CS 435 Autumn 1997 --- 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 ad 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

• Directions lights:
  Add a light center of interest for each light source. Create the light direction vector as (light_coi - light_position). Calculate the angle, beta, between the light direction vector and the vector from the light to the point being rendered. Create a falloff of the light source using cos(beta)ⁿ.
• Angle limited lights:
  Add a angle of illumination of each light and check to see if the the point being rendered is within this angular area.

 if ( cos(gamma) > cos(beta) ) 
   light_intensity = light_intensity * cosn(beta)
 else 
  light_intensity=0
    

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