export default /* glsl */`

struct PhysicalMaterial {

	vec3 diffuseColor;
	float roughness;
	vec3 specularColor;
	float specularF90;

	#ifdef USE_CLEARCOAT
		float clearcoat;
		float clearcoatRoughness;
		vec3 clearcoatF0;
		float clearcoatF90;
	#endif

	#ifdef USE_IRIDESCENCE
		float iridescence;
		float iridescenceIOR;
		float iridescenceThickness;
		vec3 iridescenceFresnel;
		vec3 iridescenceF0;
	#endif

	#ifdef USE_SHEEN
		vec3 sheenColor;
		float sheenRoughness;
	#endif

	#ifdef IOR
		float ior;
	#endif

	#ifdef USE_TRANSMISSION
		float transmission;
		float transmissionAlpha;
		float thickness;
		float attenuationDistance;
		vec3 attenuationColor;
	#endif

};

// temporary
vec3 clearcoatSpecular = vec3( 0.0 );
vec3 sheenSpecular = vec3( 0.0 );

vec3 Schlick_to_F0( const in vec3 f, const in float f90, const in float dotVH ) {
    float x = clamp( 1.0 - dotVH, 0.0, 1.0 );
    float x2 = x * x;
    float x5 = clamp( x * x2 * x2, 0.0, 0.9999 );

    return ( f - vec3( f90 ) * x5 ) / ( 1.0 - x5 );
}

// 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 V_GGX_SmithCorrelated( const in float alpha, const in float dotNL, const in float dotNV ) {

	float a2 = pow2( alpha );

	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 );

}

#ifdef USE_CLEARCOAT

	// GGX Distribution, Schlick Fresnel, GGX_SmithCorrelated Visibility
	vec3 BRDF_GGX_Clearcoat( const in vec3 lightDir, const in vec3 viewDir, const in vec3 normal, const in PhysicalMaterial material) {

		vec3 f0 = material.clearcoatF0;
		float f90 = material.clearcoatF90;
		float roughness = material.clearcoatRoughness;

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

		vec3 halfDir = normalize( lightDir + viewDir );

		float dotNL = saturate( dot( normal, lightDir ) );
		float dotNV = saturate( dot( normal, viewDir ) );
		float dotNH = saturate( dot( normal, halfDir ) );
		float dotVH = saturate( dot( viewDir, halfDir ) );

		vec3 F = F_Schlick( f0, f90, dotVH );

		float V = V_GGX_SmithCorrelated( alpha, dotNL, dotNV );

		float D = D_GGX( alpha, dotNH );

		return F * ( V * D );

	}

#endif

vec3 BRDF_GGX( const in vec3 lightDir, const in vec3 viewDir, const in vec3 normal, const in PhysicalMaterial material ) {

	vec3 f0 = material.specularColor;
	float f90 = material.specularF90;
	float roughness = material.roughness;

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

	vec3 halfDir = normalize( lightDir + viewDir );

	float dotNL = saturate( dot( normal, lightDir ) );
	float dotNV = saturate( dot( normal, viewDir ) );
	float dotNH = saturate( dot( normal, halfDir ) );
	float dotVH = saturate( dot( viewDir, halfDir ) );

	vec3 F = F_Schlick( f0, f90, dotVH );

	#ifdef USE_IRIDESCENCE

		F = mix( F, material.iridescenceFresnel, material.iridescence );

	#endif

	float V = V_GGX_SmithCorrelated( alpha, dotNL, dotNV );

	float D = D_GGX( alpha, dotNH );

	return F * ( V * D );

}

// 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

#if defined( USE_SHEEN )

// https://github.com/google/filament/blob/master/shaders/src/brdf.fs
float D_Charlie( float roughness, float dotNH ) {

	float alpha = pow2( roughness );

	// Estevez and Kulla 2017, "Production Friendly Microfacet Sheen BRDF"
	float invAlpha = 1.0 / alpha;
	float cos2h = dotNH * dotNH;
	float sin2h = max( 1.0 - cos2h, 0.0078125 ); // 2^(-14/2), so sin2h^2 > 0 in fp16

	return ( 2.0 + invAlpha ) * pow( sin2h, invAlpha * 0.5 ) / ( 2.0 * PI );

}

// https://github.com/google/filament/blob/master/shaders/src/brdf.fs
float V_Neubelt( float dotNV, float dotNL ) {

	// Neubelt and Pettineo 2013, "Crafting a Next-gen Material Pipeline for The Order: 1886"
	return saturate( 1.0 / ( 4.0 * ( dotNL + dotNV - dotNL * dotNV ) ) );

