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SYFY WIRE Bad Astronomy

Optical illusion: Shade-changing shape-shifting gray turtles

By Phil Plait
Alternating curved arrangement of lighter and darker “turtles”… or are they? Credit: Akiyoshi Kitaoki

I can't think of a better way to start off a new year than scrambling your brains. Just a little bit! But still: enough to make you scratch your head and wonder just what is wrong with that sack of wrinkly pink goo in your skull.

One of my favorite optical illusionists is Akiyoshi Kitaoki. He has created hundreds, maybe thousands, of guaranteed brain-melting illusions that will make you swear that what you're seeing is real when it really, really isn't. He has ones that appear to move, that warp your sense of shape and size, destroy your notion of color, and will make you seriously question whether your eyes and brain are talking to each other in any sort of coherent way.

He just posted a new one to Twitter, and I love it for its simplicity and efficiency: It creates two illusions at once.

Are you ready? Here it is:

Alternating curved arrangement of lighter and darker “turtles”… or are they? Credit: Akiyoshi Kitaoki

I don't know about you, but when I look at this I see alternating squarish shapes (Kitaoka called them turtles, so I'll go with that) arranged like a chessboard, with half darked and half lighter. What's disturbing immediately though is that they don't appear to be separated along straight lines. The vertical border of the turtles on the left appear to curve to the right a bit, and the ones on the right curve left. It makes it look like there's a mound or bulge in the middle of the image.

However, if you know anything about optical illusions, you know this isn't the case. In fact, the pattern is laid out along perfectly straight vertical and horizontal lines!

Don't believe me? Here's the same pattern, but I drew a straight thin line down one edge of the turtles:


Yup. Straight. So why do they look warped? The key is in the turtle 'legs.' At each vertex between turtles they form a rotated square divided into four smaller squares*. Note how they're offset from one another, giving a twist to the vertices.

But more importantly, look at the shading inside those turtles and their legs: One side is black, the other white, and they fade into the squares themselves. If you try to take in the whole grid at once, this distorts the shape of the boundaries, warping them, making them looked curved. It's really difficult to shake, unless you focus down on a smaller section of the image.

I suspect there may be a factor of whether you're looking right at the illusion or off to the side; when I look off to the side, so it's in my peripheral vision, the illusion becomes markedly stronger. Some illusions do depend on this effect. I also noticed the effect is stronger the smaller the grid appears; take a few steps back and look again, and see if that works for you, too.

But there's more! Veterans of my blog will know what's coming next: All those turtles making the chessboard pattern? Yeah. They're the same color.

Seriously! There are no lighter and darker turtles. It's gray turtles all the way down. This time, those white and black borders are throwing off your sense of shading; the white border on the inside makes a turtle look lighter, while the black border on the inside makes the turtle look darker.

Just a few pixels away from the border, towards the center, each turtle is exactly medium gray. If you don't believe me, grab the picture and look at it in an image processing program like Photoshop or Gimp. Look at the colors in the squares and you’ll find they have red:green:blue (or RGB) values of 128:128:128, precisely halfway between black and white.

One way to show this is to simply blot out the borders between them. Like this:


Boom. See? Another way, to make sure you're not fooling yourself, is to sample the colors.

I created a new picture, filling the square on the left with the color sampled from one turtle, and the right square with the color sampled from the turtle next to it. Can you see any difference?


There actually is none. They're the same color. If one looks darker that may be due to your monitor, or what other windows you have open on your computer. If you do see one as darker, flip the image horizontally and see if that one still looks darker. I bet it won't.

We tend to think of our eyes as seeing things in absolutes. This object looks red, that one blue. But we don't, not really. Much of our perception is based on comparison, contrasting one object with those near by. Sometimes that's done on a small scale that's hard for us to notice, like when the border between the turtles goes from black to white over just a couple of pixels. It's small, but enough to totally throw off our sense of contrast. And shape!

Optical illusions are actually a pretty important tool for scientists who study how our eyes work, how our brains work, and how the two work together. Or, not work together, I suppose. The machinery inside our eyes and the way it sends that information to the brain isn't perfectly understood, and sometimes the best way to figure it out is to look where it goes wrong. That can be very helpful in seeing how images are processed inside our heads.

And, of course, it teaches a very important life lesson, too: Your eyes are like a filter, and they play a game of telephone with your brain that distorts what's really out there. Photographs, drawings, even just looking out in the real world with your eyeballs — these all change reality.

And that makes your perceptions easy to fool. What you see is never, ever what you get. If someone ever tells you, "Seeing is believing," you look them right in their filtering eye and say, "Seeing is just the first step. Understanding is believing, and science is understanding."

And hey, happy new year. May you find yourself questioning everything you see and hear. Life's a lot more fun that way.

*It's hard not to notice that the offset and rotated nature of the smaller squares appears to form a swastika shape. It's also a sub-illusion; that pattern is like a negative space outline of the positive space squares. That's… unfortunate, but in this case it's just a coincidental shape, a pattern your eye picks out. I can be pretty sure it's not intended to have any political meaning.