/*
TEXTBOOK LIGHTING MODEL

A = Am * Al
A is the final ambitent color, Am is the material's ambient and Al is the light's

D = Dm * Dl * dot( normal, light_dir )
D stands for diffuse color, Dm for material's diffuse and Dl for light's

S = Sm * Sl * si     ** this shader uses a slightly different method **
S is specular color and Sm, Sl as above.
si is the specular intensity and its equal to: si = pow( dot(normal, h), material_shininess )
h is h = normalize( light_dir + eye )

C = A * D * S
C is the final color vector

*/

uniform sampler2D tex0, tex1;
varying vec3 normal, vert_pos;
varying vec4 diffuse, ambient, specular;

void main()
{
  float shininess = gl_FrontMaterial.shininess;

  vec3 lpos = gl_LightSource[0].position.xyz;
  vec3 light_dir = normalize( lpos-vert_pos );
	vec3 n = normalize(normal);
	float ndotl = dot(n, light_dir);
	vec4 color = ambient + ( diffuse * max(0.0, ndotl) );


	// add some specular lighting IF necessary
  if( ndotl > 0.0 && shininess != 0.0 )
  {
  	/*
  	TEXTBOOK METHOD:
  	the textbook way to calc spec color is as explained on the top:
  	S = Sm * Sl * si;

  	MY METHOD:
  	The "ndotl > 0.0" check removes some unwanted specular calculations but it doesnt produces color under some conditions. This problem happens when:
		1) the angle of the vert's normal and light is grater than than 90degr and
		2) the material_shininess is small (close to 8.0)
  	So we mul the si with dot(normal, light) to correct this. The result is not so correct but it looks close to the standard one and it
  	allows us to use the the "ndotl > 0.0" check without the incorrect results. So:
  	S = Sm * Sl * ( si * dot(normal, light) );
  	*/

		vec3 eye = -vert_pos.xyz;
		eye = normalize(eye);

		vec3 h = normalize( light_dir + eye );
		float spec_intensity = pow(max(0.0, dot(n, h)), shininess);
		color += specular * (spec_intensity * ndotl);
  }

	vec4 texel0 = texture2D( tex0, gl_TexCoord[0].st );
	vec4 texel1 = texture2D( tex1, gl_TexCoord[0].st );
	vec4 texel = texel0 * texel1;

  gl_FragColor = color ;
}