Nothern Lights
GLSL shader by guinetik · created 2026-02-26 · 10s loop · 2 passes
Volumetric aurora based on Baranoski et al. Three-line emission model (557.7nm green, 630.0nm red, 427.8nm blue) sampled at different spatial scales to approximate per-channel gaussian blur. Triangle-wave noise for fibrous curtain structure with polynomial ray stride.
Tags: 3d, Animation, Abstract, Noise, Layers, Symmetry, Atmosphere, Aurora, Borealis
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Shader source (GLSL)
Buffer A (iChannel0)
/**
* Aurora Borealis
* @author guinetik
* @date 2026-02-26
*
* Volumetric aurora based on Baranoski et al. "Simulating the Aurora Borealis".
* Three independent emission lines sampled at different spatial scales to
* approximate the per-channel gaussian blur from the paper.
* Camera looks straight up — lying on your back watching the sky.
*
* TECHNIQUE: Three-line emission model (Baranoski et al.)
* Real auroras have three dominant spectral lines:
* - 557.7nm green (atomic oxygen) — transition state 0.7s → moderate spread
* - 630.0nm red (atomic oxygen) — transition state 110s → very diffuse
* - 427.8nm blue (ionized nitrogen) — transition state <0.001s → razor sharp
* We sample curtain noise at different domain scales per line to fake
* per-channel gaussian blur.
*
* TECHNIQUE: Triangle-wave noise
* Triangle waves produce sharp, non-smooth fibrous patterns. Iterated with
* rotation for anisotropic band structure resembling real curtains.
*
* TECHNIQUE: Polynomial ray stride
* Sample spacing grows as pow(i, 1.4) — dense near base, sparse up high.
*/
// --- Ray marching ---
#define MARCH_STEPS 60 // volume samples per ray — more = smoother
#define SLAB_BASE 0.8 // altitude where aurora slab begins
#define STRIDE_POWER 1.4 // polynomial stride — slightly less aggressive for smoother sampling
#define STRIDE_SCALE 0.002 // base stride multiplier
#define DITHER_STRENGTH 0.006 // per-sample jitter to kill banding
#define FALLOFF_RATE 0.07 // exponential decay per sample
#define BRIGHTNESS 1.7 // final intensity multiplier
// --- Curtain noise ---
#define NOISE_ITERS 6 // triangle-wave octaves — one more for smoother detail
#define NOISE_SPEED 0.12 // animation rate — faster drift
#define NOISE_SHARPNESS 1.1 // pow exponent — slightly softer to reduce pixelation
#define NOISE_SCALE 25.0 // accumulator multiplier — slightly lower for smoother fibers
#define CURTAIN_SKEW 0.06 // domain rotation for anisotropic banding
// --- Three emission lines (Baranoski model) ---
// 557.7nm oxygen green — dominant, lower-mid altitude
#define GREEN_COLOR vec3(0.05, 0.9, 0.2)
#define GREEN_ALT_CENTER 0.25
#define GREEN_ALT_WIDTH 0.35
#define GREEN_NOISE_SCALE 1.0 // baseline (0.7s transition)
#define GREEN_INTENSITY 0.75 // strongest line
// 630.0nm oxygen red — upper altitude, very diffuse
#define RED_COLOR vec3(0.9, 0.1, 0.08)
#define RED_ALT_CENTER 0.65
#define RED_ALT_WIDTH 0.45 // wide spread (110s transition)
#define RED_NOISE_SCALE 0.6 // sample at larger scale = smoother
#define RED_INTENSITY 0.45 // weaker than green (quenching)
// 427.8nm nitrogen blue — lower border, razor sharp
#define BLUE_COLOR vec3(0.12, 0.15, 0.95)
#define BLUE_ALT_CENTER 0.08
#define BLUE_ALT_WIDTH 0.12 // narrow band (<0.001s transition)
#define BLUE_NOISE_SCALE 1.6 // sharp detail
#define BLUE_INTENSITY 0.3 // weakest of the three
// --- Camera ---
#define CAM_FOCAL 1.2 // focal length
#define CAM_TILT 0.55 // base tilt above horizon (~30°) — keeps aurora centered
#define PAN_SPEED_X 0.04 // horizontal pan speed
#define PAN_SPEED_Y 0.025 // vertical wobble speed
#define PAN_AMP_X 0.35 // horizontal pan range
#define PAN_AMP_Y 0.12 // vertical wobble range — subtle so we stay in the band
// --- Sky ---
#define SKY_ZENITH vec3(0.02, 0.02, 0.06) // straight up — deep blue-black
#define SKY_LOW vec3(0.06, 0.08, 0.15) // toward edges — slightly lighter
// --- Stars ---
#define STAR_LAYERS 4
#define STAR_THRESHOLD 0.0005
#define STAR_POINT_SHARPNESS 800.0 // gaussian falloff for star core
#define STAR_HALO_SHARPNESS 80.0 // gaussian falloff for soft halo
#define STAR_HALO_INTENSITY 0.15 // halo brightness relative to core
// ---- 2D rotation ----
mat2 rot2(float a) {
float c = cos(a), s = sin(a);
return mat2(c, s, -s, c);
}
// ---- triangle-wave noise ----
float tri(float x) {
return clamp(abs(fract(x) - 0.5), 0.01, 0.49);
}
vec2 tri2(vec2 p) {
return vec2(tri(p.x) + tri(p.y), tri(p.y + tri(p.x)));
}
mat2 noiseRot = mat2(0.9563, 0.2924, -0.2924, 0.9563);
float curtainNoise(vec2 p, float speed) {
float z = 1.8;
float z2 = 2.5;
float acc = 0.0;
p *= rot2(p.x * CURTAIN_SKEW);
vec2 bp = p;
for (int i = 0; i < NOISE_ITERS; i++) {
vec2 dg = tri2(bp * 1.85) * 0.75;
dg *= rot2(iTime * speed);
p -= dg / z2;
bp *= 1.3;
z2 *= 0.45;
z *= 0.42;
p *= 1.21 + (acc - 1.0) * 0.02;
acc += tri(p.x + tri(p.y)) * z;
p *= -noiseRot;
}
return clamp(1.0 / pow(max(acc * NOISE_SCALE, 0.001), NOISE_SHARPNESS), 0.0, 0.55);
}
// ---- hash ----
float hash21(vec2 n) {
return fract(sin(dot(n, vec2(12.9898, 4.1414))) * 43758.5453);
}
// ---- altitude envelope ----
// Asymmetric gaussian: sharp lower border, soft upper falloff.
float altitudeEnvelope(float h, float center, float width) {
float d = h - center;
// Sharp bottom, soft top (Vegard & Krogness measurements)
float spread = (d < 0.0) ? width * 0.4 : width;
return exp(-d * d / (spread * spread));
}
// ---- volumetric aurora ----
vec4 aurora(vec3 ro, vec3 rd) {
vec4 col = vec4(0.0);
vec4 avgCol = vec4(0.0);
float maxI = float(MARCH_STEPS);
for (int i = 0; i < MARCH_STEPS; i++) {
float fi = float(i);
float dither = DITHER_STRENGTH * hash21(gl_FragCoord.xy) * smoothstep(0.0, 15.0, fi);
float t = ((SLAB_BASE + pow(fi, STRIDE_POWER) * STRIDE_SCALE) - ro.y) / (rd.y * 2.0 + 0.4);
t -= dither;
vec3 pos = ro + t * rd;
float h = fi / maxI;
// Three noise samples at different scales
float densityR = curtainNoise(pos.zx * RED_NOISE_SCALE, NOISE_SPEED);
float densityG = curtainNoise(pos.zx * GREEN_NOISE_SCALE, NOISE_SPEED);
float densityB = curtainNoise(pos.zx * BLUE_NOISE_SCALE, NOISE_SPEED);
// Altitude envelopes
float envR = altitudeEnvelope(h, RED_ALT_CENTER, RED_ALT_WIDTH);
float envG = altitudeEnvelope(h, GREEN_ALT_CENTER, GREEN_ALT_WIDTH);
float envB = altitudeEnvelope(h, BLUE_ALT_CENTER, BLUE_ALT_WIDTH);
// Three independent spectral lines
vec3 emission = GREEN_COLOR * densityG * envG * GREEN_INTENSITY
+ RED_COLOR * densityR * envR * RED_INTENSITY
+ BLUE_COLOR * densityB * envB * BLUE_INTENSITY;
float totalDensity = densityG * envG;
vec4 sampleCol = vec4(emission, totalDensity);
avgCol = mix(avgCol, sampleCol, 0.5);
col += avgCol * exp2(-fi * FALLOFF_RATE - 2.5) * smoothstep(0.0, 5.0, fi);
}
// No horizon fade — we're looking up, everything is sky
return col * BRIGHTNESS;
}
// ---- stars ----
// TECHNIQUE: Spherical-coordinate star grid
// Project rd onto 2D angular coords (azimuth, elevation) for stable
// sky-dome mapping. A 3D grid on unit direction vectors causes cell-boundary
// flicker during camera pan because floor(rd*scale) is unstable near edges.
