Rendering  Illuminated Objects To render lit surfaces OpenGL must: Define normal vectors for each vertex of all the objects  Create, select, and position one or more light sources.  Create and select a lighting model Define material properties for the objects  (walk through code)     Programming Notes: Make sure each light is enabled after defining Make sure lighting is enabled Changes in lighting may be utilized with display lists for potential performance inprovements.

	Creating light souces   void glLight{if}(GLenum light, GLenum pname, TYPE param);  void glLight{if}v(GLenum light, GLenum pname, TYPE *param);  Creates the light specified by light, which can be GL_LIGHT0, GL_LIGHT1, ... , or GL_LIGHT7.

	Lighting Parameters   NOTE!  Default values of GL_DIFFUSE and GL_SPECULAR apply only to GL_LIGHT0 (0.0, 0.0, 0.0, 1.0) for the other lights

Specifying the position and color of a light    GLfloat light_ambient[] = { 0.0, 0.0, 0.0, 1.0 };  GLfloat light_diffuse[] = { 1.0, 1.0, 1.0, 1.0 };  GLfloat light_specular[] = { 1.0, 1.0, 1.0, 1.0 }; GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 };  glLightfv(GL_LIGHT0, GL_AMBIENT, light_ambient);  glLightfv(GL_LIGHT0, GL_DIFFUSE, light_diffuse);  glLightfv(GL_LIGHT0, GL_SPECULAR, light_specular);  glLightfv(GL_LIGHT0, GL_POSITION, light_position); Note: Remember to turn on each light with glEnable(). (See "Enabling Lighting" for more information about how to do this.)

Color OpenGL allows you to associate color with three different paramters: GL_DIFFUSE color correlates to the "typical" color of a light Defines the color and intensity that a light adds to the scene GL_SPECULAR color defines what color specular highlights will be GL_AMBIENT color defines the amount of ambient light that a source adds to a scene (picture of angular reflectivity)   NOTE!  We are ignoring the contribution of the alpha parameter until we discuss blending

Light Position Lights can be considered directional or positional Rays of directional lights can be considered to be parallel The sun is an example of a real-world directional light  A desklamp is an example of a positional light source GLfloat light_position[] = { 1.0, 1.0, 1.0, 0.0 };  glLightfv(GL_LIGHT0, GL_POSITION, light_position); The light position is defined as (x, y, z, w) w  = 0.0:    (x, y, z) = light direction w  != 0.0:   (x, y, z) = light position    By Default GL_POSITION = {0, 0, 1, 0} (directional, along the -Z axis) Positional lights radiate in all directions by default may be defined as spotlights

	Programming Note: Large Polygons with local (positional) lights will interpolate the light values across the polygon A problem for LARGE polygons (they may appear darker) Break the polygon into smaller polygonal pieces

Attenuation With real-world lights, intensity decreases for objects which are further away from the source Directional lights are infinately far away -- attenuation is disabled Attenuation is customizable for local lights                 k c = GL_CONSTANT_ATTENUATION  k l = GL_LINEAR_ATTENUATION  k q = GL_QUADRATIC_ATTENUATION By default kc = 1.0 kl = 0.0 and kq = 0.0      NOTE:  All lighting values are attenuated exceptglobal ambient and emmisive Because of the extra math, these calculations have lower performance -- use spairingly

	Spotlights Positional lights may have their effect constrained similiar to a spotlight                      GL_SPOT_CUTOFF selects the angle for the spotlight By default the value is 180.0 -- omnidirectional The definable range is 0.0 -> 90.0 (no definition above 90.0) Define by: glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, 45.0); A direction must be defined also for spotlights [(0, 0, -1) default]

	Spotlights (cont)  Spotlights attenuation may be defined as described previously Spotlight concentration is controled by GL_SPOT_EXPONENT Vertex intensity is attenuated by the cosine of the angle from the light source  raised to the power of the exponent