text: import {
  	BackSide,
  	BoxGeometry,
  	Mesh,
  	ShaderMaterial,
  	UniformsUtils,
  	Vector3
  } from 'three';
  
  /**
   * Based on "A Practical Analytic Model for Daylight"
   * aka The Preetham Model, the de facto standard analytic skydome model
   * https://www.researchgate.net/publication/220720443_A_Practical_Analytic_Model_for_Daylight
   *
   * First implemented by Simon Wallner
   * http://simonwallner.at/project/atmospheric-scattering/
   *
   * Improved by Martin Upitis
   * http://blenderartists.org/forum/showthread.php?245954-preethams-sky-impementation-HDR
   *
   * Three.js integration by zz85 http://twitter.com/blurspline
  */
  
  class Sky extends Mesh {
  
  	constructor() {
  
  		const shader = Sky.SkyShader;
  
  		const material = new ShaderMaterial( {
  			name: 'SkyShader',
  			fragmentShader: shader.fragmentShader,
  			vertexShader: shader.vertexShader,
  			uniforms: UniformsUtils.clone( shader.uniforms ),
  			side: BackSide,
  			depthWrite: false
  		} );
  
  		super( new BoxGeometry( 1, 1, 1 ), material );
  
  	}
  
  }
  
  Sky.prototype.isSky = true;
  
  Sky.SkyShader = {
  
  	uniforms: {
  		'turbidity': { value: 2 },
  		'rayleigh': { value: 1 },
  		'mieCoefficient': { value: 0.005 },
  		'mieDirectionalG': { value: 0.8 },
  		'sunPosition': { value: new Vector3() },
  		'up': { value: new Vector3( 0, 1, 0 ) }
  	},
  
  	vertexShader: /* glsl */`
  		uniform vec3 sunPosition;
  		uniform float rayleigh;
  		uniform float turbidity;
  		uniform float mieCoefficient;
  		uniform vec3 up;
  
  		varying vec3 vWorldPosition;
  		varying vec3 vSunDirection;
  		varying float vSunfade;
  		varying vec3 vBetaR;
  		varying vec3 vBetaM;
  		varying float vSunE;
  
  		// constants for atmospheric scattering
  		const float e = 2.71828182845904523536028747135266249775724709369995957;
  		const float pi = 3.141592653589793238462643383279502884197169;
  
  		// wavelength of used primaries, according to preetham
  		const vec3 lambda = vec3( 680E-9, 550E-9, 450E-9 );
  		// this pre-calcuation replaces older TotalRayleigh(vec3 lambda) function:
  		// (8.0 * pow(pi, 3.0) * pow(pow(n, 2.0) - 1.0, 2.0) * (6.0 + 3.0 * pn)) / (3.0 * N * pow(lambda, vec3(4.0)) * (6.0 - 7.0 * pn))
  		const vec3 totalRayleigh = vec3( 5.804542996261093E-6, 1.3562911419845635E-5, 3.0265902468824876E-5 );
  
  		// mie stuff
  		// K coefficient for the primaries
  		const float v = 4.0;
  		const vec3 K = vec3( 0.686, 0.678, 0.666 );
  		// MieConst = pi * pow( ( 2.0 * pi ) / lambda, vec3( v - 2.0 ) ) * K
  		const vec3 MieConst = vec3( 1.8399918514433978E14, 2.7798023919660528E14, 4.0790479543861094E14 );
  
  		// earth shadow hack
  		// cutoffAngle = pi / 1.95;
  		const float cutoffAngle = 1.6110731556870734;
  		const float steepness = 1.5;
  		const float EE = 1000.0;
  
  		float sunIntensity( float zenithAngleCos ) {
  			zenithAngleCos = clamp( zenithAngleCos, -1.0, 1.0 );
  			return EE * max( 0.0, 1.0 - pow( e, -( ( cutoffAngle - acos( zenithAngleCos ) ) / steepness ) ) );
  		}
  
  		vec3 totalMie( float T ) {
  			float c = ( 0.2 * T ) * 10E-18;
  			return 0.434 * c * MieConst;
  		}
  
  		void main() {
  
  			vec4 worldPosition = modelMatrix * vec4( position, 1.0 );
  			vWorldPosition = worldPosition.xyz;
  
  			gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );
  			gl_Position.z = gl_Position.w; // set z to camera.far
  
  			vSunDirection = normalize( sunPosition );
  
  			vSunE = sunIntensity( dot( vSunDirection, up ) );
  
  			vSunfade = 1.0 - clamp( 1.0 - exp( ( sunPosition.y / 450000.0 ) ), 0.0, 1.0 );
  
  			float rayleighCoefficient = rayleigh - ( 1.0 * ( 1.0 - vSunfade ) );
  
