varying float diffuse_value;

void main() {			
	// Calculate the normal value for this vertex, in world coordinates (multiply by gl_NormalMatrix)
	vec3 vertex_normal = normalize(gl_NormalMatrix * gl_Normal);
	// Calculate the light position for this vertex
	vec3 vertex_light_position = gl_LightSource[0].position.xyz;
	// Set the diffuse value (darkness). This is done with a dot product between the normal and the light
	// and the maths behind it is explained in the maths section of the site.
	diffuse_value = max(dot(vertex_normal, vertex_light_position), 0.0);

	// Set the front color to the color passed through with glColor*f
	gl_FrontColor = gl_Color;
	// Set the position of the current vertex 
	gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
}