Aurora Borealis

A shader that combines raymarching and noise texture to create the Aurora Borealis effect.

Shader code

shader_type spatial;

uniform float speed = 0.02;
uniform vec4 color : hint_color;
uniform float emission_strength = 5.0;
uniform sampler2D noise;
uniform float offset = 0.0;
uniform float smoothness = 0.15;
uniform float distort = 1.0;
uniform float scale = 1.0;

varying vec3 world_camera;
varying vec3 world_position;

const float STEP = 0.001;
const float BASE_DENSITY = 5.0;

float amplify(float value) {
	float magic_number = 0.166504;
	float output = 0.0;
	value = clamp(value, 0.0, 1.0);
	value = pow(value, 2);
	output += pow(magic_number, 4) * value;
	value = pow(value, 2);
	output += magic_number * value;
	value = pow(value, 2);
	output += value;
	return output;
}

float interpolate_noise(vec2 uv1, vec2 uv2) {
	return smoothstep(
		-smoothness, smoothness, texture(noise, uv1).r - texture(noise, uv2).r
	);
}

float random_wave(vec2 uv) {
	vec2 uv_distort = texture(noise, uv).rr * distort * 0.5;
	vec2 uv1 = uv * scale + vec2(TIME * speed, TIME * speed - offset) + uv_distort;
	vec2 uv2 = uv * scale + vec2(TIME * speed + 0.5, TIME * speed + offset) + uv_distort;
	float interpolated_noise = interpolate_noise(uv1, uv2);
	float intensity = 0.2 + clamp((0.5 - abs(interpolated_noise - 0.5)) * 1.5, 0.0, 1.0);
	float wave = amplify(intensity);
	return wave;
}

float ray_march(vec3 ray_origin, vec3 ray_direction, float start, float time) {
	float density = 0.0;
	float dist = start + STEP;
	
	while (true) {
		vec3 point = ray_origin + ray_direction * (dist);
		// Check if the point is outside of the mesh (-1 to 1 in all directions)
		if (clamp(point, vec3(-1, -1, -1), vec3(1, 1, 1)) != point) {
			break;
		}
		vec2 uv = vec2((point.x + 1.0) * 0.5, (point.z + 1.0) * 0.5);
		float wave_value = random_wave(uv);
		float extra_noise = texture(noise, vec2(point.y, 0.0)).r;
		float height_factor = (1.0 - point.y) * 0.5;
		float point_density = BASE_DENSITY * STEP * height_factor * wave_value * extra_noise;
		density += point_density;
		dist += STEP;
	}
	
	return density;
}

void vertex() {
	world_position = VERTEX;
	// Use object space
	world_camera = (inverse(MODELVIEW_MATRIX) * vec4(0, 0, 0, 1)).xyz;
}

void fragment() {
	vec3 ray_origin = world_camera;
	vec3 ray_direction =  normalize(world_position - ray_origin);
	float density = ray_march(
		ray_origin, ray_direction, length(world_camera - world_position), TIME
	);
	
	ALBEDO = color.rgb * density;
	EMISSION = color.rgb * density * emission_strength;
	ALPHA = density;
}

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