Illumination model
(1) Ambient
light (漫射)
I = la * ka * Obj(r, g, b)
la : intensity of ambient light
ka : 0.0 ~ 1.0, Obj(r, g, b): object
color
(2) Diffuse
reflection (散射)
I = Ip (r, g, b) * Kd * Obj(r, g, b) * COS(theta-angle)
Ip (r, g, b): light color
