To create images and videos in CG, there is a process called “rendering.” For those who have never worked with CG, it might seem like a mysteriously time-consuming process. Let me explain a bit.

Here, I will focus on 3D rendering.

Let’s refer to the definition of rendering from Wikipedia.

Rendering in 3D computer graphics is the process of converting a 3D scene into an image.

Even with that definition, it might not make sense. (At least, that’s how I felt when I didn’t understand 3D.)

Let me briefly explain the process of creating an image in 3D CG.

To create an image, you first sculpt a shape like clay, called a 3D model. Then, you add surface textures as if painting it, place lighting, and use a camera to “photograph” the 3D image.

In the real world, if you shine a light on a clay figurine, the illuminated side becomes bright, and shadows form on the unlit side. Light reveals the texture of objects. To explain it more technically, photons, the particles of light from the sun or a lamp, hit the object’s surface, bounce back, and enter your eyes or a camera sensor, where they are perceived as color.

In CG, the behavior of light is simulated on a computer to display textures and shading as an image. This process of simulating light is called “rendering.”

Especially for realistic animations, this light simulation process can take a lot of time. For example, in Pixar’s animated film Zootopia, it was calculated that rendering the entire movie on a single PC would take 69.18 years! （Reference）

Such long times are impractical, so to speed up rendering, better hardware (like CPUs and GPUs) is used, or multiple computers share the simulation workload to reduce rendering times.

I hope this helps you understand why rendering takes so much time and that reducing quality or using more hardware can make the process faster.