OpenGL provides two types of light sources: directional and positional. A directional light source is considered to be an infinite distance away from the objects in the scene. Thus, its rays of light are considered parallel by the time they reach the object. A positional light is near or within the scene and the direction of its rays are taken into account in lighting calculations. Positional lights have a greater performance cost than directional lights due to these additional calculations.

OpenGL uses Phong Lighting Model for Specular reflected light.

Issues in Lighting with OpenGL

• In OpenGL, we can define up to eight light sources, specified by GL_LIGHT0, GL_LIGHT1, ... GL_LIGHTX where X is equal to the constant GL_MAX_LIGHTS. Each source has various properties, which have a default value, and must be enabled (and can be disabled). Use your virtual world coordinates to specify the position of a source of light. We also have to enable lighting itself.
• The command glLightfv() is used to specify the position of the light and whether it is directional or positional. It is also used to specify the values of the color components of the light source such as ambient color, diffuse color, specular color, emissive color, and shininess.
• Once the light sources are defined, normal vectors and material properties of the objects in the scene must also be defined. An object's normal vectors define its orientation relative to light sources. Normal vectors can be specified for each vertex and/or shared by vertices. The command glNormal() is used to make this assignment.
   
• The color components specified for lights have different meaning than those specified for materials. For lights,  the numbers correspond to the percentage of full intensity of each color. Therefore, the brightest possible white  light has RGB values of (1.0, 1.0, 1.0). For materials, the numbers correspond to the reflected proportions of those colors. The command glMaterialfv() is used to define material properties by specifying the values of the color components of materials.
• The command glColorMaterial() is used to minimize the performance costs associated with changing material properties. This command should be used whenever a single material property, such as diffuse color, must be changed for most vertices in the scene. Any change to the current color made by a call to glColor() immediately updates the material property specified by glColorMaterial(). The capability state variable GL_COLOR_MATERIAL must be enabled by the call glEnable(GL_COLOR_MATERIAL).
• Lastly, lighting must be enabled by a call to glEnable(GL_LIGHTING) and each light in the scene must be enabled by calls to glEnable(GL_LIGHTi) where GL_LIGHTi is the symbolic name of the light.
   

This example is a modification of Polygons_List.c to use lighting. The three sided pyramid was changed to a four-sided pyramid to make calculating the normal vectors easier.

                                           Light.c Output 
 
 

    Light.c Source Code
    (NOTE: ABOVE CODE IS IN TK windowing NOT GLUT windowing system)
 

Notice in the source code the order in which normals are defined for GL_QUAD_STRIP. In the GL_QUAD_STRIP section, Normal A applies to vertices 1, 2, 3, and 4; Normal B applies to vertices 3, 4, 5, and 6; Normal C applies to vertices 5, 6, 7, and 8; and Normal D applies to vertices 7, 8, 9, and 10. Due to the way that OpenGL renders objects, the correct normal value must be set before the last vertex in the current object is specified so that the correct normal value is assigned to that object. 

 

The STEPS

1) Enable Lighting glEnable(GL_LIGHTING);

2) Setup each Light Source you want to create GL_LIGHTx (where 0<=x<=GL_MAX_LIGHTS)   glLightfv(GL_LIGHT1, GL_POSITION, position1); glLightfv(GL_LIGHT1, GL_AMBIENT, lightambient1); glLightfv(GL_LIGHT1, GL_DIFFUSE, lightdiffuse1);

3) Enable each Light Created in Step 2 glEnable(GL_LIGHTx)

 

Spotlights Spotlights are point sources but emit light over a narrow range of angles. Imagine a cone of light coming from the spotlight. What must we do to create one is to alter the following parameters using the glLightx() function on the light under question: We have to specify its spread. Specify the angle between the axis of the cone and a ray along its edge. GL_SPOT_CUTOFF default 180 giving a spread of 360 - no cone Values for the cutoff angle must lie between 0 and 90 (unless it is 180 in which case it is not a spotlight). We also specify a direction for spotlights. They have position and direction. GL_SPOT_DIRECTION default 0.0,0.0,-1.0 (pointing down negative z-axis) Finally, we can control the intesity of the distribution of light within the cone. This controls how concentrated the light is. The intensity of the light is highest in the centre of the cone. It is attenuated towards the edges. The higher we set the spot exponent the more focused the light source (appears - brighter, shinier, more intense) GL_SPOT_EXPONENT default=0