vec2 hash22(vec2 p) {
vec2 h = vec2(dot(p, vec2(127.1, 311.7)),
dot(p, vec2(269.5, 183.3)));
return fract(sin(h) * 43758.5453123);
}
vec3 stars(vec3 rd) {
vec2 angles = vec2(atan(rd.x, rd.z), asin(clamp(rd.y, -1.0, 1.0)));
vec3 c = vec3(0.0);
for (int i = 0; i < STAR_LAYERS; i++) {
float fi = float(i);
float scale = 25.0 + fi * 10.0;
vec2 grid = angles * scale;
vec2 id = floor(grid);
vec2 gv = fract(grid) - 0.5;
// Per-layer seed offset to avoid overlapping patterns
vec2 rn = hash22(id + fi * 100.0);
float rn3 = hash21(id + fi * 73.0);
// Randomize position within cell
vec2 starPos = (rn - 0.5) * 0.7;
float d = length(gv - starPos);
// Dual gaussian: tight core + soft halo
float star = exp(-d * d * STAR_POINT_SHARPNESS)
+ exp(-d * d * STAR_HALO_SHARPNESS) * STAR_HALO_INTENSITY;
star *= step(rn3, STAR_THRESHOLD * 80.0 + fi * fi * 0.02);
// Twinkle — slow and gentle
star *= 0.8 + 0.2 * sin(iTime * 1.5 + rn.x * 100.0);
// Warm/cool color variation
c += star * (mix(vec3(1.0, 0.49, 0.1), vec3(0.75, 0.9, 1.0), rn.y) * 0.15 + 0.85);
}
return c * 0.7;
}
// ---- main ----
void mainImage(out vec4 fragColor, in vec2 fragCoord) {
vec2 p = (fragCoord.xy - 0.5 * iResolution.xy) / min(iResolution.x, iResolution.y);
// Camera looking forward then tilted up ~30° — aurora fills the view
vec3 rd = normalize(vec3(p.x, p.y, CAM_FOCAL));
// Tilt up so we're looking across the aurora band, not straight at zenith
rd.yz *= rot2(CAM_TILT);
// Slow wandering pan
rd.xz *= rot2(sin(iTime * PAN_SPEED_X) * PAN_AMP_X + cos(iTime * 0.031) * 0.1);
rd.yz *= rot2(sin(iTime * PAN_SPEED_Y) * PAN_AMP_Y);
vec3 ro = vec3(0.0, 0.0, -6.7);
// Sky gradient
float zenithDot = max(dot(rd, vec3(0.0, 1.0, 0.0)), 0.0);
vec3 col = mix(SKY_LOW, SKY_ZENITH, zenithDot);
// Stars
col += stars(rd);
// Aurora — skip only rays pointing below horizon
if (rd.y > 0.0) {
vec4 aur = smoothstep(0.0, 1.5, aurora(ro, rd));
col = col * (1.0 - aur.a) + aur.rgb;
}
fragColor = vec4(col, 1.0);
}
Image
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