OpenGL Programming Guide

Chapter 6Lighting

Creating Light Sources

Multiple Lights

As mentioned, you can have at least eight lights in your scene (possibly more, depending on your OpenGL implementation). Since OpenGL needs to perform calculations to determine how much light each vertex receives from each light source, increasing the number of lights adversely affects performance. The constants used to refer to the eight lights are GL_LIGHT0, GL_LIGHT1, GL_LIGHT2, GL_LIGHT3, and so on. In the preceding discussions, parameters related to GL_LIGHT0 were set. If you want an additional light, you need to specify its parameters; also, remember that the default values are different for these other lights than they are for GL_LIGHT0, as explained in Table 6-1 . The following lines of code define a white attenuated spotlight:

GLfloat light1_ambient[] = { 0.2, 0.2, 0.2, 1.0 };
GLfloat light1_diffuse[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat light1_specular[] = { 1.0, 1.0, 1.0, 1.0 };
GLfloat light1_position[] = { -2.0, 2.0, 1.0, 1.0 };
GLfloat spot_direction[] = { -1.0, -1.0, 0.0 };

glLightfv(GL_LIGHT1, GL_AMBIENT, light1_ambient);
glLightfv(GL_LIGHT1, GL_DIFFUSE, light1_diffuse);
glLightfv(GL_LIGHT1, GL_SPECULAR, light1_specular);
glLightfv(GL_LIGHT1, GL_POSITION, light1_position);
glLightf(GL_LIGHT1, GL_CONSTANT_ATTENUATION, 1.5);
glLightf(GL_LIGHT1, GL_LINEAR_ATTENUATION, 0.5);
glLightf(GL_LIGHT1, GL_QUADRATIC_ATTENUATION, 0.2);

glLightf(GL_LIGHT1, GL_SPOT_CUTOFF, 45.0);
glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION, spot_direction);
glLightf(GL_LIGHT1, GL_SPOT_EXPONENT, 2.0);

glEnable(GL_LIGHT1);

If these lines were added to Example 6-1 , the sphere would be lit with two lights, one directional and one spotlight.

Try This

Try This

Modify Example 6-1 :

• Change the light to be a positional colored light rather than a directional white one.
  
  

• Add an additional colored spotlight. Hint: Use some of the code shown in the preceding section.
  
  

• Measure how these two changes affect performance.

OpenGL Programming Guide