OpenGL - Lighting

There are various types of light that can be used in the scene to illuminate a model. These are:

Ambient light exists in the environment as scattered light. It is impossible to determine its source. It is scattered equally in all directions when it strikes a surface.
Diffuse light comes from one direction so it is brighter if it hits the surface directly. It is scattered equally when it strikes a surface.
Specular light comes from a particular direction and also bounces off the surface in a particular direction. It is also called shininess
Emissive light is light that is, effectvely, emitted from the object itself, although it doesn't contribute light to the overall scene.

As well as defining a light to illuminate the object/scene, the objects in the scene have material properties similar to the light properties.

Ambient and diffuse material properties are combined with the relevant properties from each light source. These define the colour of the object and generally very similar in effect.
Specular material is combined with the properties of the light source. It is usually white, think of a light reflecting off a very shiny surface, the center of the reflection of the light is usually white, or the colour of the light source.

In order to light a scene you have to:

Define normal vectors for each vertex or polygon.

Create and position light sources.

Create a lighting model.

Define material properties.

Here is the cube from before, with default lighting:

Here is the same cube as before, with one light source:

Use these properties (in initialise routine):

  GLfloat mat_shininess[] = { 50.0 };
  GLfloat light0_position[] = { 1.0, 2.0, 5.0, 0.0 };
  GLfloat white_light[] = {1.0, 1.0, 1.0, 1.0};
  GLfloat lmodel_ambient[] = {0.1, 0.1, 0.1, 1.0};

  glClearColor( 0.0, 0.0, 0.0, 0.0 ); // Let OpenGL clear to black
  glShadeModel( GL_SMOOTH );
  glMaterialfv(GL_FRONT, GL_SHININESS, mat_shininess);
  glLightfv(GL_LIGHT0, GL_POSITION, light0_position);
  glLightfv(GL_LIGHT0, GL_DIFFUSE, white_light);
  glLightfv(GL_LIGHT0, GL_SPECULAR, white_light);
  glLightModelfv(GL_LIGHT_MODEL_AMBIENT, lmodel_ambient);

  glEnable(GL_LIGHTING);
  glEnable(GL_LIGHT0);
  glEnable(GL_LIGHT1);
  glEnable(GL_DEPTH_TEST);

add a glNormal3f() for each polygon

use glMaterialfv() in place of glColor()

Example ~ top:

  GLfloat mat_R[] = {1.0, 0.0, 0.0};
  GLfloat mat_G[] = {0.0, 1.0, 0.};
  GLfloat mat_B[] = {0.0, 0.0, 1.0};
  GLfloat mat_Y[] = {1.0, 1.0, 0.0};
  GLfloat mat_M[] = {1.0, 0.0, 1.0};
  GLfloat mat_C[] = {0.0, 1.0, 1.0};

	 glBegin(GL_POLYGON); //top
		glMaterialfv(GL_FRONT, GL_DIFFUSE, mat_M);
		glNormal3f(0.0, 1.0, 0.0);
		glVertex3f(0.0, 1.0, 0.0); //1
		glVertex3f(1.0, 1.0, 0.0); //2
		glVertex3f(1.0, 1.0, 1.0); //7
		glVertex3f(0.0, 1.0, 1.0); //6
    glEnd();

Here is an example, with two light sources:

Add another light:

  GLfloat light1_position[] = { -1.0, 2.0, 5.0, 0.0 };
  glLightfv(GL_LIGHT1, GL_POSITION, light1_position);
  glLightfv(GL_LIGHT1, GL_DIFFUSE, white_light);
  glLightfv(GL_LIGHT1, GL_SPECULAR, white_light);

Remember to check the documentation.

More OpenGL ~ 3D objects

Useful References

OpenGL Programming Guide (The Red Book).

OpenGL Index in Alphabetic Order (This is pretty long)

The OpenGL Website

The OpenGL Website - tutorials

Contact me
email: pj@cpsc.ucalgary.ca
Tel: (403) 220 7041.