Animation works because  of a natural phenomenon in our brain called  persistence of vision. This isn't a principle of animation. We'll get to those after this lesson. But it is important to help understand the how behind  animation working,  and also illuminate the why of certain things  that happen in animation, like different frame rates  that are chosen and concepts of like limited animation,  which we'll get to later on in this lesson. But first, before we jump into all that,  let's look at an example of persistence of vision. We see an object because light reflects off of that object  and into our eye by passing through the lens of the eye,  which flips this image  and projects it onto the back of the eye  where the retina is.

This flipped image is then carried up the optic nerve  to the brain where the brain flips the image right side up. If we remove the light, we can no longer see the image. But if you notice the image in our mind doesn't disappear  as quickly as the light was removed. The image persists in our mind for an instant  after we can no longer see it. So let's say we flash another image again  before the first is dimmed out of our mind. We can create the illusion of continuity  between the two images because they'll overlap in our mind.

If there's a slight difference between the two images,  like it changes position  or shape, then our brain will perceive that as motion. This is essentially an obstacle illusion  and a really good example of this is the  rubber pencil trick. So grab a pencil or a pen and try this yourself  because it's actually easier  to see in person than it is going to be to see it on camera. But grab the pencil gently in your fingers like this  and give it a very slow wobble motion. Our brain is interpreting where the pencil was  and where it is simultaneously. So it's inter interpolating between these two positions  that makes it look like the pencil is actually  bending when it's not.

That's persistence of vision at work. A more futuristic example  of this is LED light strips that spin. So if you take a look at the image of this device,  you see it's actually just one strip of LED lights. But if those lights spin fast enough so  that they kind of blend together  and the lights change quick enough,  our minds in our eye register that as a single image. So the film industry needed to take advantage of persistence  of vision to create films,  but they also wanted to reduce their costs  because making films is expensive,  especially back in the day when they landed on,  on a frame rate of 24 frames per second  around the 1920s when sound came into play in films. And everyone needed to standardize the frame rate so  that sound could sync up with the films.

So that means that for every one second of film,  there's 24 images. That's what 24 frames per second means. And 24 frames per second is about as low  as you can go in a frame rate without the human eye being  able to tell a difference between individual frames. Animation production has always been a time consuming  and expensive endeavor. And animation studios discovered  that they could actually get away with one drawing  for every two frames. So that means that they only needed 12 drawings  for every one second  or 24 frames with a very little compromise  in the smoothness of motion  that you get from persistence of vision.

So they were really pushing the boundaries of persistence  of vision to help save money and time. This is called animating on Twos. Despite these practical decisions to reduce costs  and save time, it's actually become a bit of a throwback  to animate on twos again,  to make it look like it's handmade. So you'll see this a lot in motion design now, especially  where people will be animating on a computer like in After  Effects or something like that, where by the nature  of the program you're animating on once  or every frame changes. And what they'll do  after they're done with the animation is create an effect on  their animation that was done on ones  to make it look like it was animated on twos. So it'll hold every other frame.

And that's actually something I'm doing on the titles  of this course. If you look at the lower thirds  and all the little texts that pops up, I'm animating  that in After Effects,  and then I add what's called a posturized time effect. So I basically delete every other frame and hold the one  before it so you don't have to follow along. But I just wanted to show you what  that looks like in After Effects. 'cause it's pretty simple, but  what it does is it gives something a handmade field  because we've trained our eyes  to associate something animated on twos with a,  a much more tactile, uh, authentic feeling of,  of it being handmade. So people will actually revert to that as  something in their animation,  even though they can make it smooth.

But this is what it looks like in After Effects. You basically apply what's called the poster rise time  effect to your layer,  and you change the number  to be whatever frame rate you want. In this example, it's 24 frames per second to start. So I'm setting the effect to 12,  which will create the effect of holding each image  for two frames instead of a new frame on every frame. If you're really into stop motion animation, you'll be able  to tell a huge difference between the movies  like A Nightmare Before Christmas  and the more recent stop motion movies, really any  of them out of the Leica studio, like, uh, the movie,  A Missing Link with the big difference being  that a nightmare  before Christmas is animated on twos mostly,  and the missing link is animated on ones mostly meaning  they're moving the puppets every single frame. They're not taking advantage  or trying to cut corners to save time and money.

They're animating every frame. And that's why a lot of times people will confuse their stop  motion animated movies for CGI animation. Another way animation production saves money is  by something called limited animation,  and that's when they have a static background  and only the characters change on each new drawing. That way they can save time  and money so they don't have to redraw the background every  time the character moves. And that's why you'll notice that a lot  of the earlier animations too have much more detailed  backgrounds because they're not having  to redraw it every new frame. So the characters themselves are much more simplified  because that's what the animators are gonna have  to redraw over and over and over again.

Take a look at this example to see what I mean. Check out the background here  and then look at the character. There's a huge difference in detail  and that's why there is an equivalent  that can happen in 3D animation when sometimes you'll  actually bake the lighting of a scene into the textures  of the objects so that the renderer is not having  to calculate all the light bouncing  around every time you render a frame. Animation productions have also been known  to reuse animation a lot,  so you can actually notice it in some early Disney films  where they used the same performance over and over again. Um, and that's was to help save money  and time so they could get the movies done and under budget. You'll also notice in TV animation, this happens a lot  where the character is talking and they will stick a pose  and just keep talking and only their mouth is moving,  and then they'll hit another pose  and then just keep their mouth moving.

This is also a version of limited animation so  that TV productions can crank out a lot more content  than is required on a feature and,  and under a shorter time span usually. So that's also another version of limited animation  where they try to take advantage of static poses,  even in the characters. If it was what is called full animation,  you would see the very subtle motions of a character talking  and moving quite a lot. I'm over performing right now, but you get the idea. Whereas in maybe a show like Family Guy, you'll get Peter  to just stand like this and talk, talk, talk,  and then hit another pose and talk, talk, talk, talk. Whereas in a full animation feature film,  the characters are always moving  and that's also a big difference between full animation  and limited animation.

And that's just a constraint of the time and budget  and resources that each of them have. In this lesson, we learned about persistence of vision  and how that's an important optical illusion  to understand the how and why animation and film work. Then we learned about frame rates  and why those frame rate numbers are chosen  and how animation can actually get away  with working on twos and what that means. Then we finished with understanding  what limited animation was  and how that can be used to animation's advantage  to get things done for different constraints  of different kinds of projects. So now we are going to move on to the assignment  in the next lesson,  and we are going to take what we've learned  and really push the concept of persistence of vision  by creating an animation with only two frames, two drawings,  and we are gonna trick our eye into believing  that there's motion there. So let's take that knowledge that we now have  and put it on paper.

I'll see you in that next lesson.