A One-Sided Per-Pixel Lighting Example

This example is fairly complicated so the discussion is broken up into parts. The first part will be the vertex shader. Since we're doing everything in the pixel shader, the vertex shader is fairly simple.

/**
 * \file Lighting.vert
 */
varying vec3 normal;
varying vec3 vertex;

void main()
{
    // Calculate the normal
    normal = normalize(gl_NormalMatrix * gl_Normal);
   
    // Transform the vertex position to eye space
    vertex = vec3(gl_ModelViewMatrix * gl_Vertex);
       
    gl_Position = ftransform();
}

/**
 * \file OneSidedLighting.frag
 */
void calculateLighting(in int numLights, in vec3 N, in vec3 V, in float shininess,
                       inout vec4 ambient, inout vec4 diffuse, inout vec4 specular);

varying vec3 normal;
varying vec3 vertex;

void main()
{
    // Normalize the normal. A varying variable CANNOT
    // be modified by a fragment shader. So a new variable
    // needs to be created.
    vec3 n = normalize(normal);
   
    vec4 ambient  = vec4(0.0);
    vec4 diffuse  = vec4(0.0);
    vec4 specular = vec4(0.0);

    // In this case the built in uniform gl_MaxLights is used
    // to denote the number of lights. A better option may be passing
    // in the number of lights as a uniform or replacing the current
    // value with a smaller value.
    calculateLighting(gl_MaxLights, n, vertex, gl_FrontMaterial.shininess,
                      ambient, diffuse, specular);
   
    vec4 color = gl_FrontLightModelProduct.sceneColor  +
                 (ambient  * gl_FrontMaterial.ambient) +
                 (diffuse  * gl_FrontMaterial.diffuse) +
                 (specular * gl_FrontMaterial.specular);
                
    color = clamp(color, 0.0, 1.0);
   
    gl_FragColor = color;
}

The Phong Lighting Shader