CS 435 Autumn 1998 --- Introduction to Interactive Graphics

Lights and Illumination

Colored Lights

The previous formulas assumed that we had white light.
The color of the light source will of course affect the color of light reflected from a surface.
Remember, the color of an object is a function of the incident wavelengths of light that it reflects.

Illumination with Colored Lights

To add colored lights to our illumination model, we simply need to multiply each illumination component by the color of the light.

Diffuse and Ambient with colored lights

   if (N · L) > 0
     d_a.r = ka* object_color.r + kd(N · L) * object_color.r * light_color.r
     d_a.g = ka* object_color.g + kd(N · L) * object_color.g * light_color.g
     d_a.b = ka* object_color.b + kd(N · L) * object_color.b * light_color.b

Specular Illumination with colored lights

For our specular illumination, the color of the highlight will now be the color of the light, so the formula gets changed as follows:
     if (N · L > 0)
        n_h = (MAX(0, (N · H) )spec_exp
        spec.r  = obj_ks[obj]*n_h * light_color.r;
        spec.g  = obj_ks[obj]*n_h * light_color.g;
        spec.b  = obj_ks[obj]*n_h * light_color.b;
     final_color.r = MIN(1.0, (d_a.r + (light_color.r - d_a.r)*spec.r))
     final_color.g = MIN(1.0, (d_a.g + (light_color.g - d_a.g)*spec.g))
     final_color.b = MIN(1.0, (d_a.b + (light_color.b - d_a.b)*spec.b))

Multiple Lights

Multiple lights are very easy to implement.
All you need to do is to add the illumination from each light source, making sure that you do not allow the final color values to exceed 1.0 .

Other Light Effects

Examples of Lighting Effects

Single White Light Light
Colored Lights 
Single Red Light Source Red Light Source plus a Blue Light Source  Red Light Source, Green Light Source 
Blue Light Source 
Angle and Fall-Off Effects 
Nomal Illumination Angular Fall-Off 
with exponent = 800 
Angular Cut-off (spotlight) 
Combination of Effects 
4 light sources of different colors and fall-offs 

David S. Ebert

November 1998