/*
*   Fragment shader for normal mapping and texture mapping. Basically, we 
*       do texture shading as we did before, but we use the normal specified
*       in the normal map instead of the regular normal. Note that the normal
*       that's specified in the normal map is given in TANGENT SPACE, not world space --
*       so instead of converting the normal back into eye space for lighting,
*       we converted the lighting direction into tangent space back in the vertex
*       shader. 
*
*   Alexandri Zavodny, Fall 2010 
*       for CSE 40166/60166 Computer Graphics, University of Notre Dame
*/

varying vec4 theColor;
uniform sampler2D colorTex, bumpTex;
varying vec3 dirToLight;

void main()
{
    vec2 uvCoords = gl_TexCoord[0].st;

    //grab the texture coordinate as usual
    vec4 texColor = texture2D(colorTex, uvCoords);

    //and grab the normal from the map at the same coordinates.
    vec3 normDist = vec3(texture2D(bumpTex, uvCoords));

    //the normals are packed to the range of [0,1] because images don't
    //have negative values -- we need to expand the range to [0,2] and then
    //move it to [-1.0, 1.0]
    normDist = 2.0 * normDist - vec3(1.0,1.0,1.0);

    //compute lighting as usual! using the new normal, of course.
    float lambertianIntensity = max(dot(normalize(normDist), normalize(dirToLight)),0.0);

    gl_FragColor = lambertianIntensity * texColor;
}