6.2.2 Diffuse Reflected Light

Light reaching the object from some light_source will be (partially) reflected, mainly in the direction opposite to the incoming ray with respect to the surface normal but also in all other directions in particular if the surface is rather soft like velvet.

In order to account for this later type of reflection the following clause is used in the finish statement:

	diffuse T

Which says that the fraction $T$ of the light coming (directly) from light sources will contribute to the color of the object. Note that the intensity of the light coming from the light source will also depend on the angle $\varphi$ of the incoming ray with respect to the surface normal. In fact, if the amount of light reaching the surface is $I_s$ per square unit normal to the incoming ray, then the area of the surface hit by this light is $1/\cos(\varphi )$ and hence the intensity it contributes is

$$I_d := T*I_s*\cos(\varphi ).$$