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.
Looking uncrossed at the images in the article on my phone, I can easily achieve the effect uninterrupted between fully stretched arms and about half that.
Is that true? It seems that our eyes are mechanically capable of looking in divergent directions, what's the reason that we're not able to "uncross" them beyond looking straight ahead?
(Edit: Anecdotally I can confirm for myself that I'm not able to do it, so wondering if there's anyone that can.)
From a control system standpoint, if you have one control that rotates both eyes the same number of degrees left or right to determine gaze direction and another to rotate both eyes the same (positive) number of degrees inward to control fixation distance, you can't specify the left eye rotated left of center and the right eye right, even if each eye physically can rotate that way. Not sure if that's how eyes actually work though.
That's because you have to find the distance between your eyes and the stereogram to make it work. Crossing your eyes is easier because your eyes can turn inwards far more than they can turn outwards, so it works at more distances.
