This is a dog in 1995's "Toy Story."

It was a staggering achievement at the time, but the detail in the fur just isn't there.

Compare that to this cat in "Toy Story 4."

The difference is clear.

But getting from that dog to this cat required a lot of innovation in between.

Pixar forever changed animation in 1995 with "Toy Story," the first full-length computer-animated movie.

With "Toy Story 4," it's proving that it's far from done changing the game.

Between 1995 and 2019, Pixar has made 21 feature-length films, four of those being "Toy Story" movies.

When "Toy Story" was first released in 1995, nobody had ever seen anything like it before.

And in order to bring it to life, the animators had to do some unbelievable things.

And one of the most important factors in how Pixar makes its magic happen involves rendering.

Rendering saves the computer image to the perfect finished image or video frame, with lighting and motion effects.

In order to render "Toy Story," the animators had 117 computers running 24 hours a day.

Each individual frame could take from 45 minutes to 30 hours to render, depending on how complex.

There were a total of 114,240 frames to render.

Throughout the movie, there are over 77 minutes of animation spread across 1,561 shots.

They had to invent a new software, called Renderman, to handle all this footage.

These guys are professionals.

They're the best!

According to producer Jonas Rivera, if they had to today, they could render "Toy Story" faster than you could watch the entire movie.

However, the complexity of "Toy Story 4" means it can take 60 to 160 hours to render one frame.

And there were a lot of limitations.

For instance, at this time, Pixar hadn't quite figured out how to fully animate human characters.

Animating clothes was time-consuming, so you'll notice a lot of shots of hands and feet in the movie from a toy's perspective.

Additionally, they would sometimes choose not to fully light characters, so you wouldn't see any missing details.

More on lighting characters and fully clothing them later.

When "Toy Story 2" came around in 1999, they'd had some time to work out some of the kinks, especially with 1998's "A Bug's Life" in between the two movies.

In this sequel, you'll be able to see more visible, fully formed human characters.

One key thing the animators were starting to figure out here to help them tackle humans: smoothness, which they got practice on in "A Bug's Life."

Here, you can see the improvement in just a few years.

They wouldn't be ready to have a fully human cast until 2004's "The Incredibles."

But before mastering humans, they stepped into fantasy with 2001's "Monsters, Inc."

Ooh, they're so awesome!

In "Monsters, Inc." they tackled fur head-on.

Fur is hard to animate, whether computer-generated or stop-motion.

This is because it involves animating thousands, or even millions, of individual parts of a character's body.

In the VFX world, characters are designed then rigged by adding points of movement, which can then be manually manipulated.

While limbs are typically manual based on the scene, something like fur needed to be automated, since it would take a lot more time to move each individual strand of hair.

"A Bug's Life," which centers on insects, didn't have a need for fur.

And there is a dog in "Toy Story," but as you'll notice, it's pretty smooth, as is the dog in "Toy Story 2."

Sully, who's tall and covered head to toe in fur, has over 1 million hairs on his body.

But it's not just how it looks; the animators had to get all of those hairs to move.

To do this, they created a program called Simulation, in which certain elements that are too difficult to hand-animate are motion-simulated.

See, instead of looking at Sully's hair as a whole, they looked at each strand as a distinct particle.

They had to look at every kind of force that would act on those particles, and thus how each one would move in reaction to them.

So if you want to work at Pixar, you might need to know physics.

They also found real-world fur on different animals, like llamas, for reference.

By doing this, they found the best way to make hair look and move realistically is to clump it together.

"Monsters, Inc." laid down the technical foundation, which allowed Pixar to have over 250 furry monsters in the sequel, "Monsters University."

The tools at their disposal also helped them create fur on animals seen in "Ratatouille" and "Up," the moss on the submarine from this scene in "Finding Nemo," and the grass on the ground in "Cars."

Pixar's next movie, 2003's "Finding Nemo," also required the animators to create things they'd never put on screen before.

This time, they had to figure out how to make a movie set largely underwater.

Once again, science and real-world references would help them out.

Just as they did with hair, they broke down the water as much as they could to get it right.

According to director of photography Danielle Feinberg, they started with a real-life underwater clip, re-created it in the computer, and broke it down to find the most essential elements.

The biggest one? Light, and how it travels through water.

The light they created affected both the visibility and the color of the film's elaborate underwater world.

