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Lighting Model

Currently OpenGL lighting model has three components: Global Ambient Light. Each light source can contribute ambient light to a scene. In addition, there can be other ambient light that's not from any particular source: 
  GLfloat lmodel_ambient[] = { 0.2, 0.2, 0.2, 1.0 };
  glLightModelfv(GL_LIGHT_MODEL_AMBIENT, lmodel_ambient);

Local or Infinite Viewpoint. The location of the viewpoint affects the calculations for highlights produced by specular reflectance. More specifically, the intensity of the highlight at a particular vertex depends on the normal at that vertex, the direction from the vertex to the light source, and the direction from the vertex to the viewpoint.

With an infinite viewpoint, the direction between it and any vertex in the scene remains constant. A local viewpoint tends to yield more realistic results, but since the direction has to be calculated for each vertex, overall performance is decreased with a local viewpoint. By default, an infinite viewpoint is assumed. Here's how to change to a local viewpoint: 

  glLightModeli(GL_LIGHT_MODEL_LOCAL_VIEWER, GL_TRUE);
This call places the viewpoint at (0, 0, 0) in eye coordinates. To switch back to an infinite viewpoint, pass in GL_FALSE as the argument.

Two-sided Lighting. Lighting calculations are performed for all polygons, whether they're front-facing or back-facing. Since you usually set up lighting conditions with the front-facing polygons in mind, however, the back-facing ones typically aren't correctly illuminated.

You might want to have the inside surface be fully lit according to the lighting conditions you've defined; you might also want to supply a different material description for the back faces. When you turn on two-sided lighting, as follows 

  glLightModeli(LIGHT_MODEL_TWO_SIDE, GL_TRUE);
OpenGL reverses the surface normals for back-facing polygons; typically, this means that the surface normals of visible back- and front-facing polygons face the viewer, rather than pointing away. As a result, all polygons are illumnated correctly.

glLightModel

NAME

glLightModelf, glLightModeli, glLightModelfv, glLightModeliv - set the lighting model parameters

C SPECIFICATION

void glLightModelf( GLenum pname, GLfloat param )

void glLightModeli( GLenum pname, GLint param )

PARAMETERS

pname 

Specifies a single-valued lighting model parameter. GL_LIGHT_MODEL_LOCAL_VIEWER and GL_LIGHT_MODEL_TWO_SIDE are accepted.

param 

Specifies the value that param will be set to.

C SPECIFICATION

void glLightModelfv( GLenum pname, const GLfloat *params )

void glLightModeliv( GLenum pname, const GLint *params )

PARAMETERS

pname 

Specifies a lighting model parameter. GL_LIGHT_MODEL_AMBIENT, GL_LIGHT_MODEL_LOCAL_VIEWER, and GL_LIGHT_MODEL_TWO_SIDE are accepted.

params 

Specifies a pointer to the value or values that params will be set to.

DESCRIPTION

glLightModel sets the lighting model parameter. pname names a parameter and params gives the new value. There are three lighting model parameters:

GL_LIGHT_MODEL_AMBIENT  


params contains four integer or floating-point values that specify the ambient RGBA intensity of the entire scene. Integer values are mapped linearly such that the most positive representable value maps to 1.0, and the most negative representable value maps to -1.0. Floating-point values are mapped directly. Neither integer nor floating-point values are clamped. The default ambient scene intensity is (0.2, 0.2, 0.2, 1.0).

GL_LIGHT_MODEL_LOCAL_VIEWER  


params is a single integer or floating-point value that specifies how specular reflection angles are computed. If params is 0 (or 0.0), specular reflection angles take the view direction to be parallel to and in the direction of the -z axis, regardless of the location of the vertex in eye coordinates. Otherwise specular reflections are computed from the origin of the eye coordinate system. The default is 0.

GL_LIGHT_MODEL_TWO_SIDE  


params is a single integer or floating-point value that specifies whether one- or two-sided lighting calculations are done for polygons. It has no effect on the lighting calculations for points, lines, or bitmaps. If params is 0 (or 0.0), one-sided lighting is specified, and only the front material parameters are used in the lighting equation. Otherwise, two-sided lighting is specified. In this case, vertices of back-facing polygons are lighted using the back material parameters, and have their normals reversed before the lighting equation is evaluated. Vertices of front-facing polygons are always lighted using the front material parameters, with no change to their normals. The default is 0.

In RGBA mode, the lighted color of a vertex is the sum of the material emission intensity, the product of the material ambient reflectance and the lighting model full-scene ambient intensity, and the contribution of each enabled light source. Each light source contributes the sum of three terms: ambient, diffuse, and specular. The ambient light source contribution is the product of the material ambient reflectance and the light's ambient intensity. The diffuse light source contribution is the product of the material diffuse reflectance, the light's diffuse intensity, and the dot product of the vertex's normal with the normalized vector from the vertex to the light source. The specular light source contribution is the product of the material specular reflectance, the light's specular intensity, and the dot product of the normalized vertex-to-eye and vertex-to-light vectors, raised to the power of the shininess of the material. All three light source contributions are attenuated equally based on the distance from the vertex to the light source and on light source direction, spread exponent, and spread cutoff angle. All dot products are replaced with zero if they evaluate to a negative value.

The alpha component of the resulting lighted color is set to the alpha value of the material diffuse reflectance.

In color index mode, the value of the lighted index of a vertex ranges from the ambient to the specular values passed to glMaterial using GL_COLOR_INDEXES. Diffuse and specular coefficients, computed with a (.30, .59, .11) weighting of the lights' colors, the shininess of the material, and the same reflection and attenuation equations as in the RGBA case, determine how much above ambient the resulting index is.

ERRORS

GL_INVALID_ENUM is generated if pname is not an accepted value.

GL_INVALID_OPERATION is generated if glLightModel is called between a call to glBegin and the corresponding call to glEnd.

ASSOCIATED GETS

glGet with argument GL_LIGHT_MODEL_AMBIENT
glGet with argument GL_LIGHT_MODEL_LOCAL_VIEWER
glGet with argument GL_LIGHT_MODEL_TWO_SIDE
glIsEnabled with argument GL_LIGHTING