Normal map

A normal map is a special texture that records, in colour, which way each point on a surface faces. The red, green and blue channels hold the X, Y and Z components of the surface normal, which is why most normal maps look purple-blue. When calculating light, the render engine uses these directions instead of the real geometry, and joints, grain and pores appear on a perfectly flat face.

How it differs from a bump map

A bump map is greyscale height information from which the engine derives the slope itself. A normal map gives the direction directly, so it usually delivers finer and more controllable detail. Both only fool the lighting: the silhouette stays the same, and seen edge-on the surface is still flat. For real relief you need displacement.

Practical points

• Normal maps must be read as linear data, without gamma correction.
• Software differs on the direction of the green channel (OpenGL versus DirectX); if detail looks inverted, flip it.
• Pushing the strength too far makes surfaces look plastic.

On brick walls, stone cladding or fabric, a normal map adds a lot of realism without adding to the geometry load.