Lately a animation of a woman has been going around. The animation shows a rotating silhouette, the catch is that it can be perceived to be rotating clockwise or counter-clockwise. It tends to be a bit hard to change the perception of the direction of rotation once one particular direction has been recognized (at least in my personal case), I've read that for some people it switches direction more or less randomly, after looking at it for a while.
I was curious as to why it works, whether I could reproduce the trick and if I could make myself see her rotating in one direction or the other at will.
The why it works is relatively straightforward. Whether the rotation is clockwise or counter-clockwise is impossible to say if it happens in the same plane as where the viewer's viewpoint lays and there's no feeling of depth. The brain needs the perspective in order to tell the direction for sure, perspective will make the objects that are father look smaller and the ones closer bigger, that will help the brain discriminate one direction over the other. The dancing woman has been created in such way that it appears to have some perspective, yet it's still ambiguous (and you can see things jumping strangely at rotation as a result of this composition, just pay attention at the magical stretch of the arm closer to the body when it passes in front/behind)
It's easy to reproduce, just look at this example I quickly put together. With perspective it can be easily said whether it rotates in one direction or the other. We can either display it by setting the viewpoint above or directly in front with with a large aperture angle that exaggerates the perspective.
Then, if we now we set the viewpoint in front, yet so far that the projection lines become nearly parallel so that we lose the sense of perspective. It becomes much harder to tell the direction of motion and it's even possible to see it going both directions.
Then regarding the choosing at will of one direction over the other... I figured out that given that the only thing preventing my brain from deciding is the ambiguity caused by lack of information that would bias some layer of my neural networks to decide clockwise/counter-clockwise... I went really high tech and starting moving my finger in front of the dancing woman in the direction I wanted to see her to rotate... that seems to solve the ambiguity and I can make her turn one way or another at will...
I wonder if the trick works for other people too.
Sunday, November 25, 2007
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6 comments:
Hey Ero,
Yeah, your technique seems to work for me too. Seems like the high tech finger technique disambiguates the situation sufficiently for the brain - nice!
The original animation is most definitely amusing... Seems like 9 out of 10 times, I start off seeing it "rotate" clockwise. It seems near impossible to "change direction" while still looking at it when it's "rotating" clockwise like this.
But on the other hand, on the rare occasions that I do start off seeing it "rotate" anti-clockwise, somewhere along the line it seems to "change direction" (usually under a couple of minutes into staring at it).
So perhaps there is an ongoing neural bias component that is continuously present in addition to any initial bias. I've also noticed that looking at the animation from the corner of my eyes (doesn't matter which eye) makes it easier to willfully "change direction".
Amusing indeed :-)
Cheers
Nishad
Looking from the corner of the eye works great for me too. I can reverse the direction of rotation with little effort.
Another trick to reverse the direction of rotation is by focusing on the shadow of the image.
Commenting on an old post, but the previous comment about the shadow is relevant. The shadow of one of the feet is *not* moving ambiguously, as the rest of the image is. This is because the shadow implies a specific orientation for the surface on which she is standing, and that defines one perspective in which she is rotating counter-clockwise.
Interestly, most people see her rotating clockwise at first. Then when you lower your gaze (away from the anatomical distractions...) and look at the shadow, it instantly reverses to counter-clockwise. It's neat how the brain automatically flips its perception when given an additional piece of information.
Further thought on my last comment.
This experiment says something about the hard-wired algorithm in our brains that takes a 2D image from our optical nerves and converts it to a 3D model with which our conscious mind can work. The first observation is that it fills in missing details without us being aware of that (as is evident by us not perceiving the dancer as being of ambiguous orientation and rotation).
The second is that when additional details appear that conflict with the current model, it can instantly change the output model. This doesn't seem to happen very often, which is why the dancer is so surprising to us.
This might be similar to the neural phenomena that Escher always played with. He didn't use animation, though.
Wanna know how to see it spin both ways? Just look at it from 2 different angles. Either tilt the monitor or move your head sideways.
My friend and I were looking at it and both of us always seem to see the exact opposite and then suddenly it occured to us to just turn the monitor towards the other guy, We discovered at that instant our perception of how the girl moved changed. It seems to be a trick with the angles.
But It still could have something to do with which half of the brain gets to process it first. Or it could be due to the differential delay in the propagation of the optical impulse to wherever it gets processed. Dont know what it is called. Hey we are just amateurs.
- Hemchand
http://chad.skillda.com
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