'Ultimate' optical illusion is 'so good most don't believe it'

An incredible optical illusion posted on Reddit has people divided over whether or not it's real. One user branded it "the ultimate optical illusion" due to the amount of debate it had prompted.

The post is split into two halves vertically, with the left half appearing to show a blue ball against a yellow background, covered with a pattern of purple dots. The right half appears to show a green circle against a purple background, with a pattern of yellow dots. However, the post's title claims the balls are the same colour.

The claim prompted a heated debate on the post, with one commenter writing: "Pfft no they're not! The balls on the left are purple, the right ones are yellow. OP (the original poster) is obviously colour blind."

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Another said: "They are clearly not the same colour. I'm rather confused by all the people that claim they are, using all kind of different programs too. Why do so many people claim they checked it and see the same colours?"

They added they had looked at the originally, higher resolution image, which had been posted in the comments, saying: "It seems it does have the same colours. Or at least I can't see the difference by eye. The effect is also not as spectacular as it is in this post."

However, another said: "Colour never exists in a vacuum, it always exists in relation to something else, so the same colour never looks the same because context changes."

Another user wrote: "PhD in visual perception here - what this kind of thing reveals is that vision is not about detecting wavelengths of light. It’s about figuring out what kind of surface is reflecting that light to you.

"The same reddish light may bounce off of a green tomato at sunset and a red tomato at midday. The fact that the light itself is red is not that useful. What you really want to know is what kind of surface properties an object has. Does the tomato’s surface reflect mostly green or mostly red light? Given that the light shining on it can change, the only way to figure this out is to compare it with its surroundings.

"During sunset, the landscape in general is awash with red light. Your brain will basically subtract that average redness when deciding how the tomato looks. This is what’s happening with the ball on the right in the illusion - it’s on a magenta/red field and so subtracting that colour makes it look extra green (red and green are perceptual opposites).

"The ball on the left is on a yellow field, so it gets pushed away from yellow and looks more blue. Blue and yellow being the other 'opponent pair' that your eyes are always weighing against each other."

Another user said: "I know it’s real cuz I’m colour blind in 1 eye. Specific blues and greens get cancelled out."

And someone else said: This is literally how comic book publishers were able to create the illusion of dozens of colours using only yellow, magenta, and cyan."

Someone else wrote: "OP you posted an optical illusion so good that half the people here don’t believe it. Great illusion."

The original poster replied: "It's actually pretty funny, lol. The ultimate optical illusion."