// From https://gitorious.org/wikibooks-opengl/modern-tutorials/source/mini-portal/phong-shading.f.glsl

/**
* This file is in the public domain.
* Contributors: Martin Kraus, Sylvain Beucler
*/
varying vec4 position;  // position of the vertex (and fragment) in world space
varying vec3 varyingNormalDirection;  // surface normal vector in world space
uniform mat4 m, v, p;
uniform mat4 v_inv;

struct lightSource
{
	vec4 position;
	vec4 diffuse;
	vec4 specular;
	float invLinearAttenuation;
	float spotCutoff, spotExponent;
	vec3 spotDirection;
};
lightSource light0 = lightSource(
	vec4(0.0,  3.0,  0.0, 1.0),
	vec4(1.0,  1.0,  1.0, 1.0),
	vec4(1.0,  1.0,  1.0, 1.0),
	2.0,
	180.0, 0.0,
	vec3(0.0, 0.0, 0.0)
);
vec4 scene_ambient = vec4(0.2, 0.2, 0.2, 1.0);

struct material
{
	vec4 ambient;
	vec4 diffuse;
	vec4 specular;
	float shininess;
};
material frontMaterial = material(
	vec4(0.2, 0.2, 0.2, 1.0),
	vec4(1.0, 0.8, 0.8, 1.0),
	vec4(1.0, 1.0, 1.0, 1.0),
	5.0
);

void main()
{
	vec3 normalDirection = normalize(varyingNormalDirection);
	vec3 viewDirection = normalize(vec3(v_inv * vec4(0.0, 0.0, 0.0, 1.0) - position));
	vec3 lightDirection;
	float attenuation;

	// initialize total lighting with ambient lighting
	vec3 totalLighting = vec3(scene_ambient) * vec3(frontMaterial.ambient);

	vec3 positionToLightSource = vec3(light0.position - position);
	float distanceSqr = dot(positionToLightSource, positionToLightSource);
	lightDirection = inversesqrt(distanceSqr) * positionToLightSource;
	attenuation = light0.invLinearAttenuation * inversesqrt(distanceSqr);

	vec3 diffuseReflection = attenuation 
	* vec3(light0.diffuse) * vec3(frontMaterial.diffuse)
	* max(0.0, dot(normalDirection, lightDirection));

	vec3 specularReflection;
	if (dot(normalDirection, lightDirection) < 0.0) // light source on the wrong side?
	{
		specularReflection = vec3(0.0, 0.0, 0.0); // no specular reflection
	}
	else // light source on the right side
	{
		specularReflection = attenuation * vec3(light0.specular) * vec3(frontMaterial.specular)
		* pow(max(0.0, dot(reflect(-lightDirection, normalDirection), viewDirection)), frontMaterial.shininess);
	}

	totalLighting = totalLighting + diffuseReflection + specularReflection;

	gl_FragColor = vec4(totalLighting, 1.0);
}