/*
*   Vertex shader for some simple lighting formulae. The 'mode' variable determines
*       whether we do Gourad (mode == 0), Phong (mode == 1), or Cel shading (mode == 2).
*       If we do Gourad shading, we compute the lighting equation here in the vertex
*       shader and pass it along to the fragment shader; otherwise, lighting gets
*       computed per-pixel in the fragment shader.
*
*   Alexandri Zavodny, Fall 2010 
*       for CSE 40166/60166 Computer Graphics, University of Notre Dame
*/

varying vec3 dirToLight, theNormal, theColor;
uniform int mode;
uniform vec3 ambient;


void main()
{
    //transform the normal into eye space and make sure it stays normalized
    //using normalize() here is like using glEnable(GL_NORMALIZE)!
    theNormal = normalize(gl_NormalMatrix * gl_Normal);

    //calculate the direction to the light as usual; the light source is in eye space
    //but gl_Vertex is in object space, so be sure to transform it first!
    dirToLight = normalize(vec3(gl_LightSource[0].position - gl_ModelViewMatrix * gl_Vertex));

    //pass the color along to the fragment shader
    theColor = vec3(gl_Color);

    //and compute lambertian intensity using the simple formula. note that we only
    //use this intensity if mode == 0 (which is gourad shading).
    float lambertianIntensity = max(dot(normalize(theNormal), normalize(dirToLight)),0.0);
    if(mode == 0)
        theColor = lambertianIntensity * theColor + ambient;

    gl_Position = ftransform();
}