  			// extinction (absorbtion + out scattering)
  			// rayleigh coefficients
  			vBetaR = totalRayleigh * rayleighCoefficient;
  
  			// mie coefficients
  			vBetaM = totalMie( turbidity ) * mieCoefficient;
  
  		}`,
  
  	fragmentShader: /* glsl */`
  		varying vec3 vWorldPosition;
  		varying vec3 vSunDirection;
  		varying float vSunfade;
  		varying vec3 vBetaR;
  		varying vec3 vBetaM;
  		varying float vSunE;
  
  		uniform float mieDirectionalG;
  		uniform vec3 up;
  
  		const vec3 cameraPos = vec3( 0.0, 0.0, 0.0 );
  
  		// constants for atmospheric scattering
  		const float pi = 3.141592653589793238462643383279502884197169;
  
  		const float n = 1.0003; // refractive index of air
  		const float N = 2.545E25; // number of molecules per unit volume for air at 288.15K and 1013mb (sea level -45 celsius)
  
  		// optical length at zenith for molecules
  		const float rayleighZenithLength = 8.4E3;
  		const float mieZenithLength = 1.25E3;
  		// 66 arc seconds -> degrees, and the cosine of that
  		const float sunAngularDiameterCos = 0.999956676946448443553574619906976478926848692873900859324;
  
  		// 3.0 / ( 16.0 * pi )
  		const float THREE_OVER_SIXTEENPI = 0.05968310365946075;
  		// 1.0 / ( 4.0 * pi )
  		const float ONE_OVER_FOURPI = 0.07957747154594767;
  
  		float rayleighPhase( float cosTheta ) {
  			return THREE_OVER_SIXTEENPI * ( 1.0 + pow( cosTheta, 2.0 ) );
  		}
  
  		float hgPhase( float cosTheta, float g ) {
  			float g2 = pow( g, 2.0 );
  			float inverse = 1.0 / pow( 1.0 - 2.0 * g * cosTheta + g2, 1.5 );
  			return ONE_OVER_FOURPI * ( ( 1.0 - g2 ) * inverse );
  		}
  
  		void main() {
  
  			vec3 direction = normalize( vWorldPosition - cameraPos );
  
  			// optical length
  			// cutoff angle at 90 to avoid singularity in next formula.
  			float zenithAngle = acos( max( 0.0, dot( up, direction ) ) );
  			float inverse = 1.0 / ( cos( zenithAngle ) + 0.15 * pow( 93.885 - ( ( zenithAngle * 180.0 ) / pi ), -1.253 ) );
  			float sR = rayleighZenithLength * inverse;
  			float sM = mieZenithLength * inverse;
  
  			// combined extinction factor
  			vec3 Fex = exp( -( vBetaR * sR + vBetaM * sM ) );
  
  			// in scattering
  			float cosTheta = dot( direction, vSunDirection );
  
  			float rPhase = rayleighPhase( cosTheta * 0.5 + 0.5 );
  			vec3 betaRTheta = vBetaR * rPhase;
  
  			float mPhase = hgPhase( cosTheta, mieDirectionalG );
  			vec3 betaMTheta = vBetaM * mPhase;
  
  			vec3 Lin = pow( vSunE * ( ( betaRTheta + betaMTheta ) / ( vBetaR + vBetaM ) ) * ( 1.0 - Fex ), vec3( 1.5 ) );
  			Lin *= mix( vec3( 1.0 ), pow( vSunE * ( ( betaRTheta + betaMTheta ) / ( vBetaR + vBetaM ) ) * Fex, vec3( 1.0 / 2.0 ) ), clamp( pow( 1.0 - dot( up, vSunDirection ), 5.0 ), 0.0, 1.0 ) );
  
  			// nightsky
  			float theta = acos( direction.y ); // elevation --> y-axis, [-pi/2, pi/2]
  			float phi = atan( direction.z, direction.x ); // azimuth --> x-axis [-pi/2, pi/2]
  			vec2 uv = vec2( phi, theta ) / vec2( 2.0 * pi, pi ) + vec2( 0.5, 0.0 );
  			vec3 L0 = vec3( 0.1 ) * Fex;
  
  			// composition + solar disc
  			float sundisk = smoothstep( sunAngularDiameterCos, sunAngularDiameterCos + 0.00002, cosTheta );
  			L0 += ( vSunE * 19000.0 * Fex ) * sundisk;
  
  			vec3 texColor = ( Lin + L0 ) * 0.04 + vec3( 0.0, 0.0003, 0.00075 );
  
  			vec3 retColor = pow( texColor, vec3( 1.0 / ( 1.2 + ( 1.2 * vSunfade ) ) ) );
  
  			gl_FragColor = vec4( retColor, 1.0 );
  
  			#include <tonemapping_fragment>
  			#include <encodings_fragment>
  
  		}`
  
  };
  
  export { Sky };
  