Some stereo vision illusions

[Audio Version]

While engaging in some stereo vision experiments, I found myself a little stuck. I stopped working for a while and started staring at a wall on the opposite side of the room, pondering how my own eyes deal with depth perception. I crossed my eyes to study certain facets of my visual system.

I got especially interested when I crossed my eyes so that the curtains on either side of the doorway were overlapped. I wasn't surprised to find my eyes were only too happy to lock the two together, given how similar they looked. I was, however, surprised to see how well my visual system fused various differences between the two images together into a single end product. It even became difficult to tell which component of the combined scene came from which eye without closing one eye.

I thought it worthwhile to create some visual illusions based on some of these observations. To view them, you'll need to cross your eyes so that your right eye looks at the left image and vice-versa.

Plain, no illusion.

This first figure, above, is just for practice. Both sides are identical and should form a scene that is 3D of a doorway with curtains on either side. The curtains should appear recessed slightly behind the wall. If they appear in front, your eyes are not properly crossed.

Missing Curtain.

This second figure presents an interesting dilemma for your eyes. (If you have trouble focusing, try using the upper or lower corners of the door frame to get your eyes locked into the scene.) You know there are two curtains and your vision expects them, but one is missing from just one side. You may find the "phantom" curtain floats left and right and even forward and backward as your eyes go searching for its "other half". Interestingly, you'll find that much of the time, it doesn't appear to be "half as green". Rather than appear like a dimmer version of the curtain on the right, it should typically appear to have exactly the same color. It's as though your eyes ignore the black background and accept the green curtain.

Missing dots.

This figure is quite fascinating to me. Two dots on the left are half trimmed away in one eye and another dot is totally missing in the other. As before, your visual system should accept the fact that dots really are there and that you're just having trouble finding them with one eye each. Again, the "phantom" dots are just as purple as the ones that have perfect mates on both sides. Note how the phantom can be darker or lighter than its background without impact on this effect? Also, you'll find the phantom dot on the right floats back and forth as your eyes try to find its mate, yet the same is not true for the half-dot phantoms. They seem to be solidly fixed horizontally by the rest of the dots. Interestingly, I find little chunks of these phantom dot-halves seem to come and go as my vision tries to decide if they really should be whole dots or half dots. It almost seems to compromise by concluding that they are "flatter" dots - ellipses that are as wide as the other dots but a little shorter, vertically.

Missing stripes.

This figure one is fairly straightforward. One curtain has phantom stripes just like the other one's. The stripes appear to veer back and forth a little. They also do appear to be lighter, most of the time.

Mismatched stripes.

This one is a bit more subtle than the others. The number of stripes in one of the curtains is not the same in both images. This is because one of the curtain views has the stripes significantly farther apart. Your vision will probably fight over different interpretations. One is that the lines have varying spacing from outside to inside. A variant of this is that the curtain is actually a round column. Another is that the lines are somehow behind or in front of the curtain. What's most interesting to me is that my eyes never seem to give away the fact that there is a different number of lines for the left (5) and right (4) versions. My eyes are sure they find matches for each and every line.

Bloody curtain.

This figure presents a somewhat different illusion. One of the versions of the right curtain appears to have a blood-red stain dripping down from the top. Again, the colors don't really blend. Your vision should pick one color or the other. It most likely will pick the red "stain", though I find with some effort, I can make the red stain almost completely disappear. This only works for me when I stare right at the top of the right-hand curtain, where the red is. If I stare at the bottom of either curtain, the red stubbornly remains.

Miscolored curtain.

This final figure is much more difficult to reconcile, I find. Because the right curtain's red and green alternatives are so different, my eyes frequently try shifting to find better candidates. If I stare at the top or bottom of that curtain, it helps to lock them together, though, suggesting that the corners are stronger features than the vertical edges of the curtain, alone. The color of this curtain never seems to stabilize. Curiously, when I stare at the lower part of the right curtain, it seems more likely to settle on green, yet the rest of the curtain above vacillates between green and red even while this part is stable. When I stare at the center, the whole bar is likely to go back and forth between the colors, but it's almost equally likely that the bar will appear to have shades of both colors at the same time. And blinking is almost certain to instantly disrupt whatever color it starts to settle on.

I hope you find these stereo visual illusions to be thought provoking. It seems to shed light for me on the task ahead as I continue development of stereo vision software.


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