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 );

// 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( directLight.direction, geometry.viewDir, geometry.clearcoatNormal, material.clearcoatF0, material.clearcoatF90, material.clearcoatRoughness );

	#endif

	#ifdef USE_SHEEN

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

	#endif

	#ifdef USE_IRIDESCENCE

		reflectedLight.directSpecular += irradiance * BRDF_GGX_Iridescence( directLight.direction, geometry.viewDir, geometry.normal, material.specularColor, material.specularF90, material.iridescence, material.iridescenceFresnel, material.roughness );

	#else

		reflectedLight.directSpecular += irradiance * BRDF_GGX( directLight.direction, geometry.viewDir, geometry.normal, material.specularColor, material.specularF90, material.roughness );

	#endif

	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 );

}
`;
