float punctualLightIntensityToIrradianceFactor( const in float lightDistance, const in float cutoffDistance, const in float decayExponent ) {

#if defined ( PHYSICALLY_CORRECT_LIGHTS )

	// based upon Frostbite 3 Moving to Physically-based Rendering
	// page 32, equation 26: E[window1]
	// https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf
	// this is intended to be used on spot and point lights who are represented as luminous intensity
	// but who must be converted to luminous irradiance for surface lighting calculation
	float distanceFalloff = 1.0 / max( pow( lightDistance, decayExponent ), 0.01 );

	if( cutoffDistance > 0.0 ) {

		distanceFalloff *= pow2( saturate( 1.0 - pow4( lightDistance / cutoffDistance ) ) );

	}

	return distanceFalloff;

#else

	if( cutoffDistance > 0.0 && decayExponent > 0.0 ) {

		return pow( saturate( -lightDistance / cutoffDistance + 1.0 ), decayExponent );

	}

	return 1.0;

#endif

}

vec3 BRDF_Diffuse_Lambert( const in vec3 diffuseColor ) {

	return RECIPROCAL_PI * diffuseColor;

} // validated

vec3 F_Schlick( const in vec3 specularColor, const in float dotLH ) {

	// Original approximation by Christophe Schlick '94
	// float fresnel = pow( 1.0 - dotLH, 5.0 );

	// Optimized variant (presented by Epic at SIGGRAPH '13)
	// https://cdn2.unrealengine.com/Resources/files/2013SiggraphPresentationsNotes-26915738.pdf
	float fresnel = exp2( ( -5.55473 * dotLH - 6.98316 ) * dotLH );

	return ( 1.0 - specularColor ) * fresnel + specularColor;

} // validated

// Microfacet Models for Refraction through Rough Surfaces - equation (34)
// http://graphicrants.blogspot.com/2013/08/specular-brdf-reference.html
// alpha is "roughness squared" in Disney’s reparameterization
float G_GGX_Smith( const in float alpha, const in float dotNL, const in float dotNV ) {

	// geometry term (normalized) = G(l)⋅G(v) / 4(n⋅l)(n⋅v)
	// also see #12151

	float a2 = pow2( alpha );

	float gl = dotNL + sqrt( a2 + ( 1.0 - a2 ) * pow2( dotNL ) );
	float gv = dotNV + sqrt( a2 + ( 1.0 - a2 ) * pow2( dotNV ) );

	return 1.0 / ( gl * gv );

} // validated

// Moving Frostbite to Physically Based Rendering 3.0 - page 12, listing 2
// https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf
float G_GGX_SmithCorrelated( const in float alpha, const in float dotNL, const in float dotNV ) {

	float a2 = pow2( alpha );

	// dotNL and dotNV are explicitly swapped. This is not a mistake.
	float gv = dotNL * sqrt( a2 + ( 1.0 - a2 ) * pow2( dotNV ) );
	float gl = dotNV * sqrt( a2 + ( 1.0 - a2 ) * pow2( dotNL ) );

	return 0.5 / max( gv + gl, EPSILON );

}

// Microfacet Models for Refraction through Rough Surfaces - equation (33)
// http://graphicrants.blogspot.com/2013/08/specular-brdf-reference.html
// alpha is "roughness squared" in Disney’s reparameterization
float D_GGX( const in float alpha, const in float dotNH ) {

	float a2 = pow2( alpha );

	float denom = pow2( dotNH ) * ( a2 - 1.0 ) + 1.0; // avoid alpha = 0 with dotNH = 1

	return RECIPROCAL_PI * a2 / pow2( denom );

