You’ve likely heard about how with each of its movies, Pixar has improved some element
of its process, or mastered some new skill.
With Monster’s Inc. they mastered how to create fur and hair, in Finding Nemo they
mastered underwater animation and lighting, in The Incredibles they mastered animating
humans, in Cars, they mastered reflective surfaces, and in Soul they mastered making
me fundamentally question the meaning and purpose of my life.
But as the technology has progressed, one thing has stayed remarkably constant: it typically
takes years to render one of their movies.
Which raises the question: why?
Rendering is the process by which computers spit out the actual, final frames that will
create an animated movie.
It’s a complex process, but it can ultimately be thought of very simply—the computer must
answer one, basic, question: what color is this pixel?
The problem is, the computer has to answer that question, like, a lot.
Let’s say Pixar makes a movie in HD—which they probably wouldn’t, but for the sake
of simplicity, let’s say they did.
You might sometimes hear this called 1080p.
That’s because it’s 1080 pixels by 1920 pixels, for a total of 2,073,600 pixels in
a frame.
Pixar’s movies are in 24 fps, or frames per second, so that’s 49,766,400 pixels
per second, meaning for a 90 minute, or 5,400 second, movie, the computer has to answer
the question “what color is this pixel” 268,738,560,000 times.
To put into perspective how big 268,738,560,000 is, if I ate 268,738,560,000 hot dogs, I would
die.
So to answer the fundamental question for understanding rendering, how does the computer
figure out what color this pixel should be, let’s start at the beginning.
Characters at Pixar start, of course, as story and art—artists and writers come up with
weird little guys who would do funny little things that would make us weep uncontrollably.
The next step is modeling, where animators create a wireframe of points and lines that
define a character’s shape.
Basically, it’s a way to turn the writers and artists’ ideas and drawings into a model
that is fundamentally mathematical, so a computer can understand it.
The next step is rigging, where the model forms relationships to how it moves.
In rigging, character models are fitted with virtual bones, muscles, joints, and so on
to give the animators the control points they use to move characters around.
They also use tools called deformers and displacers to define how the character’s elements have
relationships to each other: that way, if a character smiles, their mouth moves, but
also their chin goes down, their cheeks get lines, their eyes widen, their eyebrows raise,
and whatever else happens when you smile—I don’t totally know, as I refuse to smile
until Jet Lag gets a gold play button.
Next they animate surfaces, which are basically all about figuring out how a surface interacts
with light—is it shiny, or matte, or ooey gooey sticky icky—which is calculated using
a mathematical function called BRDF, which stands for Big Rockin’ Deflection Fingy.
Afterwards, virtual 3D sets are built, lighting designers add in virtual light sources, and
virtual cameras are added into those virtual sets to determine what parts of the shot will
actually end up in the frame of the movie you see.
Then, to add things like fur, hair, and clothing, there’s simulation.
Basically, something like fur has potentially millions of hairs, which can’t be individually
animated, so physics models have to be built to simulate their motion.
Lastly, animators finally come in to fine tune and craft a bunch of programmed motions
into a talking fish so emotionally resonant that it makes you call your parents and tell
them you should have appreciated them more.
And then, finally, once this is all done—essentially, the movie is made—it’s time to render.
Rendering time is a function of several different factors.
One is, pretty simply, detail, both in terms of the number of objects and the detail on
each object.
For example, if I have a shot with a bunch of… what would Pixar make a movie about…
talking socks overcoming nihilism—the more talking socks, the more render time, and the
more detail put into rendering the fabric on each sock, adding more individual threads
that are taken into account, the longer.
Another key aspect is the number of samples per pixel, which is really complicated, so
I’m going to simplify in a way that hits the perfect mix of simultaneously annoying
animators and still confusing you.
Let’s start here: if you had to guess, you’d probably assume that to render a shot, the
computer looks at the lighting sources rigged up by the lighting designers, then simulates
having that light get shot down and reflected and refract off the surfaces, and then it
would see what light reaches the camera.
The thing is, if the computer did that, most of the light wouldn’t reach the camera,
making that super inefficient.
So instead, at least in several of its movies, Pixar uses ray tracing, where the computer
sort of shoots a theoretical beam of light from one of the pixels in its camera’s frame,
onto the scene, and then sees if, when it bounces off whatever it hits, it reaches a
light source.
If it doesn’t the pixel is dark, if it does, it’s bright.
But light can bounce many different ways, so if you only simulate this once per pixel,
most of your theoretical light beams won’t hit a lighting source, so you’ll end up
with a bunch of dark spots that shouldn’t really be dark.
Instead, the computer runs multiple samples per pixel, to see potentially hundreds or
thousands of ways the light could bounce, and then averages those to create a crisp
image.
The computer also must determine how many times to let the light bounce, which adds
significantly to render time as well.
So, going back to the title I so cleverly engineered to trick you into clicking this
video: why do Pixar movies take years to render?
Well, movies have to be rendered at massively high levels of detail, with a huge number
of samples per pixel, and with several light bounces, ultimately having to answer the question
“what color is this pixel” over 260 billion times.
Arguably, though, the biggest part of why rendering takes so long is that Pixar is always
pushing the limits of what’s possible.
With Pixar’s modern technology—their special rendering software, called Renderman, and
their render farm that has 2,000 machines and 24,000 cores, which by the way makes it
one of the 25 largest supercomputers in the world—they could render the original Toy
Story in about an hour.
But the complexity of the shots they choose to put in their movies now means there are
individual frames in Toy Story 4 that took 30 hours just to render.
That’s pretty inefficient, so if you want to protest Pixar’s rendering inefficiency
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