Although an object illuminated by ambient light is lit in proportion to the ambient intensity, it is uniformly illuminated across the surface.

Dull or matte surfaces, like chalk, exhibit diffuse reflection (also known as Lambertian reflection).

These surfaces appear equally bright from all viewing angles.

However, the brightness changes based on the angle between the surface normal n and the direction to the light source, s.  This is conveniently expressed as a dot product where n and s are normalised.



A light’s diffuse component ld is reflected by a surface.  The amount of light reflection at each vertex is based on surface normal n, the light source direction s, and the surface diffuse material, md.

The colour resulting from the diffuse reflection is:

High

Normally, all vectors used in lighting are normalised
So n Ÿ· s = ||n|| · ||s|| cos(θ) = cos(θ)

Let’s consider a few cases:



If n ·Ÿ s < 0, no diffuse reflection is modelled