Normally, local light sources shine equally in all directions.
Using spotlight options, you can have a light shine in a particular direction.
A spotlight's beam is a cone starting from the light position and pointing in the spotlight direction (GL_SPOT_DIRECTION).
The angle of the cone is defined by the GL_SPOT_CUTOFF value.
GL_SPOT_EXPONENT makes the brightness of the spotlight fall off toward the edges.
glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION, direction) glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, angle) glLightf(GL_LIGHT0, GL_SPOT_EXPONENT, exponent)
All lighting calculations are done only at the vertices.
Spotlights, like specular highlights, will not work well with large polygons.
The geometry must be finely tesselated for a good spotlight effect.
In the real world, the amount of illumination from a light source decreases the farther away you are from it.
By default, OpenGL lighting is the same, no matter how far you are from a light source.
Attenuation simulates the falloff of light with distance.
OpenGL attenuation uses a formula with 3 adjustable coefficients, so that you can vary light realistically, or semi-realistically:
1
brightness = -----------------------
Kc + Kl * d + Kq * d^2
These coefficients are controlled by the following calls:
glLightf(GL_LIGHT0, GL_CONSTANT_ATTENUATION, Kc); glLightf(GL_LIGHT0, GL_LINEAR_ATTENUATION, Kl); glLightf(GL_LIGHT0, GL_QUADRATIC_ATTENUATION, Kq);
Some example results:
| Kc, Kl, Kq | d = 0 | d=0.5 | d=1 | d=10 | d=100 |
|---|---|---|---|---|---|
| 1, 0, 0 | 1 | 1 | 1 | 1 | 1 |
| 1, 0.1, 0 | 1 | 0.95 | 0.91 | 0.5 | 0.09 |
| 1, 0, 0.1 | 1 | 0.98 | 0.91 | 0.09 | 0.001 |
| 0.1, 0.1, 0 | 10 | 6.67 | 5 | 0.91 | 0.1 |
Two-sided lighting affects how the backs of polygons are rendered.
It is enabled with glLightModel:
glLightModelf(GL_LIGHT_MODEL_TWO_SIDE, GL_TRUE)
A separate material can be specified for the backs of polygons:
glMaterialfv(GL_BACK, GL_DIFFUSE, [1, 1, 1, 1])
The front of a polygon is the side where the vertices appear in counter-clockwise order.
| Front | Back |
|---|---|
 |  |
When two-sided lighting is enabled:
if front side of polygon faces camera:
use GL_FRONT material
use normal as given
else if back side faces camera:
use GL_BACK material
reverse normal
When two-sided lighting is not enabled, the back of a polygon is illuminated exactly the same as the front - both sides will show the same color.
Fog is a useful effect for realistic large-scale scenes.
It provides an additional depth cue for distant objects - aerial perspective.
It can also be used to hide the edges of a 3D world.
OpenGL fog mixes the color of a fragment with the color of the fog. The degree of mixing is a function of the distance from the camera to the fragment.
finalColor = (1-f) * polyColor + f * fogColor
Example:
glEnable(GL_FOG)
glFogi(GL_FOG_MODE, GL_LINEAR)
glFogfv(GL_FOG_COLOR, [1, 1, 1, 1])
glFogf(GL_FOG_START, 5.0)
glFogf(GL_FOG_END, 40.0)
There are three different fog modes:
