Why Compute Vertex Normals in Three.js Custom Geometry

In Three.js, computing vertex normals for custom geometries is essential for achieving realistic lighting, shading, and reflections. This article explains how vertex normals dictate how light interacts with 3D surfaces and why calculating them is a crucial step when building custom shapes from scratch.

When you define a custom geometry using BufferGeometry in Three.js, you manually specify the positions of the vertices to construct faces. However, position data alone is not enough for the rendering engine to understand how light should bounce off the object. To calculate lighting, the renderer needs to know the direction each vertex is facing. This direction is represented by a 3D vector called a vertex normal.

Without vertex normals, any light-sensitive material—such as MeshStandardMaterial, MeshPhongMaterial, or MeshLambertMaterial—cannot render correctly. The mesh will either appear completely flat, solid black, or fail to respond to light sources in the scene. Computing vertex normals ensures that light interacts with the geometry realistically, creating realistic highlights, shadows, and depth.

Additionally, vertex normals are critical for smooth shading. When Three.js renders a mesh, it uses the vertex normals to interpolate lighting across the faces of the polygons. This interpolation tricks the eye into seeing a smooth, curved surface rather than a faceted, blocky one, even if the underlying geometry has a relatively low polygon count.

In Three.js, you can easily calculate these vectors by calling the geometry.computeVertexNormals() method after defining your custom geometry’s vertices and index. This built-in function automatically analyzes the faces of your geometry and generates the necessary normal attributes, saving you from performing complex mathematical vector calculations manually.