A few sections on this page use normals, so it's worth explaining what they are before getting into the techniques.
Every surface in 3D has a direction it's pointing in. Take a flat plane: you can describe which way it faces by drawing a line sticking straight out of it. That line is called a normal.
A 3D model is made up of many of these planes combined together. Each one has its own normal, and the 3D software calculates them automatically. But you can also edit them manually to create various different effects. Some of them are being covered later.
Standard 3D shading creates a smooth gradient from lit to dark across every surface. That gradient is what makes things read as three-dimensional. A cel shader on the other hand, limits the shading to one light and one dark color and three things determine what parts of the model are lit or unlit.
The Brightness Threshold determines how bright an area needs to be to be considered “lit”.
The Light Direction describes the direction the light is coming from.
The Normal Direction is the direction an area of the model is facing.
There are two ways to set this up, depending on how much control you want.
The beginner shader uses a Diffuse BSDF node feeding into a Shader to RGB node, with a Greater Than node creating the cel shading look. Blender handles the light direction calculation for you based on whatever lights are in your scene. It's quicker to understand and set up, but it isn’t as versatile. For 3D Anime rendering you want as much control as possible over the final look. The Diffuse BSDF node includes a few features that are hard to control and usually not needed. It also only works in Eevee.
