stays perfectly still. This causes instant double vision, and relaxing how crossed the eye is lets me line the images up very quickly. It's fun to do in places with a repetitive wall texture, too - seeing something in real life while adding a faux 3D effect on top of it is kind of trippy. Probably my most useless skill, but a fun one regardless.

Jokes on you, my eyes don't work together so I can only see out of one at a time!

I am so remarkably one-eye dominant (my left) that the first image just looks like the right cat to me about 98% of the time. Every once in a bit I'll see the flicker, but even then it's faint and hard to notice.

Thanks to this article I have successfully taken a two-hour break from HN and my computer screen, on behalf of the massive headache I acquired at the same time as the aforementioned superpower.

More things you can use this for:

”Are these two things the same size?”

”Are these things that are supposed to be evenly spaced actually evenly spaced?”

”Are all these things straight/at the same angle?”

”Is the wallpaper pattern aligned everywhere?”

”Is that surface using a repeating texture?”

I have strabismus and have always been good at these games. I wonder if it helps me for similar reasons? The examples on his page were pretty easy for me, though the universe one took maybe 45 seconds.

Instead of crossing your eyes and attempting to focus, what can really help relax your ocular muscles is to do the opposite: look "past" the images into the distance until the images overlap.

This same technique can be used for a 3D effect: https://old.reddit.com/r/CrossView/

Any recommendations for when you can't get the images to quite overlap? I feel like I can get 75% of the way there, but then they start going the other direction. I can do magic eye easily.

Wow that's interesting — trying to cross my eyes produces hellish jitter.

I suspect it's because my left eye is slighty lazy.

But I was able to superimpose the right cat picture onto the left one (it's a lot harder for the more complex sky resort picture). It's pretty eerie, the right picture just slides right up the left one (I did need to figure out the right distance for it).

It doesn't help me pick out the differences though, I mostly only see the right picture, and if try to focus my left eye, the right picture slides out. Still, intersting.

I taught myself how to free-view stereo side-by-side images on my phone but after I did so I found that I would get strong stereo impressions involuntarily just looking at certain repeating patterns in normal circumstances like watching TV with side-by-side images or when looking at patterned wallpaper.

My visual system is pretty weird in general so I don’t know how common an experience this is with others. It’s not bothersome at all because I know what it is but it was a little startling the first few times.

By the way... There is a nice old good article about making real time stereograms.

https://developer.nvidia.com/gpugems/gpugems/part-vi-beyond-...

I have no idea how that is supposed to work. If I cross my eyes I see 3 images and the middle one is a superposition of the two but I don't see any part standing out.

Open the image in the Gimp. Duplicate layer adjust opacity then drag it sideways until they line up. Toggle layer visibility.....

Wait, why isn't there a service for this? Or is there?

I'm amazed that there's so many comments and yet not a single one is pointing out that the coffee bean is missing from the _right_ side of the image. Not the left.

Incredible! This technique is also used for the 3d visualization of protein structures, it was called "cross viewing": https://imgur.com/cross-views-are-commonly-used-to-view-prot...

You cross your eyes to get the two images to line up, hold it there and then try to adjust the focus of your eyes. It's a neat skill to have.

The superpower being writing clickbait titles for trivial posts

I remember doing this as a child on our TV that had a picture-in-picture setting. I would set the same channel twice and cross my eyes pretending that it was 3d TV.

Wow! It really works. The missing bean "pops" out at you. The hardest part is getting your brain to focus on the cross-eyed virtual center image.

de-clickbaited: to spot minor differences between two images, view them like stereograms

Autostereogramming it doesn't help me lol

If it's perfect, the overlapping regions just merge in color, i.e. the cat's paw becomes off-white. If it's not perfect, I still have to attend to which parts are popping in and out. In both cases I still have to compare the merged view to the left and right hand sides.

Although it is very nice for illustrating each eye's contributions to the merged view. Just not an attention-saver.

You can also use this to display 3-D images. You have a stereographic projection of your image (like those kids' view-masters) and then just cross the eyes and look at the middle image. Only since the 2 images are slightly different, you can have the middle image be 3-D. It takes a bit of practice though and causes eye strain (at least for me)

It's not a great way of showing the image, but it'll do in a pinch.

I found it almost instantly, which was by dumb luck.

