Count the colours

Here's a killer optical illusion for you:

Spiral optical illusion

The "green" and "blue" spirals in this image are actually the same colour. It's like the classic same colour illusion, except, you know, colourful.

(I don't know how this illusion will affect people with one or another kind of colour-blindness. Please comment if you don't see the effect, so the rest of us can all float theories about why you're so weird and unlovable.)

After marveling at this for a while, I spent 15 minutes straightening the illusion out into parallel stripes:

Blue and green stripes optical illusion

As you can see, it still works.

If you look closely, you can see that the leftmost and rightmost "blue" vertical stripes look distinctly greener than the rest of the "blue" stripes, because they're only flanked by magenta on one side. But they still look pretty blue, despite being exactly the same colour as the "green" stripes.

There are only three colours in this image - magenta, orange and green.

(See also.)

20 Responses to “Count the colours”

  1. TwoHedWlf Says:

    You are so full of shit... I'll prove it! I'll just go into MS paint because that's all I have here at work, use the eyedropper tool and...Wait...


  2. Anne Says:

    For most people it's probably true that there are only three colours in the image. But not necessarily everyone: if you're viewing the image on a CRT TV, say, or through a VNC connection that uses JPEG compression, or through a video screencast, it's quite possible that there really is blue on the image. JPEG compression, video compression, and analog TV standards all take advantage of the fact that you need less detail in the hue channel than you do in the other two, and in practice they have effectively less resolution in that dimension. So for those, the same perceptual weirdness that makes the lines look blue is incorporated into the compression scheme in such a way that there may actually be blue pixels on the screen. Which says something about how hard it is to design a good compression scheme...

  3. Daniel Rutter Says:

    Well, in a sense, there's blue in every computer-displayed hue that has a B value above zero, because all colour computer screens, and all colour televisions, use RGB phosphor-dot or subpixel triads to render colours.

    But you could use exactly three colours of marker pen, or paint (or Lego bricks) to draw either of these images on a piece of paper (or Lego baseplate), and the illusion would still work.

    (The two images themselves also contain exactly three colours of pixel; they're not lossily compressed, so there's nothing in between. There's actually also black in the colour table for the spiral image, but there are no actual black pixels in the image. Your browser or image-viewing program will probably create fuzzy intermediate hues if you magnify or shrink the image, as will the other things you mentioned, but none should appear in normal 1:1 magnification.)

    The important part is that no matter what combination of pigments or little dots or light wavelengths are rendering the colours, the "green" and the "blue" in the image are actually exactly the same. It's only the interleaved orange and magenta that create the extraordinarily strong illusion of different colours.

  4. Stark Says:

    For those who want a nice visual confirmation that the blue bars are in fact green - take 4 post-it note and create a mask that only allows you to see one bar at a time on the straightened image (took me about 30 seconds to get right, you could do it on the spiral image but I imagine it might be a bit harder to construct the mask). Put it up to the screen so you isolate a single 'blue' bar and indeed they are green. Also fun to leave one side off the mask so you can slide it along the blue/magenta bar to the last bar and watch it suddenly snap from blue to green.

  5. atarisan Says:

    There are four lights! I mean colors... I mean that's just eerie.

  6. rbluff Says:

    I'm red-green colour blind, and the "blue" stripes look like washed out green to me. So there.

  7. TwoHedWlf Says:

    Yeah, but, rbluff, you're even more broken than the rest of us people who are hallucinating colors that aren't there.:P

  8. Buckermann Says:

    To my shame I must admit that at first I didn't trusted Dan's word that the green and blue parts are in fact both green. Clearly they are different colours, everybody can see that.
    I thought that maybe he wanted to teach us a lesson on authority, that we shouldn't trust the word of some stranger on the internet. Even when he has himself proved as a credible source for all kinds of information in the past. Then I started Gimp and instead I learned that Dan is still a nice guy who would never mislead his audience.

    I'm sorry Dan! I should never have doubted you! :(

  9. ImaFish Says:

    "I'll just go into MS paint because that's all I have here at work, use the eyedropper tool..."

    That's exactly what I did.

  10. matguy Says:

    Semi-color blind impression:

    I see kind of sea-green and magenta stripes in the center bar, except right at the termination between the vertical stripes I see a super small blue vertical stripe, just a sliver, probably equivalent to 1 screen pixel wide. (The stripe is to the right of magenta, left of green, but not on the next termination when they're opposite; although in full disclosure, my right eye is also half blind, but it doesn't change if I close that eye.) It could be an artifact of my monitor too, when I blow it up the absolute physical size of the tiny blue stripe stays the same. Another monitor artifact I notice is that if I drag it around the screen, the area with magenta and green turns to a fairly solid blue bar as I presumably wait for the LCD pixels to snap to their commanded brightnesses (along with the fairly solid green and magenta bars above and below it.) If I focus away from the image to blur it, I see kind of blue-ish and sea-green stripes, then from about 10 feet away it turns more blue.

    Similar to rbluff, I'm slightly red-green color-blind. My version generally shows up as making it hard to tell the difference between browns and slightly green-browns, but it's not like I have any excuse for running red lights.

  11. corinoco Says:

    Architectural Fact: If you paint your walls and ceilings like this you will go stark raving mad within 3 days.

  12. TwoHedWlf Says:

    No way, 1 day at most.

  13. shmahoo Says:

    You'll be mad even before you finish painting the first wall !

  14. matguy Says:

    I would think that the initial decision to paint a wall like this could be an indication of pre-existing issues.

  15. Boxy Says:

    I am color blind but have over the years discovered that there are many more types of color problems than there are tests. (the closest test I ever took was a yellow/blue test, but even that wasn't very accurate) So I have long since given up on testing.

    So the Illusion?

    First band I see brown and purple.
    Second band I see red and green.

    Forth band i see green and blue.

    /Offtopic kinda, I was once arrested. Had my prints taken, picture taken.
    Cop stares at my pic for a loong time. Asks -
    "what color are your eyes?" my reply -
    "well, I'm color blind but people tell me my irises are blue around the edges and green closer in."
    "Ok, I'm entering brown."
    "Er, okay?"

    And yes, thats how they look. :)

  16. AdamW Says:

    The two people in the house with me utterly refused to believe this. Even after isolating the colors in GIMP (no Paint handy...), still flatly refused. Which just goes to show how good it is!

  17. RichVR Says:

    I proved it to the girlfriend with a post-it note that I cut with a razor. When she finally saw it she slapped me in the head and went to bed.

    Needless to say it wasn't the good "I'm going to bed" thing.

    Thanks, Dan.

  18. RichVR Says:

    Having said that, has anyone played with Mixtikl? As I write this I'm listening to an ambient mix that I set up in a few clicks on my iPod touch. You can get it for just about anything from Macs to Linux.

    Highly recommended.

    End threadjack here ......................................................

  19. RichVR Says:

    What did I put in my last post that didn't make it through the filter?

  20. macro Says:

    Having been shown that she can't always trust the evidence provided by her own senses, your girlfriend was probably just trying to get you to "see" the little flashy spots of light that happen when your brain bounces off the inside of your cranium.

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