}

vec3 BRDF_Sheen( const in vec3 lightDir, const in vec3 viewDir, const in vec3 normal, vec3 sheenColor, const in float sheenRoughness ) {

	vec3 halfDir = normalize( lightDir + viewDir );

	float dotNL = saturate( dot( normal, lightDir ) );
	float dotNV = saturate( dot( normal, viewDir ) );
	float dotNH = saturate( dot( normal, halfDir ) );

	float D = D_Charlie( sheenRoughness, dotNH );
	float V = V_Neubelt( dotNV, dotNL );

	return sheenColor * ( D * V );

}

#endif

// This is a curve-fit approxmation to the "Charlie sheen" BRDF integrated over the hemisphere from 
// Estevez and Kulla 2017, "Production Friendly Microfacet Sheen BRDF". The analysis can be found
// in the Sheen section of https://drive.google.com/file/d/1T0D1VSyR4AllqIJTQAraEIzjlb5h4FKH/view?usp=sharing
float IBLSheenBRDF( const in vec3 normal, const in vec3 viewDir, const in float roughness ) {

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

	float r2 = roughness * roughness;

	float a = roughness < 0.25 ? -339.2 * r2 + 161.4 * roughness - 25.9 : -8.48 * r2 + 14.3 * roughness - 9.95;

	float b = roughness < 0.25 ? 44.0 * r2 - 23.7 * roughness + 3.26 : 1.97 * r2 - 3.27 * roughness + 0.72;

	float DG = exp( a * dotNV + b ) + ( roughness < 0.25 ? 0.0 : 0.1 * ( roughness - 0.25 ) );

	return saturate( DG * RECIPROCAL_PI );

}

// Analytical approximation of the DFG LUT, one half of the
// split-sum approximation used in indirect specular lighting.
// via 'environmentBRDF' from "Physically Based Shading on Mobile"
// https://www.unrealengine.com/blog/physically-based-shading-on-mobile
vec2 DFGApprox( const in vec3 normal, const in vec3 viewDir, const in float roughness ) {

	float dotNV = saturate( dot( normal, 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 fab = vec2( - 1.04, 1.04 ) * a004 + r.zw;

	return fab;

}

vec3 EnvironmentBRDF( const in vec3 normal, const in vec3 viewDir, const in vec3 specularColor, const in float specularF90, const in float roughness ) {

	vec2 fab = DFGApprox( normal, viewDir, roughness );

	return specularColor * fab.x + specularF90 * fab.y;

}

// Fdez-Agüera's "Multiple-Scattering Microfacet Model for Real-Time Image Based Lighting"
// Approximates multiscattering in order to preserve energy.
// http://www.jcgt.org/published/0008/01/03/
#ifdef USE_IRIDESCENCE
void computeMultiscatteringIridescence( const in vec3 normal, const in vec3 viewDir, const in vec3 specularColor, const in float specularF90, const in float iridescence, const in vec3 iridescenceF0, const in float roughness, inout vec3 singleScatter, inout vec3 multiScatter ) {
#else
void computeMultiscattering( const in vec3 normal, const in vec3 viewDir, const in vec3 specularColor, const in float specularF90, const in float roughness, inout vec3 singleScatter, inout vec3 multiScatter ) {
#endif

	vec2 fab = DFGApprox( normal, viewDir, roughness );

	#ifdef USE_IRIDESCENCE

		vec3 Fr = mix( specularColor, iridescenceF0, iridescence );

	#else

		vec3 Fr = specularColor;

	#endif

	vec3 FssEss = Fr * fab.x + specularF90 * fab.y;

	float Ess = fab.x + fab.y;
	float Ems = 1.0 - Ess;

	vec3 Favg = Fr + ( 1.0 - Fr ) * 0.047619; // 1/21
	vec3 Fms = FssEss * Favg / ( 1.0 - Ems * Favg );

	singleScatter += FssEss;
	multiScatter += Fms * Ems;

}

#if NUM_RECT_AREA_LIGHTS > 0

	void RE_Direct_RectArea_Physical( const in RectAreaLight rectAreaLight, const in GeometricContext geometry, const in PhysicalMaterial material, inout ReflectedLight reflectedLight ) {

		vec3 normal = geometry.normal;
		vec3 viewDir = geometry.viewDir;
		vec3 position = geometry.position;
		vec3 lightPos = rectAreaLight.position;
		vec3 halfWidth = rectAreaLight.halfWidth;
		vec3 halfHeight = rectAreaLight.halfHeight;
		vec3 lightColor = rectAreaLight.color;
		float roughness = material.roughness;

		vec3 rectCoords[ 4 ];
		rectCoords[ 0 ] = lightPos + halfWidth - halfHeight; // counterclockwise; light shines in local neg z direction
		rectCoords[ 1 ] = lightPos - halfWidth - halfHeight;
		rectCoords[ 2 ] = lightPos - halfWidth + halfHeight;
		rectCoords[ 3 ] = lightPos + halfWidth + halfHeight;

		vec2 uv = LTC_Uv( normal, viewDir, roughness );

		vec4 t1 = texture2D( ltc_1, uv );
		vec4 t2 = texture2D( ltc_2, uv );

		mat3 mInv = mat3(
			vec3( t1.x, 0, t1.y ),
			vec3(    0, 1,    0 ),
			vec3( t1.z, 0, t1.w )
		);