But in the following few moments, seeing two nearly identical photos side by side soon made me think of stereograms, since I'm into them, and have shot a few in my lifetime.

I then used my eyes to overlap the images.

In binocular overlapped view, the difference loudly draws attention to itself, because it flickers between the two eyes.

It's almost as if there were a blinking LED saying "here it is!"

There's a video game of this called QuickSpot. Back in 2008, this was one of my favorite games for the Nintendo DS. It trains the ability to spot little differences quickly. Sadly, outside of winning a barely known video game, it's not a super useful skill.

https://en.wikipedia.org/wiki/QuickSpot

Back when I studied media and communications, around when the first avatar movie created a hype for 3D movies. Me and a couple of friends decided to try and make a 3D movie for class. We made a custom camera rig out of metal brackets and edited the movies cross eyed since we had no actual way of editing in 3D, worked like a charm, altho, it was really hard to get the 3D right.

Dammit!

I can't uncross, now!

https://www.youtube.com/watch?v=qaz2hxZLycY

I've done this on these game machines 30 years ago when I was 10. I'm baffled that there are still people who have to figure this out.

“Impossible mode” was the easiest for me - took a few seconds. Probably due to the aspect ratio and the size of the images on my phone screen.

I have been doing this forever, if you get the high score over a certain threshold you can get a free game (400,000?)

I would usually get accused of memorizing all the pictures.

You will get bored or a headache before you stop getting free games using this technique.

You can get stifled by the older machines with faded CRT screens. The newer LCD (that's how old these games are...) are usually better to play on.

This blog post was nostalgic. Growing up in TamilNadu India, a Tamil weekly magazine called Anandha Vikatan use to have one page of stereogram where an object will be hidden in it. I found it so exciting to do the eye crossing trick to find the image pop up, made me feel great as a kid when my family could not find the image :) fun times

This whole time it's been a Magic Eye problem? Geeeeez. This would've won me so many random challenges over the years.

i didn't use the trick, or read what it was until after i tried for myself. it took me about 7 seconds at a conservative overestimate...

then again, i did outperform my entire national cohort at school in almost every subject by a wide margin... an outlier.

the trick however is very clever, but it wont work in more complicated scenarios where attention to detail matters.

EDIT: after doing the first one near the top i tried the rest. with a bit of warm up its very fast. no tricks needed. maybe a relic of playing these games when younger and having a "once in a generation" level of learning power coupled with training it when very young when learning speed is multiplied by a huge scale factor. i had to zoom the last one, but the other two were incredibly fast, close to immediate. sub second.

EDIT2: the warm up was doing the first image once after reading the first few sentences.

EDIT3: this is not a superpower.

What an unexpected and fun read to find on HN.

Bonus tip: while focused on the overlapping image in the middle, jiggle your screen, and the diff will move around while the rest remains static. This helped me solve the impossible challenge instantly.

I'd love to learn more about the underlying mechanism here. Anyone can point me in the right direction?

I've always thought that ability to unfocus the eyes might be related to mental focus. If you have difficulty staying on topic, or a diagnosis of ADD/ADHD or whatever {I'm not equate these} AND find it really hard to do magic eye images... Then please take part in my 'totally scientific study'TM.

Am I weird for almost immediately spotting the missing bean as soon as the image showed up? It certainly would've taken me longer to adjust my eyes for the cross-eyed trick.

Even the impossible one was harder when trying to use the cross-eyed trick than just visually comparing the two by quickly moving my eyes back and forth.

The most mind-bending example of the magic eye effect I've found is people using it in videos.

Of course, the internet being what it is, someone made a version of Bad Apple with it: https://www.youtube.com/watch?v=RLuGJGyCS90

I that not the entire point with those "find the difference" pictures? To teach kids how to do just this ?

A ton of comments in here saying this is easy/old hat for them.

Well I'll be the one to say this blew my mind. somehow creating the third middle image, being able to relax my eyes and even scan around this composite image actually made me giggle out loud on my laptop, a very rare occurrence. Thank you to the author.

I can get the overlap to lock in, but the simmering effect is weak and difficult to pick out. Probably still faster than exhaustive search, but I don't feel like I'm at superpower level. Maybe my brain favors one eye more than the other? I know I have different glasses prescriptions for each eye.

