precision highp float;

uniform float u_time;
uniform vec2 u_resolution;

vec3 permute(vec3 x) { return mod(((x * 34.0) + 1.0) * x, 289.0); }

float snoise(vec2 v) {
  const vec4 C = vec4(0.211324865405187, 0.366025403784439, -0.577350269189626,
                      0.024390243902439);
  vec2 i = floor(v + dot(v, C.yy));
  vec2 x0 = v - i + dot(i, C.xx);
  vec2 i1 = (x0.x > x0.y) ? vec2(1.0, 0.0) : vec2(0.0, 1.0);
  vec4 x12 = x0.xyxy + C.xxzz;
  x12.xy -= i1;
  i = mod(i, 289.0);

  vec3 p =
      permute(permute(i.y + vec3(0.0, i1.y, 1.0)) + i.x + vec3(0.0, i1.x, 1.0));

  vec3 m = max(
      0.5 - vec3(dot(x0, x0), dot(x12.xy, x12.xy), dot(x12.zw, x12.zw)), 0.0);
  m = m * m;
  m = m * m;

  vec3 x = 2.0 * fract(p * C.www) - 1.0;
  vec3 h = abs(x) - 0.5;
  vec3 ox = floor(x + 0.5);
  vec3 a0 = x - ox;
  m *= 1.79284291400159 - 0.85373472095314 * (a0 * a0 + h * h);

  vec3 g;
  g.x = a0.x * x0.x + h.x * x0.y;
  g.yz = a0.yz * x12.xz + h.yz * x12.yw;
  return 130.0 * dot(m, g);
}

vec3 spectral_colour(float l) {
  float r = 0.0, g = 0.0, b = 0.0;
  if ((l >= 400.0) && (l < 410.0)) {
    float t = (l - 400.0) / (410.0 - 400.0);
    r = +(0.33 * t) - (0.20 * t * t);
  } else if ((l >= 410.0) && (l < 475.0)) {
    float t = (l - 410.0) / (475.0 - 410.0);
    r = 0.14 - (0.13 * t * t);
  } else if ((l >= 545.0) && (l < 595.0)) {
    float t = (l - 545.0) / (595.0 - 545.0);
    r = +(1.98 * t) - (t * t);
  } else if ((l >= 595.0) && (l < 650.0)) {
    float t = (l - 595.0) / (650.0 - 595.0);
    r = 0.98 + (0.06 * t) - (0.40 * t * t);
  } else if ((l >= 650.0) && (l < 700.0)) {
    float t = (l - 650.0) / (700.0 - 650.0);
    r = 0.65 - (0.84 * t) + (0.20 * t * t);
  }
  if ((l >= 415.0) && (l < 475.0)) {
    float t = (l - 415.0) / (475.0 - 415.0);
    g = +(0.80 * t * t);
  } else if ((l >= 475.0) && (l < 590.0)) {
    float t = (l - 475.0) / (590.0 - 475.0);
    g = 0.8 + (0.76 * t) - (0.80 * t * t);
  } else if ((l >= 585.0) && (l < 639.0)) {
    float t = (l - 585.0) / (639.0 - 585.0);
    g = 0.82 - (0.80 * t);
  }
  if ((l >= 400.0) && (l < 475.0)) {
    float t = (l - 400.0) / (475.0 - 400.0);
    b = +(2.20 * t) - (1.50 * t * t);
  } else if ((l >= 475.0) && (l < 560.0)) {
    float t = (l - 475.0) / (560.0 - 475.0);
    b = 0.7 - (t) + (0.30 * t * t);
  }

  return vec3(r, g, b);
}

void main() {
  vec2 fragCoord = gl_FragCoord.xy;
  vec2 iResolution = u_resolution;
  float iTime = u_time;

  vec2 uv = fragCoord / iResolution;

  // Generate aurora wave pattern
  float amplitude = 1.0;
  float height =
      snoise(vec2(uv.x * 2.0 + iTime * 0.05, iTime * 0.1)) * 0.5 * amplitude;
  height = exp(height);
  height = (uv.y * 2.0 - height + 0.2);
  float intensity = 0.6 * height;

  // Create smooth falloff
  float midPoint = 0.20;
  float blend = 0.5;
  float auroraAlpha =
      smoothstep(midPoint - blend * 0.5, midPoint + blend * 0.5, intensity);

  // Use spectral color based on position and time
  float wavelength = uv.x * 150.0 + 450.0 + sin(iTime * 0.3) * 20.0;
  vec3 rampColor = spectral_colour(wavelength);

  vec3 auroraColor = intensity * rampColor;

  gl_FragColor = vec4(auroraColor * auroraAlpha, auroraAlpha);
}
Aurora
