Stefan Röttger

varying vec3 normal;
varying vec3 vertex_to_light_vector;

void main()
{
   const vec4 tone1 = vec4(1.0, 0.5, 0.5, 1.0);
   const vec4 tone2 = vec4(0.6, 0.3, 0.3, 1.0);
   const vec4 tone3 = vec4(0.4, 0.2, 0.2, 1.0);
   const vec4 tone4 = vec4(0.2, 0.1, 0.1, 1.0);

   vec3 normalized_normal = normalize(normal); // renormalization

   float diffuseTerm = clamp(dot(normal, vertex_to_light_vector), 0.0, 1.0);

   if (diffuseTerm>0.95) gl_FragColor = tone1;
   else if (diffuseTerm>0.5) gl_FragColor = tone2;
   else if (diffuseTerm>0.25) gl_FragColor = tone3;
   else gl_FragColor = tone4;
}