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By on March 12, 2006, with 36 Comments

The squares marked A and B are the same shade of gray, yet they appear different. Read inside this article to get proof and explanation why this illusion occurred.

Colored Squares Illusion

Proof:

By joining the squares marked A and B with two vertical stripes of the same shade of gray, it becomes apparent that both squares are the same.

Colored Squares Illusion


Explanation:

The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray “paint” that belongs to the surface.

The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.

A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.

The “paintness” of the checks is aided by the form of the “X-junctions” formed by 4 abutting checks. This type of junction is usually a signal that all the edges should be interpreted as changes in surface color rather than in terms of shadows or lighting.

As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.

Comments

36 Responses
  1. Mel says:

    I’ve seen this illusion so many times. I’ve seen it as just a picture, I’ve seen it so you can drag a black cover thing over everything else but the two squares and I’ve also seen one where you click on the B square and drag a line over to the A and I still don’t believe it!!

  2. Dean says:

    I had to dig out xScope and compare the RGB values of the colors to believe it! Incredible!

  3. Meg says:

    That’s just wicked. I’m glad someone took the time to find the explanation for the way this works on the eyes.

  4. avn says:

    explanation? neurons responsible for detecting the contrasts.

  5. Osophos says:

    Do neurons detect the contrast? How? Don’t the rod and cone cells in the retina take in the emf according to contrast? Don’t the resulting patterns get sent up the optical nerve to the brain? Doesn’t the resulting pattern end up as “lines and dots” in the neurons? Would appreciate Avn giving a specific process step by step.

  6. Lauuuu says:

    The only way i see them as being the same is when i look up to the screen instead of straight on. However, I don’t understand how it makes sense otherwise.

  7. Hai says:

    whuuuuu…..im squninting like hell and i still dont get it!! #0

  8. M 2 the H O says:

    ive tried it on paint and its true, hard 2 believe, and even when we no they are the same colour we still cant see it,
    the explanation is 2 long 4 me 2 read so i wont bother,
    i just cant believe my own brain cud decieve me like that

  9. M 2 the H O says:

    hey if u tilt ur head so that ur looking straight up at the moniter so a + b are really close, u can see that they are the same colour

  10. MMM says:

    really weird!

  11. Nachos Yummy says:

    CoOoOl!

  12. Pandas says:

    my god, this thing is all over the internet. tell me that u guys have seen it b4

  13. Soupy says:

    holy fishpaste batman. that can’t be true. the colors are changed in the line connector thinger. i dont beleive it. MY BRAIN WOULDN”T LIE TO ME!!! just kidding… but it is pretty unbelievable

  14. Andrew says:

    Now THAT is a cool illusion. Well done!

  15. DoNTPLaYFaiR says:

    I really couldn’t understand the explanation, but I verified it myself with a tiny tool called eye dropper which gives you the colour code of any pixel you lay the pointer on. and HELL, both were #787878!!!

    It might be necessary not to believe in all what you see, yet you have to believe it only when its verifiable. If I couldnt verify it in anyways, I would’ve still thought they are not the same colour even after reading the explanation mumbo jumbo to me.

  16. harajukugirl says:

    dats awesome

  17. meeow says:

    very kOoL!

  18. I have the explanation!!!!!!!!!!!!!! says:

    The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray “paint” that belongs to the surface.

    The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.

    A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.

    The “paintness” of the checks is aided by the form of the “X-junctions” formed by 4 abutting checks. This type of junction is usually a signal that all the edges should be interpreted as changes in surface color rather than in terms of shadows or lighting.

    As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.

  19. Caz says:

    Whoa..! This is cool!
    I even chacked it and the colours of them two squares are #787878!

  20. Anonymous says:

    easier explaination for those who don’t want to read the long one ;)

    in the real world, this wouldn’t be the same grey, as B square is in the shadow of the column, so our smart brain corrects what he receives from the eye and optic nerve, and tells us that B square “must” (should?) be lighter than A square.

    a lighter thing in shadow will give the same wave-lenght as darker thing in light. But our brain won’t care of wave-length, will only care about what it must be in the physical world (non-screen). (think you remove the column!!)

    If our brain wasn’t able to correct like that, we wouldn’t be able to see things with shadow, thus wouldn’t be able to see 3D.

  21. oscar says:

    Illusions like this takes me to a sci-fi plot, where all you are seeing (and believing) is not as you think at all !! Don’t you wonder what could have done Escher exploring this tools to the limit?

  22. Anonymous says:

    After staring at the image for 3 hours and doing a complete analysis, i have finally concluded that my Boxers are Green!!

  23. Anonymous says:

    i’ve been looking at optical illustions all day. All i see is blue.T_T.

  24. Anonymous says:

    in photoshop select the b square and copy it and move it back and forth between the A and B. You can actually see it changing in front of your eyes. If you leave it in the middle it looks like a gradient.

  25. Anonymous says:

    :O!

    OMG!!!! i compared the colour values in photoshop. really can’t see both are the same colour even though i know the have the same colour value.

    O_O

  26. William says:

    It works if you invert the colors too

  27. moost146@hotmail.coo.uk says:

    The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray “paint” that belongs to the surface.

    The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.

    A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.

    The “paintness” of the checks is aided by the form of the “X-junctions” formed by 4 abutting checks. This type of junction is usually a signal that all the edges should be interpreted as changes in surface color rather than in terms of shadows or lighting.

    As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.

  28. Ady says:

    Strange but true indeed… I opened the image on Dreamweaver and compared the values… and unbelievably, the values are same (RGB 120,20,120) for both A & B. incredible!

  29. This is yet another proof for me that we actually don’t even know what we see so to speak. Our eyes are quite unreliable I would say. This proves to me that neighter Nessy or Bigfoot exist. People just don’t really know what they see

  30. Come on, this is great!

  31. jemmima says:

    I dont get it

  32. Eizabeth says:

    wow thats cool
    =)

  33. john john says:

    what a crap.

  34. Rick says:

    My favourite illusion EVER. Seriously.

  35. replay says:

    okay i get the whole thing the problem i want to use this trick for a project but theres no information on how to do it

  36. AG says:

    What Sorcery is this!?!?

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