}

// GGX Distribution, Schlick Fresnel, GGX-Smith Visibility
vec3 BRDF_Specular_GGX( const in IncidentLight incidentLight, const in GeometricContext geometry, const in vec3 specularColor, const in float roughness ) {

	float alpha = pow2( roughness ); // UE4's roughness

	vec3 halfDir = normalize( incidentLight.direction + geometry.viewDir );

	float dotNL = saturate( dot( geometry.normal, incidentLight.direction ) );
	float dotNV = saturate( dot( geometry.normal, geometry.viewDir ) );
	float dotNH = saturate( dot( geometry.normal, halfDir ) );
	float dotLH = saturate( dot( incidentLight.direction, halfDir ) );

	vec3 F = F_Schlick( specularColor, dotLH );

	float G = G_GGX_SmithCorrelated( alpha, dotNL, dotNV );

	float D = D_GGX( alpha, dotNH );

	return F * ( G * D );

} // validated

// Rect Area Light

// Real-Time Polygonal-Light Shading with Linearly Transformed Cosines
// by Eric Heitz, Jonathan Dupuy, Stephen Hill and David Neubelt
// code: https://github.com/selfshadow/ltc_code/

vec2 LTC_Uv( const in vec3 N, const in vec3 V, const in float roughness ) {

	const float LUT_SIZE  = 64.0;
	const float LUT_SCALE = ( LUT_SIZE - 1.0 ) / LUT_SIZE;
	const float LUT_BIAS  = 0.5 / LUT_SIZE;

	float dotNV = saturate( dot( N, V ) );

	// texture parameterized by sqrt( GGX alpha ) and sqrt( 1 - cos( theta ) )
	vec2 uv = vec2( roughness, sqrt( 1.0 - dotNV ) );

	uv = uv * LUT_SCALE + LUT_BIAS;

	return uv;

}

float LTC_ClippedSphereFormFactor( const in vec3 f ) {

	// Real-Time Area Lighting: a Journey from Research to Production (p.102)
	// An approximation of the form factor of a horizon-clipped rectangle.

	float l = length( f );

	return max( ( l * l + f.z ) / ( l + 1.0 ), 0.0 );

}

vec3 LTC_EdgeVectorFormFactor( const in vec3 v1, const in vec3 v2 ) {

	float x = dot( v1, v2 );

	float y = abs( x );

	// rational polynomial approximation to theta / sin( theta ) / 2PI
	float a = 0.8543985 + ( 0.4965155 + 0.0145206 * y ) * y;
	float b = 3.4175940 + ( 4.1616724 + y ) * y;
	float v = a / b;

	float theta_sintheta = ( x > 0.0 ) ? v : 0.5 * inversesqrt( max( 1.0 - x * x, 1e-7 ) ) - v;

	return cross( v1, v2 ) * theta_sintheta;

}

vec3 LTC_Evaluate( const in vec3 N, const in vec3 V, const in vec3 P, const in mat3 mInv, const in vec3 rectCoords[ 4 ] ) {

	// bail if point is on back side of plane of light
	// assumes ccw winding order of light vertices
	vec3 v1 = rectCoords[ 1 ] - rectCoords[ 0 ];
	vec3 v2 = rectCoords[ 3 ] - rectCoords[ 0 ];
	vec3 lightNormal = cross( v1, v2 );

	if( dot( lightNormal, P - rectCoords[ 0 ] ) < 0.0 ) return vec3( 0.0 );

	// construct orthonormal basis around N
	vec3 T1, T2;
	T1 = normalize( V - N * dot( V, N ) );
	T2 = - cross( N, T1 ); // negated from paper; possibly due to a different handedness of world coordinate system

	// compute transform
	mat3 mat = mInv * transposeMat3( mat3( T1, T2, N ) );

	// transform rect
	vec3 coords[ 4 ];
	coords[ 0 ] = mat * ( rectCoords[ 0 ] - P );
	coords[ 1 ] = mat * ( rectCoords[ 1 ] - P );
	coords[ 2 ] = mat * ( rectCoords[ 2 ] - P );
	coords[ 3 ] = mat * ( rectCoords[ 3 ] - P );

	// project rect onto sphere
	coords[ 0 ] = normalize( coords[ 0 ] );
	coords[ 1 ] = normalize( coords[ 1 ] );
	coords[ 2 ] = normalize( coords[ 2 ] );
	coords[ 3 ] = normalize( coords[ 3 ] );

