text: import { ShaderNode,
  	add, addTo, sub, mul, div, saturate, dot, pow, pow2, exp2, normalize, max, sqrt, negate,
  	cond, greaterThan, and,
  	transformedNormalView, positionViewDirection,
  	diffuseColor, specularTint, roughness,
  	PI, RECIPROCAL_PI, EPSILON
  } from '../ShaderNode.js';
  
  export const F_Schlick = new ShaderNode( ( inputs ) => {
  
  	const { f0, f90, dotVH } = inputs;
  
  	// Original approximation by Christophe Schlick '94
  	// float fresnel = pow( 1.0 - dotVH, 5.0 );
  
  	// Optimized variant (presented by Epic at SIGGRAPH '13)
  	// https://cdn2.unrealengine.com/Resources/files/2013SiggraphPresentationsNotes-26915738.pdf
  	const fresnel = exp2( mul( sub( mul( - 5.55473, dotVH ), 6.98316 ), dotVH ) );
  
  	return add( mul( f0, sub( 1.0, fresnel ) ), mul( f90, fresnel ) );
  
  } ); // validated
  
  export const BRDF_Lambert = new ShaderNode( ( inputs ) => {
  
  	return mul( RECIPROCAL_PI, inputs.diffuseColor ); // punctual light
  
  } ); // validated
  
  export const getDistanceAttenuation = new ShaderNode( ( inputs ) => {
  
  	const { lightDistance, cutoffDistance, decayExponent } = inputs;
  
  	return cond(
  		and( greaterThan( cutoffDistance, 0 ), greaterThan( decayExponent, 0 ) ),
  		pow( saturate( add( div( negate( lightDistance ), cutoffDistance ), 1.0 ) ), decayExponent ),
  		1.0
  	);
  
  } ); // validated
  
  //
  // STANDARD
  //
  
  // 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
  export const V_GGX_SmithCorrelated = new ShaderNode( ( inputs ) => {
  
  	const { alpha, dotNL, dotNV } = inputs;
  
  	const a2 = pow2( alpha );
  
  	const gv = mul( dotNL, sqrt( add( a2, mul( sub( 1.0, a2 ), pow2( dotNV ) ) ) ) );
  	const gl = mul( dotNV, sqrt( add( a2, mul( sub( 1.0, a2 ), pow2( dotNL ) ) ) ) );
  
  	return div( 0.5, max( add( gv, gl ), EPSILON ) );
  
  } ); // validated
  
  // 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
  export const D_GGX = new ShaderNode( ( inputs ) => {
  
  	const { alpha, dotNH } = inputs;
  
  	const a2 = pow2( alpha );
  
  	const denom = add( mul( pow2( dotNH ), sub( a2, 1.0 ) ), 1.0 ); // avoid alpha = 0 with dotNH = 1
  
  	return mul( RECIPROCAL_PI, div( a2, pow2( denom ) ) );
  
  } ); // validated
  
  
  // GGX Distribution, Schlick Fresnel, GGX_SmithCorrelated Visibility
  export const BRDF_Specular_GGX = new ShaderNode( ( inputs ) => {
  
  	const { lightDirection, f0, f90, roughness } = inputs;
  
  	const alpha = pow2( roughness ); // UE4's roughness
  
  	const halfDir = normalize( add( lightDirection, positionViewDirection ) );
  
  	const dotNL = saturate( dot( transformedNormalView, lightDirection ) );
  	const dotNV = saturate( dot( transformedNormalView, positionViewDirection ) );
  	const dotNH = saturate( dot( transformedNormalView, halfDir ) );
  	const dotVH = saturate( dot( positionViewDirection, halfDir ) );
  
  	const F = F_Schlick( { f0, f90, dotVH } );
  
  	const V = V_GGX_SmithCorrelated( { alpha, dotNL, dotNV } );
  
  	const D = D_GGX( { alpha, dotNH } );
  
  	return mul( F, mul( V, D ) );
  
  } ); // validated
  
  export const RE_Direct_Physical = new ShaderNode( ( inputs ) => {
  
  	const { lightDirection, lightColor, directDiffuse, directSpecular } = inputs;
  
  	const dotNL = saturate( dot( transformedNormalView, lightDirection ) );
  	let irradiance = mul( dotNL, lightColor );
  
  	irradiance = mul( irradiance, PI ); // punctual light
  
  	addTo( directDiffuse, mul( irradiance, BRDF_Lambert( { diffuseColor } ) ) );
  
  	addTo( directSpecular, mul( irradiance, BRDF_Specular_GGX( { lightDirection, f0: specularTint, f90: 1, roughness } ) ) );
  
  } );
  
  export const PhysicalLightingModel = new ShaderNode( ( inputs/*, builder*/ ) => {
  
  	// PHYSICALLY_CORRECT_LIGHTS <-> builder.renderer.physicallyCorrectLights === true
  
  	RE_Direct_Physical( inputs );
  
  } );
  