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Because it's Friday: Illusions in shadow

I tweeted out the image below earlier this month, and it quickly went viral: Mind = blown. These two blocks are exactly the same shade of grey. Hold your finger over the seam and check. pic.twitter.com/OqAnforGqs — David Smith (@revodavid) December 5, 2013 I stumbled across the illusion above while trying to find an explanation for this "shady" optical illusion. Carlos Scheidegger pointed me in the direction of the Cornsweet Illusion, which in turn linked to this figure from American Scientist, which is where I stumbled across the image.  I was surprised that the illusion went viral, but on reflection it's easy to see why it was so popular. It really is mind-blowing: despite being told that the two blocks are the same shade of grey, your mind rejects the possibility until you actually compare the on-screen colors directly. Phil Plait's Bad Astronomy blog gives a great explanation of the illusion, and finds its original source: Purves Lab. I had several people suggest via Twitter that this isn't really an illusion at all: in "real" life, those blocks are in fact, white and dark: it's just the shadow on the white block and the lighting on the dark block that make them the same shade in the on-screen representation, and this is obvious to anyone who's ever studied color theory. But I think this is exactly the point that makes the illusion so suprising: for anyone (like me) who hasn't studied color theory, it's a real suprise to find the disparity between what our brain interprets as color, and the frequency of light that enters our eyes. I certainly have a greater appreciation for the skill of artists in mixing paint to match a scene. (Check out this TEDx talk for more about color and illusions, which features the Purves illusion above, and also the surprising news that 10 percent of women have four color receptors, and can therefore see more colors than the rest of us who only have three.) But what about that "shady" illusion that started this all off? I don't actually think the Cornsweet Illusion is the explanation here, as there doesn't seem to be any variation in lighting between the diamonds. Anyone got another explanation?   If you'd like to see more illusions, check out our archive of Because it's Friday posts. That's all from the blog for now — see you next week.

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More Stories By David Smith

David Smith is Vice President of Marketing and Community at Revolution Analytics. He has a long history with the R and statistics communities. After graduating with a degree in Statistics from the University of Adelaide, South Australia, he spent four years researching statistical methodology at Lancaster University in the United Kingdom, where he also developed a number of packages for the S-PLUS statistical modeling environment. He continued his association with S-PLUS at Insightful (now TIBCO Spotfire) overseeing the product management of S-PLUS and other statistical and data mining products.<

David smith is the co-author (with Bill Venables) of the popular tutorial manual, An Introduction to R, and one of the originating developers of the ESS: Emacs Speaks Statistics project. Today, he leads marketing for REvolution R, supports R communities worldwide, and is responsible for the Revolutions blog. Prior to joining Revolution Analytics, he served as vice president of product management at Zynchros, Inc. Follow him on twitter at @RevoDavid

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