	// calculate vector form factor
	vec3 vectorFormFactor = vec3( 0.0 );
	vectorFormFactor += LTC_EdgeVectorFormFactor( coords[ 0 ], coords[ 1 ] );
	vectorFormFactor += LTC_EdgeVectorFormFactor( coords[ 1 ], coords[ 2 ] );
	vectorFormFactor += LTC_EdgeVectorFormFactor( coords[ 2 ], coords[ 3 ] );
	vectorFormFactor += LTC_EdgeVectorFormFactor( coords[ 3 ], coords[ 0 ] );

	// adjust for horizon clipping
	float result = LTC_ClippedSphereFormFactor( vectorFormFactor );

/*
	// alternate method of adjusting for horizon clipping (see referece)
	// refactoring required
	float len = length( vectorFormFactor );
	float z = vectorFormFactor.z / len;

	const float LUT_SIZE  = 64.0;
	const float LUT_SCALE = ( LUT_SIZE - 1.0 ) / LUT_SIZE;
	const float LUT_BIAS  = 0.5 / LUT_SIZE;

	// tabulated horizon-clipped sphere, apparently...
	vec2 uv = vec2( z * 0.5 + 0.5, len );
	uv = uv * LUT_SCALE + LUT_BIAS;

	float scale = texture2D( ltc_2, uv ).w;

	float result = len * scale;
*/

	return vec3( result );

}

// End Rect Area Light

// ref: https://www.unrealengine.com/blog/physically-based-shading-on-mobile - environmentBRDF for GGX on mobile
vec3 BRDF_Specular_GGX_Environment( const in GeometricContext geometry, const in vec3 specularColor, const in float roughness ) {

	float dotNV = saturate( dot( geometry.normal, geometry.viewDir ) );

	const vec4 c0 = vec4( - 1, - 0.0275, - 0.572, 0.022 );

	const vec4 c1 = vec4( 1, 0.0425, 1.04, - 0.04 );

	vec4 r = roughness * c0 + c1;

	float a004 = min( r.x * r.x, exp2( - 9.28 * dotNV ) ) * r.x + r.y;

	vec2 AB = vec2( -1.04, 1.04 ) * a004 + r.zw;

	return specularColor * AB.x + AB.y;

} // validated


float G_BlinnPhong_Implicit( /* const in float dotNL, const in float dotNV */ ) {

	// geometry term is (n dot l)(n dot v) / 4(n dot l)(n dot v)
	return 0.25;

}

float D_BlinnPhong( const in float shininess, const in float dotNH ) {

	return RECIPROCAL_PI * ( shininess * 0.5 + 1.0 ) * pow( dotNH, shininess );

}

vec3 BRDF_Specular_BlinnPhong( const in IncidentLight incidentLight, const in GeometricContext geometry, const in vec3 specularColor, const in float shininess ) {

	vec3 halfDir = normalize( incidentLight.direction + geometry.viewDir );

	//float dotNL = saturate( dot( geometry.normal, incidentLight.direction ) );
	//float dotNV = saturate( dot( geometry.normal, geometry.viewDir ) );
	float dotNH = saturate( dot( geometry.normal, halfDir ) );
	float dotLH = saturate( dot( incidentLight.direction, halfDir ) );

	vec3 F = F_Schlick( specularColor, dotLH );

	float G = G_BlinnPhong_Implicit( /* dotNL, dotNV */ );

	float D = D_BlinnPhong( shininess, dotNH );

	return F * ( G * D );

} // validated

// source: http://simonstechblog.blogspot.ca/2011/12/microfacet-brdf.html
float GGXRoughnessToBlinnExponent( const in float ggxRoughness ) {
	return ( 2.0 / pow2( ggxRoughness + 0.0001 ) - 2.0 );
}

float BlinnExponentToGGXRoughness( const in float blinnExponent ) {
	return sqrt( 2.0 / ( blinnExponent + 2.0 ) );
}
