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blue black dress
That dress three ways: the eye is easily fooled with perceiving colour (Source: swiked/Tumblr)

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Six ways the blue and black dress scrambles your brain

Tuesday 17th March 2015 11:04 am

It’s a very rare event when visual neuroscience and textile technology combine to take over the interwebs, but it almost happened in February, 2015. Yup, I’m talking about the famous ‘blue dress’.

No, it’s not the infamous blue dress belonging to Monica Lewinsky and apparently carrying some bodily fluids of the President of the United States, Bill Clinton. Instead, it’s the blue dress that changes colour all by itself, from blue to white — or more accurately, it’s the photo that does this. Yup, it’s a lovely optical illusion.

It all began with a wedding on the Scottish island of Colonsay. The mother of the bride wore a dress bought from a British retailer, Roman Originals, for 50 pounds. The bride posted a photo of the soon-to-be-famous blue dress online.

Very rapidly, this optical illusion became famous. At its peak, over two-thirds of a million people were looking at this photo at the same time on Buzzfeed.

Celebrities were polarised in their views. Taylor Swift apparently said she was “scared and confused” and saw a blue dress. Kim Kardashian saw a white dress, while her husband, Kanye West saw a blue one.

So what’s going on? It’s a six-part answer — with five definite do-knows, and one don’t-know.

First, the dress is actually blue. If you use the picture-editing app called Photoshop, you can analyse individual pixels of the photo and see that the blue dress is in fact blue.

dressgate

Blue and black dress by pixel colour
Second, the eye is easily fooled with perceiving colour.

The eye-brain combination is NOT good at judging the absolute colour of anything, but it’s very good at comparing.

So while you can’t accurately estimate that the wavelength of a colour is exactly 575 nanometres, you can tell whether it’s more red than another colour. There are so many optical illusions that use this. For example, the same chess piece can look black or white, depending on its background colour.

Third, in fact, the eye-brain combination tries really hard to maintain what the visual neuroscientists call ‘colour constancy’.

Consider a white sheet of paper. It just reflects whatever the ambient light colour is. It will be white in bright sunlight, but under the red lights of a night club it will be red.

But this change of colour bothers your brain. So your brain has evolved colour constancy, where it ‘adjusts’ or compensates for the ambient light, removes the reddish influence of the night club lights, and suddenly the sheet of paper looks white — even though it’s reflecting red light and actually looks red.

Colour constancy is a survival advantage. A red apple always looks the same colour, whether that particular food item is in the shade, or in sunlight.

So what you ‘see’ is a combination of three factors — what the true colour of an object is, plus any colours right next to it, plus the overall ambient lighting.

Fourth, the photo of the now-famous blue dress is, purely by accident, beautifully ambiguous. There is absolutely no bare skin — and bare skin always gives you a good idea as to the true colour. There are no other dresses, such as a white wedding gown, which could give you a clue.

All you get is the fabric of the blue dress — and an out-of-focus band of background brightness on the right side of the photo. This might make you think that the front of the blue dress is in shadow.

But, at the top of the dress is a panel of shiny fabric that is partly reflective. This is essential for helping to create this optical illusion.

Visual neuroscientists call these mirror-like reflections on the shiny part of an object ‘specularities’. It turns out that specularities can give you the best clue as to the actual colour of the ambient light. And in this case, the specularities give you the impression that the dress was well-illuminated from the front.

Fifth, we can now put it all together.

If you assume that the front of the dress is in shadow (thanks to the bright blurry background light), your brain will apply colour constancy and remove the blueish hue of the shadow — and bingo, the dress is white.

But if you assume that the front of the dress is well lit (thanks to the shiny reflections on the top panel of the dress), you will see the dress as blue.

So that’s what we know.

And finally for item six — which is what we don’t know.

Why do some people assume shadow and a white dress, while others assume brightness and a blue dress? We don’t know. As far as we know, it’s not related to your emotional state of mind, or your emotional or intellectual intelligence.

As in all visual illusions, we’ve been blinded by the light …

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This blog first appeared on Dr Karl's Great Moments in Science

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  1. Kristy says:

    So far nobody has explained why two people standing side by side see the dress differently coloured. I think it has the answer to how we perceive reality and need to understand why some see it the colours they do. It looks like something science cannot answer :)

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