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I saw a picture of a dancer spinning in a netizen's blog. What is the principle?
First of all, I want to say that under normal perspective, there is no ambiguity in this picture, and the dancer's real rotation direction is counterclockwise. The reason is the rotation direction of the shadow. If you notice the dancer's reflection, you will find that the toe movement track in the reflection is an ellipse. Under the normal visual angle, this ellipse is the trajectory produced by the circular motion of the toes under the visual error of near and far. The upper part of the ellipse is the trajectory when the toes are far away from the observer, and the lower part is the trajectory when the toes are close to the observer. In the picture, the lower part of the ellipse is hidden, and in the upper part, the movement direction of toe reflection is counterclockwise, so the dancer's rotation direction is counterclockwise.

Of course, under this observation, we can still imagine the upper part of the ellipse as a part close to us, but this imagination does not conform to the normal perspective. Because in such a spatial perspective, the picture should not present a bright background of "brighter in the middle and darker around".

The next question is, why does this picture "look ambiguous"? Although I know nothing about the process of making pictures, through observation, I think the problem lies with the dancers. Most people think that after all-black processing, the dancer's rotating images are exactly the same in two directions, which leads to blurring. But I found that if you only look at your legs, it will be easy to change your direction (in fact, many people focus on their legs to change their direction); But when I look at the whole person, it is difficult to see the counterclockwise direction. So I think, in fact, counterclockwise and clockwise images are not exactly the same (especially in some rotating joints, such as buttocks ...). At the same time, I also think that the dancer's own part is a clockwise image, which leads most people to see clockwise rotation at first sight.

I have no strong argument at this point. My reason comes from my skill of changing direction every 180 degrees. In fact, as long as I see the shadow on the upper ellipse sliding from right to left, I will immediately see the counterclockwise rotation; But when the shadow is hidden in the ellipse below, I can easily see the clockwise rotation, so I realized the direction change of 180 degrees. In addition, I also noticed that at the beginning of the picture, the toes were near the right end and moved to the right end, just when there was no shadow, so most people would think that the dancer was rotating clockwise for the first time. If I have photo processing software such as photoshop, I can do an experiment: cut the first few frames of the picture so that the toes are at the far right at first, and then the shadow slides from right to left in the upper part of the ellipse for the first time. I think a considerable proportion of people will think it is counterclockwise at first sight after this treatment.

Finally, discuss the relationship between the rotation direction and the left and right brain. To sum up, if I judge correctly, this picture is composed of a human image rotating clockwise and a reflection rotating counterclockwise. Because the human body is completely black, it is reasonable to think that the human body rotates counterclockwise (both directions are reflected in the two-dimensional image as swinging from side to side, and there are at most local deformation and "unimaginable" problems), but because it is reflected on the floor and becomes an ellipse, there is only one direction, counterclockwise, under the illumination background with bright middle and dark surroundings.

The following question is, why do you say that the right brain sees things clockwise and the left brain sees things counterclockwise? I think it should be like this. Because the right brain is good at observation and spatial imagination, when we don't pay attention to the direction of the shadow, the right brain will be keenly aware of the subtle difference between clockwise and counterclockwise in the image, thus outputting the signal of clockwise rotation. After noticing the direction of the shadow, the left brain draws a conclusion through reasoning that clockwise is impossible. At this time, the right brain needs to synthesize the shadow and human image and build a model of counterclockwise rotation in the brain. At this point, you can see that it is counterclockwise. The reason why we can't switch freely is because the brain has a famous persistent suggestion effect on visual signals. In order to overcome this effect, a trick is to blink continuously when you want to switch directions, which will help to weaken this hint effect.