#version 330

out vec4 FragColor;

in vec3 eyeNorm;
in vec4 eyePosition;

struct LightInfo {
    vec4 Position; // Light position in eye coords.
    vec3 La; // Ambient light intensity
    vec3 Ld; // Diffuse light intensity
    vec3 Ls; // Specular light intensity
};

uniform LightInfo Light;

struct MaterialInfo {
    vec3 Ka; // Ambient reflectivity
    vec3 Kd; // Diffuse reflectivity
    vec3 Ks; // Specular reflectivity
    float Shininess; // Specular shininess factor
};

uniform MaterialInfo Material;

vec3 phongModel( vec4 position, vec3 norm )
{
    vec3 s = normalize(vec3(Light.Position - position));
    vec3 v = normalize(-position.xyz);
    vec3 r = reflect( -s, norm );
    vec3 ambient = Light.La * Material.Ka;
    float sDotN = max( dot(s,norm), 0.0 );
    vec3 diffuse = Light.Ld * Material.Kd * sDotN;
    vec3 spec = vec3(0.0);
    if( sDotN > 0.0 )
        spec = Light.Ls * Material.Ks * pow( max( dot(r,v), 0.0 ), Material.Shininess );
    return ambient + diffuse + spec;
}

void main() {

    vec3 FrontColor = phongModel( eyePosition, eyeNorm );
    vec3 BackColor = phongModel( eyePosition, -eyeNorm );

    if( gl_FrontFacing ) {
        FragColor = vec4(FrontColor, 1.0);
    } else {
        FragColor = vec4(BackColor, 1.0);
    }
}