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Cool Optical Illusions:

This blog displays lots of them, including my favorite, the color contrast illusion:

Would you believe that the seemingly blue tiles on top of the left cube and the seemingly yellow tiles on top of the right cube are actually identical? For proof, and more on this, see color the site. Check out the archives as well (linked to in the left margin, a few pages down).

UPDATE: Visitor Again wins.

FantasiaWHT:
That's great! I just set the rotating snakes picture as my desktop background. Anything to make the law students sitting behind me distracted!
2.28.2008 1:07pm
Archit (www):
Here and here Dan Ariely makes the connection between optical illusions and behavioral economics, both are about tricks your mind plays on you.
2.28.2008 1:13pm
Matty G:
Very cool. I actually prefer this version of the same illusion, only because it doesn't have the seemingly artificial use of the yellow and black "frame" over the whole image:

http://en.wikipedia.org/wiki/Same_color_illusion
2.28.2008 2:32pm
Visitor Again:
The devil's work.
2.28.2008 3:58pm
Duffy Pratt (mail):
This assumes that a "color" is defined by a machine measurement, and not by what the brain, via the eye, interprets. If you did color correction in photography, and relied solely on machine numbers, you would soon be out of a job.

The short answer to this "illusion" is that if a color "seems" to be blue, then it is blue. You can say that your eyes are lying to you. But why not say that the numbers are lying? Both statements are equally correct, depending on what you want to do with the colors.
2.28.2008 4:41pm
TGGP (mail) (www):
Check out this blog for more optical illusions: http://scienceblogs.com/mixingmemory/visual_illusions/
2.28.2008 11:35pm
Thoughtful (mail):
Duffy: The short answer to this "illusion" is that if a color "seems" to be blue, then it is blue. You can say that your eyes are lying to you. But why not say that the numbers are lying?

Try this: The short answer to this "illusion" is that if the pencil in a glass of water "seems" to be bent, then it is bent. You can say that your eyes are lying to you. But why not say that the pencil's shape changes as it enters water?

Was it your intent to make a general argument against the impossiblity of "illusion"? Because for your argument to make sense, that would seem to be the requirement.

The answer in both cases, Duffy, is that we've developed objective measurements for these sorts of things, and if we choose not to trust them despite the fact we developed them because we learned we can't always trust our senses, we're throwing out a lot of science and logic.

While it's true that I can't know with certainty that "my" yellow matches "your" yellow, I should be able to know that "my" yellow is still the same color just because changing its context makes it look blue. And that's what the objective measurements confirm. And I can even align my subjective assessment by simple adjustments, like masking the rest of the image and just focusing on the square of interest.
2.29.2008 1:00am
Duffy Pratt (mail):
Thoughful:

Go take a picture of a bride in a wedding dress at sunset, with the sun lighting one side of the dress, and the other side of the dress in deep shadows. A camera, or any other recording device, will show the brides dress as a warm color (orangish red) on the sunlit side, and it will make the shadows very blue.

If you had a job as a wedding photographer, and left the shots as the camera depicted the dress, you would stop getting much business in fairly short order. People see some of the color imbalance that is caused by the warm sun and the reflection of the blue sky, but they temper the colors by a process known as chromatic adaption.

On the one hand, the dress is "really" white. On the other hand, the camera or other measuring device sees it as a fairly wide range from blue to red, and that's what the dress "really" is. For human observers, its neither. As a photographer, I can assure you that in these sorts of situations the camera is wrong, and nearly 100% of the people who see a camera original vx. a corrected print that tempers the blues and reds will agree that the corrected print is a more accurate depiction of the scene.

To take an even more trivial example, in the dark all cats are black. Again, in photography this has a practical implication. Humans are very bad at discerning colors in deep shadow areas. If you have a picture of a wooded mountainside, you are safe at picking the darkest areas of the woods, the areas where you no longer care about detail, and setting them as black. This is so even though the camera probably still sees these areas as a very dark green, or a very dark brown (which is a kind of red).

In short, I'm not arguing a vague philosophical point, but a very practical point, one that I deal with almost every day as an amateur photographer. People are still the ultimate judge of what colors are, not machines. This has all sorts of implications for the science of color and color reproduction. And where people almost universally agree on something about color, for most purposes it makes sense to say that people are right.

As for your pencil analogy. If I were teaching a drawing class, I'd tell people that if the pencil seems bent, then its bent. I'd come to a different conclusion if I were trying to teach spear fishing. That's why I said that both statements were equally correct, depending on what you wanted to do.
2.29.2008 5:30am
kipp (mail):
This effect works for all kinds of colors. If you rotate the hue so that the left and right are green &red, you see red and green squares that are the same shade of grey.

-|
2.29.2008 2:47pm
Duffy Pratt (mail):
kipp:

They probably don't use the red/green as an example because of the number of people who are red/green colorblind.
2.29.2008 5:00pm
Patterico (mail) (www):
If I were teaching a drawing class, I'd tell people that if the pencil seems bent, then its bent.

Who did you vote for in 2004, Duffy, if you don't mind my asking?
3.1.2008 2:04am
Mike Gallo (mail):
I'm with Duffy on this one. Color is subjective - if you paint a wall with two colors next to another, and they both assume different perceived colors because of that, it is not an optical illusion, because the colors are what we see. I'd like to know what a machine would think of my house, as my wife and I used all intermediate colors juxtaposed to give exactly such effects as you walk or look from one room to another.
3.3.2008 6:52pm