This Picture Has No Red Pixels—So Why Do the Strawberries Still Look Red?
Color constancy continues to confound us.
This weekend marked the two-year anniversary of The Dress: the unfathomably viral photo of a dress that divided the internet for more than a week in 2015 over whether it was blue and black, or white and gold. So it's appropriate that, on this auspicious date, an equally maddening photo recently started making the rounds online:
The photo was created by Akiyoshi Kitaoka, a Professor of Psychology at Ritsumeikan University in Japan, who specializes in creating optical illusions (his twitter feed will blow your mind). As you can see in the tweet above, this photo has no red pixels in it, even though the strawberries pictured clearly appear red. Though plenty of twitter users tried to argue this fact, another person demonstrated that the pixels we're seeing as red are really grey (and a little green):
While this time everybody is seeing the same thing, the optical illusion is created through a similar phenomenon that caused so much turmoil with The Dress. It's called color constancy. It's your brain's way of color correcting the world when it's filtered through different light.
When you look around the world, the light that enters your eye is made of different wavelengths that come from both the pigments of the objects around you and the light that illuminates them.
"If you imagine walking around outside under a blue sky, that blueness is, in some sense, color-contaminating everything you see," explained Bevil Conway, an expert on visual perception from the National Eye Institute. "If you take a red apple outside under a blue sky, there are more blue wavelengths entering your eye. If you take the apple inside under a fluorescent or incandescent light without that same bias, the pigments in the apple are exactly the same but because the spectral content of the light source is different, the spectrum entering your eye that's reflected off the object is different."
Since all this color contamination from light sources isn't really useful (it would be super confusing if a ripe banana looked yellow in the morning but green at midday, for example), our brains have evolved to color correct. It allows the colors we see to look the same no matter the lighting.
"In this picture, someone has very cleverly manipulated the image so that the objects you're looking at are reflecting what would otherwise be achromatic or grayscale, but the light source that your brain interprets to be on the scene has got this blueish component," Conway told me. "You brain says, 'the light source that I'm viewing these strawberries under has some blue component to it, so I'm going to subtract that automatically from every pixel.' And when you take grey pixels and subtract out this blue bias, you end up with red."
Conway said this illusion is also helped out by the fact that we recognize the objects as strawberries, which we very strongly associate with the color red, so our brain is already wired to be looking for those pigments.
Color constancy was the big reason why people saw 'The Dress' differently: since the light source was really unclear, people's brains corrected for different kinds of light, causing them to see the dress differently. You'd think we'd have learned our lesson by now, but there's something about the mysteries of color and perception that continue to fascinate our collective conscious. Especially when we can argue about it.
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