About Persistence of Vision and Animation

A blue eye

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Let's all be honest, animation works like magic. You're a wizard of the page. How does animation work though? Let's break it down into some detail so we can understand what dark arts we are drawing upon.

The prevailing idea for a long time was the persistence of vision was the reason animation worked. While it is partially true, now we understand that there is more in play than simply just persistence of vision. But what is the persistence of vision?

Persistence of Vision

Persistence of vision is the fact that your eyes seem to retain an image for a split second after the image has vanished from your view. It's kind of like when you look out a window on a sunny day and close your eyes real tight, you can still kind of see the basic shapes of what you were looking at. It's not that same exact principle since that has to do with light more and your retinas readjusting to the dark, but it's the same idea.

Remember those old bird and cage toys? Like the one that Johnny Depp's mom shows him in Sleepy Hollow. Those are called thaumatropes. Don't worry that won't be on the final exam, those work by the principle of persistence of vision. Your eye retains that there is both a bird and a cage slightly after they switch images, causing the illusion that the bird is inside the cage when really they are two separate pictures.

Animation Strings a Series of Images Together

Now in animation, we have a series of images that string together to make a movement. For a long time, people presumed it was because of the persistence of vision, that our mind would retain the frame for a split second as we blended it with the new frame to create the movement. Nowadays though, at least among our animation nerd community, that isn't quite the full explanation.

So you know when you're walking down the street and you blink and you're like; "Whoa, where did everything go?!" No? Well, that's good because that'd be a huge pain and very scary. Luckily for us, our brain ignores all of those blinks so we don't constantly see a flash of black every few seconds. A film projector works very similarly to a human eye, it has a rolling shutter that blocks out the picture while the image is changing. That way we only see the full frames and not any weird half-frames as the film advances.

The Brain Ignores Blank Frames

So why is it that when we watch a movie we don't see all of those blank frames like we're watching a strobe light? Our brain ignores them just like it ignores all of our blinks. But now that everything is digital the process still remains the same, it's just happening at a much faster rate.

Instead of a rolling shutter, it works by refreshing either half the screen at a time, interlaced, or from top to bottom, progressive. Have you ever noticed when you see a YouTube clip of someone filming their TV screen there are always weird bars sliding around the screen? That's the refresh area of the screen.

What Causes Animation to Seem Continuous and Smooth

Again, it goes at such a speed that our eye ignores it. So the combination of your brain retaining the split-second image from before, as well as ignore the black or half frames is what causes the animation to seem like one continuous smooth movement. You can see it all start to break down once we get past shooting 1s and 2s and start shooting in 4s or 5s, the animation starts to break down and become choppier and choppier because it's getting outside the sweet spot of the human eye.

So there is a brief history of the persistence of vision and how crazy the human eye is as well as how animation works. Really though if you have to explain it to anyone just say you found a goat that turned into a wizard and granted you magical powers, it's much quicker than explaining all this.

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