And while they need science to animate certain elements of a movie, there are times where they can use artistic license.

For instance, they made the water in Sydney Harbour look fairly green to fit the mood of the scene.

In reality, it would not be that color.

Once the environment was created, they had to populate the world.

Perhaps the most challenging sea creature they had to create was Hank the octopus from the 2016 sequel, "Finding Dory."

It was literally impossible for them to put Hank in "Nemo," and you can see why.

Creating just one scene with him supposedly took about two years.

Character supervisor Jeremy Talbot explained that they had to break down an octopus and piece it back together again, which sounds a lot like how Pixar mastered fur and water.

One thing they discovered was that octopus tentacles don't bend but almost unfurl.

An engineer spent six months just getting the curve of one of his tentacles right, and this was even before they mastered his camouflage.

And as Pixar got better at developing the natural world, it also improved on the man-made world.

By the time 2006's "Cars" came around, Pixar had about 1,000 times the computing power it did on "Toy Story."

"Cars" gave the animators a chance to hone their skills creating metal surfaces.

As they did with the water in "Finding Nemo," they took time to make the light reflect off Lightning McQueen.

Those metal surfaces would then be rusted up and seen in 2008's "Wall-E," often considered one of Pixar's most visually stunning works.

Then when "Ratatouille" rolled around in 2007, Pixar combined its ability to work with fur from "Monsters, Inc." and with water in "Finding Nemo" to display wet fur.

Lighting is one of the most important factors in making CG animation look real.

It takes a lot of rendering time to get it right.

And it's not just one or two lights we're talking about.

This one shot alone in "Ratatouille" contained 230 lights.

But that's nothing.

Jump ahead to 2017's "Coco."

When Miguel enters the Land of the Dead, he's laying his eyes on about 8 million lights.

And even with a movie as visually ambitious as this one, something as simple as clothes can be the biggest challenge.

A lot of the characters that wore clothes in "Coco" were actually skeletons.

The animators found that while simulating clothing, the cloth would often get caught between individual bones, creating a wedgie of sorts.

For this, it implemented a technique called continuous collision detection, which allowed the animators to spot the clothes getting caught, even at moments where it was difficult to notice.

A year later, when the long-awaited "Incredibles 2" came out they were back to working with humans: skin, bones, and all.

There was a 14-year gap between the two "Incredibles" movies and the benefits of improved technology actually allowed them to make Jack-Jack look even cuter than he did in the first movie.

All of these movies would eventually lead to "Toy Story 4."

Twenty-four years after the original was released, it seems like this sequel is trying to do a lot of things the original just couldn't.

The differences couldn't be more stark.

While "Toy Story" used a 1.85:1 aspect ratio, "Toy Story 4" expanded to 2.39:1, so what you saw went from this to this.

Now that Pixar had figured out how to put fur on one character and then hundreds more, it could make the animals in this movie eye-popping.

Fans pointed out that a cat in a clip for the movie looked 100% real.

Compare that with Sid's dog in the original if you really want to know how far Pixar has come.

Even just compare how much different it looks from the cat in "Coco."

In the same vein, creating stuffed animals covered in fur was not the challenge it once was, and it allowed Pixar to give Ducky and Bunny starring roles.

In addition, rain scenes are not easy to animate.

Just like with the water in "Finding Nemo," the animators did a physics-centered, frame-by-frame analysis of raindrops.

Luckily, they'd done enough creating rivers in "The Good Dinosaur" that bringing a torrential downpour to life was a little less challenging.

Like with Jack-Jack in "Incredibles 2," the animators use sequels as an excuse to improve upon their classics.

They didn't just give Bo Peep a new outfit; they were also able to make her look like a much more convincing doll.

Years of perfecting shiny surfaces allowed them to really bring out Bo's porcelain skin.

To effectively light this material, the director of photography referenced lighting of female characters in movies made from the 1930s through the 1960s.

The team also realized they couldn't use square lights on Bo.

Only circular lights without sharp edges would reflect well on porcelain.

"Toy Story 4" also gave Pixar a chance to do something surprising: go back to basics.

Look no further than Forky.

I don't belong here!

There are no tentacles, no fur, no shiny surfaces here, just a plastic fork.

But he couldn't be more real.

And Pixar isn't slowing down anytime soon.

In fact, they're moving "Onward," literally.