		// LTC Fresnel Approximation by Stephen Hill
		// http://blog.selfshadow.com/publications/s2016-advances/s2016_ltc_fresnel.pdf
		vec3 fresnel = ( material.specularColor * t2.x + ( vec3( 1.0 ) - material.specularColor ) * t2.y );

		reflectedLight.directSpecular += lightColor * fresnel * LTC_Evaluate( normal, viewDir, position, mInv, rectCoords );

		reflectedLight.directDiffuse += lightColor * material.diffuseColor * LTC_Evaluate( normal, viewDir, position, mat3( 1.0 ), rectCoords );

	}

#endif

void RE_Direct_Physical( const in IncidentLight directLight, const in GeometricContext geometry, const in PhysicalMaterial material, inout ReflectedLight reflectedLight ) {

	float dotNL = saturate( dot( geometry.normal, directLight.direction ) );

	vec3 irradiance = dotNL * directLight.color;

	#ifdef USE_CLEARCOAT

		float dotNLcc = saturate( dot( geometry.clearcoatNormal, directLight.direction ) );

		vec3 ccIrradiance = dotNLcc * directLight.color;

		clearcoatSpecular += ccIrradiance * BRDF_GGX_Clearcoat( directLight.direction, geometry.viewDir, geometry.clearcoatNormal, material );

	#endif

	#ifdef USE_SHEEN

		sheenSpecular += irradiance * BRDF_Sheen( directLight.direction, geometry.viewDir, geometry.normal, material.sheenColor, material.sheenRoughness );

	#endif

	reflectedLight.directSpecular += irradiance * BRDF_GGX( directLight.direction, geometry.viewDir, geometry.normal, material );

	reflectedLight.directDiffuse += irradiance * BRDF_Lambert( material.diffuseColor );
}

void RE_IndirectDiffuse_Physical( const in vec3 irradiance, const in GeometricContext geometry, const in PhysicalMaterial material, inout ReflectedLight reflectedLight ) {

	reflectedLight.indirectDiffuse += irradiance * BRDF_Lambert( material.diffuseColor );

}

void RE_IndirectSpecular_Physical( const in vec3 radiance, const in vec3 irradiance, const in vec3 clearcoatRadiance, const in GeometricContext geometry, const in PhysicalMaterial material, inout ReflectedLight reflectedLight) {

	#ifdef USE_CLEARCOAT

		clearcoatSpecular += clearcoatRadiance * EnvironmentBRDF( geometry.clearcoatNormal, geometry.viewDir, material.clearcoatF0, material.clearcoatF90, material.clearcoatRoughness );

	#endif

	#ifdef USE_SHEEN

		sheenSpecular += irradiance * material.sheenColor * IBLSheenBRDF( geometry.normal, geometry.viewDir, material.sheenRoughness );

	#endif

	// Both indirect specular and indirect diffuse light accumulate here

	vec3 singleScattering = vec3( 0.0 );
	vec3 multiScattering = vec3( 0.0 );
	vec3 cosineWeightedIrradiance = irradiance * RECIPROCAL_PI;

	#ifdef USE_IRIDESCENCE

		computeMultiscatteringIridescence( geometry.normal, geometry.viewDir, material.specularColor, material.specularF90, material.iridescence, material.iridescenceFresnel, material.roughness, singleScattering, multiScattering );

	#else

		computeMultiscattering( geometry.normal, geometry.viewDir, material.specularColor, material.specularF90, material.roughness, singleScattering, multiScattering );

	#endif

	vec3 totalScattering = singleScattering + multiScattering;
	vec3 diffuse = material.diffuseColor * ( 1.0 - max( max( totalScattering.r, totalScattering.g ), totalScattering.b ) );

	reflectedLight.indirectSpecular += radiance * singleScattering;
	reflectedLight.indirectSpecular += multiScattering * cosineWeightedIrradiance;

	reflectedLight.indirectDiffuse += diffuse * cosineWeightedIrradiance;

}

#define RE_Direct				RE_Direct_Physical
#define RE_Direct_RectArea		RE_Direct_RectArea_Physical
#define RE_IndirectDiffuse		RE_IndirectDiffuse_Physical
#define RE_IndirectSpecular		RE_IndirectSpecular_Physical

// ref: https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf
float computeSpecularOcclusion( const in float dotNV, const in float ambientOcclusion, const in float roughness ) {

	return saturate( pow( dotNV + ambientOcclusion, exp2( - 16.0 * roughness - 1.0 ) ) - 1.0 + ambientOcclusion );

}
`;
