Sunday, October 21, 2007

Explanation of the spinning woman demo

In view of feedback I've received, I feel obliged to make it clear that the demo presented in my offbeat article entitled Right brain versus left brain [display] is merely an amusing and innocuous hoax, which has nothing to do with the viewer's brain. My presentation of the demo and my subsequent comments were deliberately facetious: a big joke! It would have been neither more nor less silly to claim that viewers who see the girl spinning in a clockwise direction have right-wing political beliefs, and vice versa.

Only the final two sentences in my post [where I suggest that interested observers should examine the individual images in this animated GIF, using a graphics tool such as Fireworks] are to be taken seriously. The truth of the matter is that everybody sees exactly the same animation, which does indeed change directions suddenly, before reverting to the initial direction. The demo is ingenious in that the 34 fixed images composing the animation have been designed and drawn in an exceptionally skillful manner. Viewers are intrigued by the fact that, when the woman changes directions, she does so in such a smooth and seamless fashion that we have the impression that this change has taken place—like the perception of beauty—in "the eye of the beholder". This is an illusion. The change has well and truly occurred in the animation, not in our brains. In most of the silhouettes in the animation, various visual features of the woman—including her face, her breasts and her pony-tail hair—provide explicit clues as to the direction in which she is spinning. But I have extracted a unique image, shown here, in which all these visual features are missing:

Here, the viewer is unable to decide between two perfectly plausible possibilities:

— We are facing the silhouette of a woman poised on her left leg and spinning in a clockwise direction.

— The silhouette is a rear view of a woman poised on her right leg and spinning in an anticlockwise direction.

Consequently, when this pivotal image occurs in the animated sequence, the direction of spinning can be either maintained [as is usually the case] or reversed [exceptionally] in a totally seamless fashion. And this is why the woman seems to spin in a clockwise direction for a while, then suddenly change directions, and finally revert to the initial direction.

Some readers might not be familiar with the concept of animated GIFs. Back in the early Internet days, people often included an animated image of this kind in the email contact section of their rudimentary websites, showing a letter being folded, place in an envelope and slipped into a mailbox. Animated GIFs provide a good example of software gadgets—a little out of fashion nowadays—that are relatively laborious to create, but simple to borrow and use.


  1. Neither was my comment - it was supposed to be a joke as well!

    Thank you for your explanation, at least now I understand how it works.
    It's a great animation.

  2. But why two people seeing the gif at the same time perceive the change in rotation at different times?

  3. I disagree with the above explanation. It's a very simple and effective, yet well thought out, illusion. The direction of spinning is actually ambiguous - in other words you can either interpret it to be clockwise or counter-clockwise at any point in the animation. So yes, the direction of her spinning does depend on your interpretation of the image.

    To understand how this illusion works, you could imagine a simple square rotating such like the silhouette. The first still animation of this rotating square would just simply look like a square. With each subsequent frame of the animation this square will shrink width wise ever so slightly until it is a mere line. At this stage of the animation the square would have rotated 90 degrees. As the square continues to rotate, it will expand width wise until it is back to its original square shape. So the square has now rotated half a cycle as the back of the square is now facing you.

    Either the square rotated clockwise or counter clockwise, whichever way your brain interpreted it. The direction is ambiguous because the element of depth has been left out here. In real life, as the square rotates, the side that has come closer to you would have appeared to become larger. But in this case, the square only shrinks width wise so that the direction of rotation is unknown.

    If you look at the individual stills, in each of them, the leg that is lifted can either be her left or her right. Infact with a bit of brain control, you really can make her "rotate" in the other direction at will.

  4. I agree that the spinning is in the animation. I'm not a computer scientist, animation expert, or Ph.d; but if you scroll your screen up and look just at the animation the shadow of her feet, you can tell it's programed. The shadow of her lifted foot either passes to the front or the rear of her body showing the circular motion. Without the spinning body in sight, you can plainly see when she changes direction!

  5. I find it amazing that innocent observers are still commenting upon this illusion, of zero interest. It's a cunningly-designed animated GIF. That's all there is to say. As they say in France, we should not seek midday when it's already two'clock in the afternoon... I repeat: This animated GIF of a spinning woman is a totally uninteresting illusion. I end up regretting that I ever joked about this gadget.