Shader for a sky sphere that adds a starry pattern~~ driven by layers of perlin noise

StarrySkyShader.js

const StarrySkyShader = {

  vertexShader: `
    varying vec3 vPos;

    void main() {
      vPos = position;
      vec4 mvPosition = modelViewMatrix * vec4( vPos, 1.0 );
      gl_Position = projectionMatrix * mvPosition;
    }

  `,

  fragmentShader: `
    vec4 permute(vec4 x){return mod(((x*34.0)+1.0)*x, 289.0);}
    vec4 taylorInvSqrt(vec4 r){return 1.79284291400159 - 0.85373472095314 * r;}
    vec3 fade(vec3 t) {return t*t*t*(t*(t*6.0-15.0)+10.0);}

    float cnoise(vec3 P){
      vec3 Pi0 = floor(P); // Integer part for indexing
      vec3 Pi1 = Pi0 + vec3(1.0); // Integer part + 1
      Pi0 = mod(Pi0, 289.0);
      Pi1 = mod(Pi1, 289.0);
      vec3 Pf0 = fract(P); // Fractional part for interpolation
      vec3 Pf1 = Pf0 - vec3(1.0); // Fractional part - 1.0
      vec4 ix = vec4(Pi0.x, Pi1.x, Pi0.x, Pi1.x);
      vec4 iy = vec4(Pi0.yy, Pi1.yy);
      vec4 iz0 = Pi0.zzzz;
      vec4 iz1 = Pi1.zzzz;

      vec4 ixy = permute(permute(ix) + iy);
      vec4 ixy0 = permute(ixy + iz0);
      vec4 ixy1 = permute(ixy + iz1);

      vec4 gx0 = ixy0 / 7.0;
      vec4 gy0 = fract(floor(gx0) / 7.0) - 0.5;
      gx0 = fract(gx0);
      vec4 gz0 = vec4(0.5) - abs(gx0) - abs(gy0);
      vec4 sz0 = step(gz0, vec4(0.0));
      gx0 -= sz0 * (step(0.0, gx0) - 0.5);
      gy0 -= sz0 * (step(0.0, gy0) - 0.5);

      vec4 gx1 = ixy1 / 7.0;
      vec4 gy1 = fract(floor(gx1) / 7.0) - 0.5;
      gx1 = fract(gx1);
      vec4 gz1 = vec4(0.5) - abs(gx1) - abs(gy1);
      vec4 sz1 = step(gz1, vec4(0.0));
      gx1 -= sz1 * (step(0.0, gx1) - 0.5);
      gy1 -= sz1 * (step(0.0, gy1) - 0.5);

      vec3 g000 = vec3(gx0.x,gy0.x,gz0.x);
      vec3 g100 = vec3(gx0.y,gy0.y,gz0.y);
      vec3 g010 = vec3(gx0.z,gy0.z,gz0.z);
      vec3 g110 = vec3(gx0.w,gy0.w,gz0.w);
      vec3 g001 = vec3(gx1.x,gy1.x,gz1.x);
      vec3 g101 = vec3(gx1.y,gy1.y,gz1.y);
      vec3 g011 = vec3(gx1.z,gy1.z,gz1.z);
      vec3 g111 = vec3(gx1.w,gy1.w,gz1.w);

      vec4 norm0 = taylorInvSqrt(vec4(dot(g000, g000), dot(g010, g010), dot(g100, g100), dot(g110, g110)));
      g000 *= norm0.x;
      g010 *= norm0.y;
      g100 *= norm0.z;
      g110 *= norm0.w;
      vec4 norm1 = taylorInvSqrt(vec4(dot(g001, g001), dot(g011, g011), dot(g101, g101), dot(g111, g111)));
      g001 *= norm1.x;
      g011 *= norm1.y;
      g101 *= norm1.z;
      g111 *= norm1.w;

      float n000 = dot(g000, Pf0);
      float n100 = dot(g100, vec3(Pf1.x, Pf0.yz));
      float n010 = dot(g010, vec3(Pf0.x, Pf1.y, Pf0.z));
      float n110 = dot(g110, vec3(Pf1.xy, Pf0.z));
      float n001 = dot(g001, vec3(Pf0.xy, Pf1.z));
      float n101 = dot(g101, vec3(Pf1.x, Pf0.y, Pf1.z));
      float n011 = dot(g011, vec3(Pf0.x, Pf1.yz));
      float n111 = dot(g111, Pf1);

      vec3 fade_xyz = fade(Pf0);
      vec4 n_z = mix(vec4(n000, n100, n010, n110), vec4(n001, n101, n011, n111), fade_xyz.z);
      vec2 n_yz = mix(n_z.xy, n_z.zw, fade_xyz.y);
      float n_xyz = mix(n_yz.x, n_yz.y, fade_xyz.x);
      return 2.2 * n_xyz;
    }

    varying vec3 vPos;
    uniform float skyRadius;
    uniform vec3 noiseOffset;

    uniform vec3 env_c1;
    uniform vec3 env_c2;

    uniform float clusterSize;
    uniform float clusterStrength;

    uniform float starSize;
    uniform float starDensity;

    void main() {
      float freq = 1.1/skyRadius;
      float noise = cnoise(vPos * freq);
      vec4 backgroundColor = vec4(mix(env_c1, env_c2, noise), 1.0);

      float scaledClusterSize = (1.0/clusterSize)/skyRadius;
      float scaledStarSize = (1.0/starSize)/skyRadius;

      float cs = pow(cnoise(scaledClusterSize*vec3(vPos)+noiseOffset),1.0/clusterStrength) + cnoise(scaledStarSize*vec3(vPos));

      cs =clamp(pow(cs, 1.0/starDensity),0.0,1.0);
      vec4 starColor = 0.5*vec4(cs, cs, cs, 1.0);

      gl_FragColor = backgroundColor;
      gl_FragColor += starColor;
    }
  `,

};

export default StarrySkyShader;