Almost certainly a lot of the people saying they are ”crossing” their eyes are actually ”uncrossing” their eyes; focusing the eyes straighter than what would normally happen on the surface where the image is laid out.

This is also how the legendary ”Magic Eye” books were supposed to be viewed. Not by crossing the eyes.

This is effectively how Pluto was discovered (not cross-eyed, but with a tool to help): https://airandspace.si.edu/stories/editorial/finding-pluto-b...

I used to use that trick with some arcade game that was popular in bars in 00s Spain. People were just impressed!

Using this trick, I wrote a program for 3d plot visualization in the 80-s. It computed two projections of the plot, as if looked by left and right eye, and printed them side to side on the thermal printer. If you looked hard enough, you can bring the two pictures together and see a 3d picture!

Is the technique to this exactly the same as the technique to view a 3d stereogram image?

I was able to get a 3rd image to be clearly visible in the middle doing this, on the 2nd image I could definitely seem some spots appear that lead me straight to 3 of them but didn't work for me on the other 2 images.

I've been using this to do quick source code diffs for years. I started back on the days of printouts.

The first (easy mode) image worked like magic - I crossed eyes, let them rest and the differences shimmered in a very static 3rd/middle image. Couldn't for the life of me rest my eyes on 2nd and 3rd though. [edit - tried to zoom out, worked on 2nd image too]

Doesn't work if you have lazy-eye, unfortunately! I generally just see whatever my stronger eye sees. It might be a good exercise to improve my weaker eye's sight, though.

I spent weeks doing this, looking at stereoscopic (?) images of protein structures, while a grad student in molecular biophysics. I got so that I could see the overlapped images pretty much instantly. But I'm having a hard time getting it now, even on the easy one.

I tried crossing my eyes, but it’s not working for me; I keep seeing things blurry. Maybe I’m doing it wrong. However, I solved the first two puzzles. For the last one, I just guessed randomly. My guess wasn’t exactly correct, but it was close, just a little distance away.

There are some great examples and detailed explanations IMO of this general phenomenon in the book "How the Mind Works" by Stephen Pinker. It essentially discusses how your brain is doing statistical work to build the 3D model from these 2 stereoscopic images.

I picked this up during the magic eye craze of the 90s, and I will never not find it hilarious how people get shocked at my ability to find the differences. I always share the skill too, it’s one of those things people find impossible until they get it and it’s easy.

Doesn't work for me. Just like stereograms. I just don't know how to "tell" my eyes to cross. Maybe similar to how I didn't figure out until I was 20-something how finger snipping works. Maybe by the time I'm 50 I can cross my eyes...!

One of the coolest "immediate novelty" things I've read in a while

Sent it to my whole family

The way your mind locks in and focuses on the middle one seems a testament to the brain processing behind vision we take for granted

Neat. If this isn't working for you, try on mobile. It's easier if the images are small.

This trick had been used in practice to detect fake banknotes and coins, with a device like a two-sided periscope. It allowed a bank worker to put a real coin on one side and the tested sample on the other, so that any differences can be immediately apparent.

I recently learned about this from one of my favourite German podcasts: https://freakshow.fm/fs286-zurueck-zur-vision (Starting at 2:50h)

I always feel like I'll permanently see cross eyed if I keep doing that. It doesn't help that I was accidentally hit in the head by my double's partner racket in tennis and spent like a minute or two walking around seeing double. Not fun.

Claims have been made (outside the medical mainstream) that regularly practicing crossing your eyes helps stave off presbyopia. One does get better at seeing stereograms with practice, so it seems like it at least improves some type of muscle control.

Amazing! Tiny bit of feedback, it might help to add to the article that there's 1 difference to spot in impossible mode (the other two difficulties say there's 3 and 8 diffs respectively, which is useful to know when attempting).

I couldn't do the cross-eyed thing, but it took me maybe 20 seconds to spot the difference just by looking at sections of the images. But I'm not sure that would have worked had the missing bean been buried in the denser part of the photo.

My Piano teacher used to have this book on her coffee table with images like this. You could blur and cross your eyes, and the image would combine to become 3D.

But I never knew this technique could be used to spot the difference between images. Very cool discovery!

This is how I help my family when they're stuck on "spot the difference" steam games. It also takes literally any fun out of them, the actual game has to come from not (just) spotting differences, because that task is trivial.

