LightDir

LightDir is a vector representing the direction of incoming light with respect to a point on a surface in shading calculations. It is typically a normalized three-dimensional vector and is used to determine how much light a surface receives. In practice, lightDir may be defined in world space, view space, or tangent space, depending on the rendering pipeline.

Conventions and computation vary. For a directional light, lightDir is constant across the scene. For a point light, lightDir is usually computed as the normalized vector from the surface point to the light position, i.e., normalize(lightPos - fragPos). The sign convention matters: many shading models treat lightDir as the direction from the surface toward the light, so the diffuse term is max(dot(N, lightDir), 0). If lightDir is defined as from the light toward the surface, the calculation uses max(dot(N, -lightDir), 0).

LightDir is commonly used in lighting models such as Lambertian diffuse reflection and specular terms in Phong or Blinn-Phong models. In Blinn-Phong, the half-vector H is defined as normalize(lightDir + viewDir), and the specular term depends on dot(N, H). In Phong, the reflection vector R is used with dot(R, viewDir). Properly transforming lightDir into the same coordinate space as the normal N is essential for correct results.

In shader code, lightDir is frequently supplied as a uniform or computed per fragment and named consistently across languages.