All optical illusions fascinate but some astonish. They also teach us things about how human vision operates and how well adapted it is to helping us survive.
For my fiftieth blog post I’ve chosen to look at the celebrated Adleson checkerboard illusion. It’s been published many times and you may have seen it before. But even if you already know it, the illusion retains the power to surprise. It appears almost paradoxical.
Which square is the darker shade of grey, A or B?
Of course it’s a trick question. Although they look quite different in fact they are the same. It seems almost incredible. But if you don’t believe me, I printed two copies of the picture and cut square A out. Watch while I slide the cut-out square from A to B.
These are genuine photographs. Although the cut-out is the same throughout we see it miraculously changing colour.
What’s going on and what does this show? Our visual perception is very good at taking the information that comes from our eyes and using it to construct a view of the world.
It processes the information using systems that have evolved over many millions of years and which it is very hard for computer vision to come anywhere near. For example smartphone apps that attempt to recognise colours, such as those designed to assist the blind or VI, only have limited success. But at the same time the human ability to interpret what actually falls on the retina means we can be misled sometimes.
You can find the original image of the illusion and read Adelson’s explanation here. If you are like me then however many times you look at the illusion A and B still look quite different. So do what I did – print off two copies, cut A out and do some experiments. You will be as astonished as I was!