Learned it from Magic Eye books too! But using it for spot-the-difference is way more practical - saves so much time compared to staring at two pictures. Kind of feels like having X-ray vision for finding differences

I can't seem to do this, it seems impossible for me to cross my eyes. Hmmm.

I appreciate the sentiment but "overlay the images by crossing your eyes" receiving that kind of incredulous reaction is really funny and kind of sad for me. I hope it's just amateur editorializing.

It's funny, it really works.

Was also able to spot the difference on the coffee beans image: https://i.imgur.com/0TcWvWJ.png

Very cool. After a minute I was able to focus on the false “third” image of the overlap of each side, but I couldn’t notice any difference. It did appear to be closer to me than the screen though.

This is called 'vdiff' in the Jargon File: https://jargonfile.johnswitzerland.com/vdiff.html

This is also how I used to do Magic Eye images when I was a kid. Although the stereoscopic image was inverted on the z axis, it was a lot easier than to cross eyes by looking further out into the distance

I've used this method as a diff tool for years. Be it code or product comparison, like CPU specs, or these "spot the difference" images, crossing the eyes can be really useful.

Fascinating!

I was able to grab focus for the first image. Found it hard for the second and third image. I focused on the first one and then scrolled down to the second and third - that made it easier for me.

Wow so cool! It really just bounces out. I used to love those Magic Eye books. There's something meditative and calming about that sort of focus. This post was really cool!

After a short period of training, I got to the point where I can see the third image which I can focus on. However, the differences are very subtile and don't stick out at all :/

my grandmother used to tell me if i went cross-eyed too much, they'd get stuck like that. it took me until a google search at age 15 to see that was an old wives' tale...

Validates a claim in the predictive processing paradigm. The diff between actual and expected is what matters to error correction. That's where all the relevant information is.

Some repeating tile patterns like stripes will cause my eyes to do this automatically and it's really weird and annoying because everything else gets blurry. Fun trick though.

Remember the old magic eye posters? Like the static you'd see on an old untuened TV, in monochrome (but some odd colour like puce, cyan or bile)

"It's a schooner"

Feel like people who are old enough to remember that 6 week period in the 90's where those Magic Eye 3d Stereogram pictures were everywhere have a big advantage here.

That's so cool, thanks for sharing this. I managed to do all of them, in the impossible one I identified correctly the area, but couldn't pinpoint the difference.

You can also now free-view stereo image pairs. Congratulations.

I instantly knew as soon as I saw the image. It's the Magic Eye trick. The same one you use to view stereoscopic images. The differences will just flicker.

I feel old knowing https://en.wikipedia.org/wiki/Magic_Eye

This does not work even in the slightest. When I cross my eyes, the image becomes wayyyy too blurry and its hard to keep it in the center for more than a second

It works for me like every tenth time I cross my eyes to look at a "spot the difference" picture. I don't know how it works for people instantly.

That's how I used to help my grandmother when I was a little child. She always used to remark that the last one (5th) was the most difficult to find.

I was hoping to get a new superpower today, but when I was young I was cross eyed. I got this corrected through surgery and can no longer cross my eyes.

Funny. I crossed my eyes and found the difference before reading the article.

Somehow expect most people to know this, but I guess that's not the case.

i certainly used this trick back in my college days, prompted by a similar technique for "seeing" 3d stereoscopes on a computer monitor. I feel like i learned it somewhere on Eric Weisstein's Mathworld because the 3d objects viwer app let you split the image into two stereo images. Unfortunately java applets have been banished from the internet landscape.

I'm not sure this works if you have astigmatism

This trick is interesting to me as someone who mixes records. I (and mixers in general) have a lot of different tricks for making mono audio objects in the mix feel stereo or have a sense of width or depth.

Examples include:

1. Delaying the left or right channel by a few ms (Haas effect).

2. De-tuning one of the channels by a few cents

3. Boosting an EQ band on one channel, with a complimentary cut on the opposite channel.

...and many more.

These are usually very subtle changes that our stereoscopic ears have no problem detecting.

In any case, when we need to do some forensic searching for possible differences between two near-identical channels we'll invert the polarity on one channel and then sum them. The resulting delta sticks out like a sore thumb and highlights even then tiniest differences between the files.

So it's fascinating to discover that we can easily do something similar with our eyes to find the differences.

