To reduce eye strain, don’t cross your eyes, but relax them (so, the other way). Instantly clear and snaps together as if magnetic.

A simple trick to doing this, in case it's not clear how to do it, is to try focusing on an imaginary point behind the screen as you look at the images. You will see a third image between the two start to come into focus. Now relax your eyes and look at that image. Simple, and quite a bit more relaxing than crossing your eyes.

The only disadvantage to this method is that it seems there is a limit to how wide the middle image can be, i.e., the original images may not completely overlap.

If you do want to cross your eyes but do not know how to do it, do the opposite of the above: try to focus on an imaginary point closer to you than the screen as you look at the images. This method is far more taxing on the eyes though.

I was still unable to do this, not sure what I am doing wrong, but I can't get over the sense that I am always directing only one of my eyes. I can't move them independently.

In this case, you aren't directing your eyes, but instead looking through the object. Start with your finger about 6-8 inches from your face, and look at your screen. You'll see two fingers. Now try to look past your screen. The farther you are from the object the harder it is.

> You'll see two fingers

I don't follow this part, still. Do you mean computer screen / phone screen behind my finger which I would try to look at?

I can switch between my left and right eye when I concentrate, but I can't see both at once. I can try to rapidly switch from left to right to back, but it takes a lot of effort, and it's never the same image at once?

My right eye is dominant, I can switch to left eye if I focus hard, but I can't merge both or can I?

Edit:

I tried putting a paper between my eyes, so right eye would only see right part and left eye only left, am I supposed to see the whole image at once then combining the inputs from both eyes at the same time? Because I can only see left or right, as if my other eye was closed, even though it's not.

I'm wondering if something is wrong with me.

For me, when I look past my finger, I see two and each is sort of transparent as the image of each eye is transposed over the other. Maybe you can only see one at a time?

Finally after some time of trying I was kind of able to see 2 fingers, but I think I have some sort of condition that means I mostly have monocular vision compared to binocular vision, which most people seem to have. I am still trying to understand how different I see things from most people. But this is something I never realized.

My brain really prefers to process from only one eye at a time, it seems. I wasn't able to get further than the 2 fingers even after more than a hour of trying. E.g. I wasn't able to duplicate anything on my screen.

Weird that I only now realize this after decades of living. I wonder what impacts has it had on my depth perception etc.

This is called “divergence” [1] and is 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

The only problem with divergence is that you can't go too much farther out than the distance between your eyes, whereas convergence works for larger images as well.

Convergence highlighted the differences for me in all four images.

Divergence only worked for me in the cat bear image. For the others, I could see a combined image but I could not see any differences highlighted, even though I knew what to look for.

The problem I have with this is that instead of the images completely overlapping they overlap a section in the middle. I can't get both images to completely overlap and am getting some eye strain from trying to force them.

By ready your comment, I just realised that this is what I’m doing instinctively, instead of crossing my eyes :o

I'll second the blink comparator method as a simple diff checker, or when comparing two chunks of code that are structured exactly the same way but somehow behave differently. (e.g. "what's the difference between these two functions" or "how is this yaml block different from that yaml block"?)

Line them up as two tabs in the editor, flip very rapidly between the two repeatedly, and usually the difference is apparent in 5-6 flips.

Fun fact: it's a different device, but the principle is the same as the device used in the documentary Tim's Vermeer. It results in the images overlapping between your left and right eye and you simply paint until the difference goes away.

A blink comparator would be a much better GUI tool for comparing photos (for example original vs. post-processed) than the standard "left half-right half" with a movable divider. Alas, it’s rarely used (eg. Lightroom’s compare view doesn’t have one).