Vertex

A vertex (plural vertices) is "a single point or position in 3D space" and "the most elementary part of a mesh" (Blender 3.1 Manual: Mesh Structure). Every edge runs between two vertices, and every face has vertices at its corners.

#Why vertices matter in games

In a modeling tool a vertex is a position you can grab and move. Blender draws them as "small dots" in Edit Mode (Blender 3.1 Manual: Mesh Structure). Move a vertex and every face that uses it changes shape.

In a game engine a vertex holds more data: a position plus "optional additional attributes" (Unity Manual 2022.2: Anatomy of a mesh). Those attributes include the normal, tangent, one or more UV coordinates and an optional colour (Unity Manual 2022.2: Anatomy of a mesh). The engine stores all of them for every vertex, so vertices cost memory.

#One corner, several vertices

Each engine vertex can hold only one value per attribute. In a Polycount discussion, one artist wrote: "A single vert can only have one normal and one UV coordinate" (Polycount forum: Vertex Count vs Triangle Count). So "a vertex must be split if it has multiple normals, UV coordinates or vertex colors" (Unity Manual: Modeling characters for optimal performance).

That is why the engine's count "is usually higher than the count given by the 3D application," which only counts "distinct corner points" (Unity Manual: Modeling characters for optimal performance). Hard edges, UV seams and per-face vertex colours all add copies (Polycount forum: Vertex Count vs Triangle Count). The anatomy of a polygon mesh page walks through the classic case of a cube going from 8 to 24 vertices.

#Vertices and topology

How many edges meet at a vertex matters too. A vertex with three, five or more edges is a pole, and edge loops stop there "because there is no unique way to continue a loop from a pole" (Blender 3.1 Manual: Mesh Structure). Where you put poles is a big part of topology and edge flow.

#Common mistakes

• Duplicate vertices. Two vertices sitting at the same spot look like one, but the mesh is not joined there. Meshy's glossary lists overlapping, unmerged vertices as a cause of non-manifold geometry (Meshy: Manifold). Blender's 3D Print Toolbox suggests merging by distance to clean up degenerate geometry (Blender 3.6 Manual: 3D Print Toolbox), and Unity's importer has a "Weld Vertices" option that will "combine vertices that share the same position in space" (Unity Manual 2018.2: Model tab).
• Lone vertices. A vertex with no edges does nothing visible but still counts as stray geometry. See manifold and non-manifold geometry.
• Judging cost by the modeling-tool count. Check the engine's number, which includes splits (Unity Manual: Modeling characters for optimal performance).
• Painting vertex colours per face by accident. Per-face colour values split vertices, just like hard edges (Polycount forum: Vertex Count vs Triangle Count). Use smooth gradients unless you need a hard break. See vertex painting and blending.

#In different tools

Maya's separate UV count is a useful hint. If the UV number is much higher than the vertex number, many vertices sit on seams and will split in the engine.

#Related terms

edge · face · normals · pole · anatomy of a polygon mesh · smoothing, hard edges and smoothing groups · UV seams and islands

#Further reading and watching