Guide to Game Development/Theory/Lighting/Light normals

To show how light should interact with the geometry, you should use a light normal vector.

Face normals vs vertex normals

Left image: Face normals(a), Vertex normals(b). Right image: Vertex normals on a shape with angles less than 90°. Note that the geometry of the image is appearing like a face normal despite the use of vertex normals (replacement for more intuitive image maybe needed).

This is where you calculate the normal vector of each face and directly apply it onto each of the face's vertices. See a of the left image on the right for an illustration. Face normals are easier to compute and they give a 'retro' look to the geometry.

${\displaystyle {\overrightarrow {V_{1}}}+{\overrightarrow {V_{2}}}\dots +{\overrightarrow {V_{n}}}={\overrightarrow {V}}}$, then normalise it: ${\displaystyle {\hat {V}}={\frac {\overrightarrow {V}}{|{\overrightarrow {V}}|}}}$.
${\displaystyle {\hat {V}}={\frac {{\overrightarrow {V_{1}}}+{\overrightarrow {V_{2}}}\dots +{\overrightarrow {V_{n}}}}{{\bigg |}{\overrightarrow {V_{1}}}+{\overrightarrow {V_{2}}}\dots +{\overrightarrow {V_{n}}}{\bigg |}}}}$