What has happened to me? How am I able to create a combined image and have it be -stable-? This totally works. I’ve been mutated.

Stereograms have been used in structural biology publications for many decades now. Google 'stereogram structural biology'

I used to play this game with a friend when we were at the pub, once we start struggling spotting the differences we know we’re drunk

I can produce the third image by crossing my eyes, but one eye dominates and all I see is a cat with three stripes on its head. :(

I’ve tried this in the past without luck, but suddenly I can do it now after reading about the subtle “shimmer” effect. Very cool!

As a kid I learned to see stereograms in 3D by crossing my eyes. Exact same technique. Comes in handy for Mars images today.

Those kinds of puzzles are part of the CCAT test (annoying test for "cognitive aptitude") and I used this trick.

It just feels like I'm going to harm something if I leave them crossed more than a split second and I stop.

That's fun, it worked for me as well. I could spot the difference in all images, including the 'impossible mode'.

I've never been able to 'lock' the two images together like this with my eyes focus. That was incredible.

Please stop using that RSS icon on your "Subscribe" button if you don't intend to provide an RSS feed.

I knew some people who used it to compare aerial photos (though generally a special device with mirrors was used).

half-off-topic..

i have ~1 diopter shortsightness. Was less before, slowly going up. So screens are getting blurrier. Have glasses but still try avoid using them.

If i put the (flat edged) TV remote control at about 10cm from my face so it horizontally shadows lower half of both eyes, i see perfectly (without any glasses).

go figure..

Semester 1 when getting a BFA in painting the prof told us to do this to help tighten composition

As someone with a lazy eye it's kind of hilarious to me that this isn't common knowledge.

human stereo vision processing has incredibly sophisticated noise filtering capabilities that are still hard to replicate in software. The shimmering effect people report is essentially your visual cortex highlighting areas where the stereo correspondence fails.

I was really good at the random dot stereograms. This is a really cool recasting of that skillset.

I can do the 3d images in kids books but cannot make it work for spotting the difference

Am I the only one who cant do that? Or does me having aphantasia has anything to do with it?

Every 80’s kid knows this trick from those old books where you cross your eyes to reveal images.

I found two differences in the last picture. Turns out the second was dust on my monitor. XD

Wait, this is news? that's how I always solved those puzzles, I thought everyone did that

I was able to do it for the first time after reading this webpage with the technique, cool!

Dumb question: How do I cross my eyes? can someone explain this method in detail?

As usual, this doesn't work for me because I don't have stereoscopic vision.

I've been doing this for years and always considered it my secret little hack!

Image worked but when I tried with text it did not work. Is there some trick to it?

Great read. Not sure it was worth being stuck cross-eyed for the evening, though.

This is exactly the same skill needed to see 3d pictures (aka autostereograms).

related: if you have a side-by-side 3d image/video you can do the crosseye to see it in 3D without any special display/glasses.

it's just that your eyes are gonna be tired after a movie session.

I can overlap. But still need to match overlap with anyone.anyway interesting

Easily defeated by arranging the images vertically.

Or at least, makes it a LITTLE bit harder.

1. Get a bit closer to the screen so you can see both images clearly.

2. Now, cross your eyes and aim to overlap both images.

3. Draw the rest of the fucking owl.

Seriously. Ever since my physics teacher in high school tried to get the class interested in stereograms, everyone and every article I see talking about it treat "crossing your eyes" as an atomic, trivial step. It isn't. I for one have no first clue how to do it, it's not a distinct operation I know how to perform. Perhaps this is because I am nearsighted and wear glasses.

Still, I wish articles like these focused on explaining how to do the whole cross-eye thing, because once you master that, everything else becomes instantly self-apparent and doesn't need further explanation (I know because I did manage to accidentally cross my eyes once or twice while looking at a stereogram, so I know how the effect looks like).

EDIT: FWIW, I compensate by using another trick for diffing documents with Mark I Eyeball - get them printed on separate pieces of paper, put one on top of the other, and hold in front of you with some bright light behind you (Sun, or your phone's flashlight, will do). Not as good as crossing your eyes, but something I can reliably do.

Learned this at a young age with the Highlights magazines at the dentist.

This is a game mechanic in one of the trials in Ace Attorney Chronicles.

