Seeing is not believing －视觉错觉系列（翰唐整理）
Stare at the plus sign on the left for about 30 seconds. As you do this you probably will see some colors around the blue and green circles. After about 30 seconds, shift your gaze to the plus sign on the right. What did you see?
You probably saw a yellow and desaturated reddish circle. Did you notice that they were not there before you tried this experiment? Furthermore, unlike the colored circles on the left, the ones you saw on the right moved around as your eyes moved, which proves that these perceptions are the result of afterimages in your visual system and not some trick of the stimulus presentation on the right.
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Do you see a ball moving back and forth at the top and below this moving ball do you see two stationary balls?
Would it surprise you if I said that the moving ball is really one ball presented first on the left over the stationary ball and then on the right over the right stationary ball? This is true even though it appears like one ball moving across the screen.
This is an old and famous illusion called the Phi Phenomenon. If you think about it, you've seen this principle many times on movie marquees. The principle behind this phenomenon also lies behind the moving pictures you see at the cinema. Did you realize that at the movies you are looking at a rapid sequence of discrete images?
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Close your right eye and look directly at the number 3. Can you see the yellow spot in your peripheral vision? Now slowly move towards or away from the screen. At some point, the yellow spot will disappear.
Note how far you are from the screen when the yellow spot vanishes. Repeat the experiment looking at a larger and then again at a smaller number. Did you notice the difference in distance from the video screen when the spot disappeared?
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Examine these figures closely. Note the yellow areas on the left and on the right. Now step back from your computer screen about 6 or 8 feet and look at them again. Do they still appear as they did close up? Most people would probably see the shape of the right yellow area defined by the black squiggly line.
It is generally assumed that luminance is mediated primarily by the long and middle wavelength cones. There is little luminance contrast between the yellow and white areas. This implies that the excitations of L and M cones are approximately equal and the isoluminant contrast is mediated by the short wavelength cone system.
The filling-in phenomenon confirms that the spatial resolution of the short wavelength cone system is poor. Also, it demonstrates that areas of color, mediated by the y-b opponent system tend to be constrained by contours mediated by the long and middle wavelength sensitive cones.
Boynton predicted this illusion before he produced it for the first time.
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Take two small pieces of paper and place them on the video screen so that you can see only one of the steps. You will probably notice that this step appears uniformly bright across its horizontal extent. However, when you view all the steps at once, each one appears lighter on the left and darker on the right.
The dashed red line represents the brightness function of the average physical luminance of each step. The solid line represents the apparent brightness of the steps.
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This figure is from Dr. Irtel .
Take two pieces of paper and place their edges as close as possible to both sides of one red diagonal. Note that the red square in the middle is the same as all the other squares on that diagonal. Now move your pieces of paper to expose mainly the red squares on the other diagonal. On this diagonal all the red squares, including the center one, are again the same color red. Because the center square is common to both diagonals this exercise proves that all the red squares in both diagonals are exactly the same red color. Yet, when the pattern is seen as a whole, the two red diagonals appear different from each other.
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Which line is longer, the red one or the green one? Use a ruler to find out. You will probably be surprised.**
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**If you don't have a ruler handy, mark the length of one line on a piece of paper and compare it to the other line.
Ouchi Apparent Motion Illusion
If you look at the left (black/white) pattern you will probably notice that the vertical pattern in the center appears to move relative to the horizontal pattern in the surround.
This diagram was adapted from a design presented by a Japanese artist whose name is Ouchi.
If you see relative motion on the right pattern than you can conclude that the illusion also works with color patterns.
Perhaps not. It might be that if the lumiances of the red and green rectangles were equal then the illusion might not be as strong or prehaps would even disappear.
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Which colored block appears the largest and which the smallest? Take a ruler and measure them, you may be surprised at what you find.
I hope you measured the blocks. If you did you will have noticed that they are all the same size. Why then do they appear to increase in size as they get further away? Think about it. Maybe you can guess why.
Take close look at the pattern on the floor, walls and ceiling. Can you see how they tend to converge on to a common point in the distance? This is called perspective. The further way objects are the smaller the visual angle they subtend on the retina. However, if you were actually standing in the hallway containing the blocks the floor pattern and those on the walls and ceiling would not seem to converge. Another example is when you stand in the middle of rail road tracks (beware of trains) the parallel tracks continue to appear parallel for a long way into the distance even though they project onto your retina in perspective.
Artists are well aware of perspective and use it to create the impression of distance in their painting and drawings.
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