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.
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!
I had never done the parallel view before either- spent 5 or so minutes at it and finally got it. For me it's still takes a fair amount of effort to maintain it (unlike cross view that takes effort to stop seeing it instead) but the 3d looks way more impressive somehow. Like the Toronto crowd one- hadn't seen so much depth in a "magic eye" before
I believe it’s better because it is more natural to eyes. You may also want to play with the perspective. E.g. the hall here looks much better from 0.4m than up close on my phone due to the picture’s perspective. https://triaxes.com/docs/3DTheory-en/522ParallelCrosseyedvie...
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.
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.
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