Blue Dress Illusion

3 June 2015

It's a very rare event when optical physics, visual neuroscience and textile technology combine to take over the Interwebs, but it almost happened in February 2015.

I'm talking about the famous Blue Dress. It's the one that two people can look at, but one sees it as blue and black while the other sees it as white and gold. More accurately, this happens only in this photo. The blue and black dress looks blue and black in the shop window.

(Spoiler Alert - it's 'just' 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 and black dress online. Very rapidly, this optical illusion became famous and even warranted its own hashtag #TheDress. 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 Tweeted that she was "confused and scared" and saw a blue and black dress. Kim Kardashian saw a white and gold dress, while her husband, Kanye West saw a blue and black 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 and black. If you use the picture-editing app called Photoshop, you can analyse individual pixels and see that the dress is in fact blue and black.

Second, the eye is easily fooled with perceiving shades and colours. The eye-brain combination is NOT good at judging the absolute colour of anything, but it's excellent at comparing. So while you can't accurately estimate that the wavelength of a colour is exactly 575 nanometres, you can say 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 nightclub 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 nightclub 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 and black dress is, purely by accident, beautifully ambiguous. There is no bare skin - which always gives you a good idea as to the true colour. There are no other dresses in the photo, such as a white wedding gown, which could give you a clue.

All you get is the fabric of the blue and black 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 and black dress is in shadow.
But, at the top of the dress is a bolero jacket of shiny fabric that is partly reflective. This is essential for helping to create this optical illusion. Visual neuroscientists call the mirror-like reflections on the shiny part of an object 'specularities'. Specularities can give you the best clue as to the actual colour of the ambient light. 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 and gold.

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 and black.

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 intelligence or emotional intelligence.

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

© Karl S. Kruszelnicki Pty Ltd 2015