Corporate needs you to find the differences between theses two images

I've discovered I can't cross my eyes. Any techniques ?

using the same trick, watch this video: https://www.youtube.com/watch?v=zBa-bCxsZDk

You can also use this method to watch 3d movies without vr googles.

This is incredible! Works unbelievably well. Thanks for sharing!

Wow. That’s insane! With the trick I can somehow solve them all!

Who's been doing this since they were maybe 7 years old :)

Very cool. Even the impossible mode one was relatively easy!

This makes my eyes hurt.

And it's also not really a usefull life or even primitive skill, just a byproduct of our double eyeballs, which are meant for redundancy and depth measuring.

Pass.

(Although it was cool to do it once and see the third image in the middle.)

For some reason when attempting this my neck starts cramping

It's a sailboat!

Hah, old trick, I read pull requests this way for years

Super cool. It works for me. I love magic eye stuff.

yep it’s just like those magic eye pictures that were big in the 90s, once you learn how to see those instantly you can do this trick also

Wonderful! with your write-up, I was able to “see”.

TIL you can learn super powers on the internet

Woah that is amazing how quickly I was able to apply this.

So cool!

This is the coolest thing I have seen on here!!

all i managed to do was make myself very dizzy

Wife was not impressed. That hit home haha.

A bunch of math is like this too.

dang it i just did this on a zoom meeting. hopefully no one saw me trying to cross my eyes

This is literally a superpower

Damn, it works exactly that well.

ITT: everyone telling us their stories about how great they are at stereograms. We get it, you're all super special.

I can't believe this has 1587 points (already). Has no one here grown up with those Magic Eye books?

Do people not know about this?

This is exactly why I read HN

The "impossible mode" example of a starfield is literally how astronomers have found new bodies (planets, asteroids, comets, supernovae, etc.) since the invention of photography.

Typically this utilised a negative photographic plate (so that bright objects appeared as dark against a light background making their presence easier to detect for our visual system), and the plates were rapidly flipped. The object which moved or suddenly appeared and disappeared was the new element.

This works for nearby moving objects (planets, asteroids, comets) which would track against the stellar background, or distant variable ones (supernovae, variable stars, pulsars) which would appear and disappear (or brighten and fade) over time.

Now of course this is automated and direct sensor readings can be interpreted, but the childs' game was at one time Best Scientific Practice.

Described here, "How are asteroids discovered?" <https://catalina.lpl.arizona.edu/faq/how-are-asteroids-disco...>.

There are archives of these astronomical plates, and there are projects which utilise older observations so recorded to make new discoveries even now (or in the recent past). I'd first learned of this visiting the Lick Observatory on Mt. Hamilton near San Jose some years back. Key was that old plates were used to compare current observations using the same 'scope, an otherwise now-archaic instrument but not without its advantages. As I recall, the astronomer doing the study was the same one who'd made the original plates many decades earlier, and was doing this as his own retirement project. There's a description of similar work (including Lick observations) here:

"Astronomy's Photographic Glass Plates: Demonstrating Value Through Use Cases"

<https://journals.library.ualberta.ca/istl/index.php/istl/art...>

On plate archives:

"Preserving an Astronomical Legacy"

<https://skyandtelescope.org/astronomy-news/preserving-an-ast...>

"APPLAUSE: Archives of Photographic PLates for Astronomical USE"

<https://www.plate-archive.org/applause/info/>

(dylan604 noted this use case earlier: <https://news.ycombinator.com/item?id=42657956>)

A parallel processor is you!

This is just incredible!

Wow! That was amazing!

okay, this is so cool. thank you for the new superpower!

It's a sailboat

Fuck it does work...

I cannot believe this is unknown to so many people that it's news

Not working for me

Wow, I didn’t know I could do that!!

I’m not sure how this new learned trick, can apply to other stuff… but I’m happy I tried it and it did amazed me haha thanks for the share dude!

Come on, I've been doing this since I was like 4 years old, this can't be news for anyone, Am i right???

Thanks! now I have a migraine

and now my eyes are stuck this way forever

I came into this rolling my eyes thinking he acquired some ability to learn and be humble or some programming or management topic... the typical bs you see on hacker news.

Boy was I wrong. This is an actual super power.

A new way of doing git diff, leave both versions of the code side by side and cross your eyes.

tldr: blogger learns how to cross his eyes for fun and "profit".

Would AI not be able to crush this game?