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https://github.com/Merlin1809/Eclipse-Shader.git
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Merlin1809
113 lines
3.6 KiB
GLSL
113 lines
3.6 KiB
GLSL
// from https://www.shadertoy.com/view/XtGGRt, edited
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// Auroras by nimitz 2017 (twitter: @stormoid)
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// License Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License
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// Contact the author for other licensing options
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/*
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There are two main hurdles I encountered rendering this effect.
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First, the nature of the texture that needs to be generated to get a believable effect
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needs to be very specific, with large scale band-like structures, small scale non-smooth variations
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to create the trail-like effect, a method for animating said texture smoothly and finally doing all
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of this cheaply enough to be able to evaluate it several times per fragment/pixel.
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The second obstacle is the need to render a large volume while keeping the computational cost low.
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Since the effect requires the trails to extend way up in the atmosphere to look good, this means
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that the evaluated volume cannot be as constrained as with cloud effects. My solution was to make
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the sample stride increase polynomially, which works very well as long as the trails are lower opcaity than
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the rest of the effect. Which is always the case for auroras.
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After that, there were some issues with getting the correct emission curves and removing banding at lowered
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sample densities, this was fixed by a combination of sample number influenced dithering and slight sample blending.
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N.B. the base setup is from an old shader and ideally the effect would take an arbitrary ray origin and
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direction. But this was not required for this demo and would be trivial to fix.
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*/
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float hash_aurora(float p)
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{
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p = fract(p * .1031);
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p *= p + 33.33;
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p *= p + p;
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return fract(p);
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}
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mat2 mm2(in float a) {
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float c = cos(a), s = sin(a);
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return mat2(c, s, -s, c);
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}
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const mat2 m2 = mat2(0.95534, 0.29552, -0.29552, 0.95534);
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float tri(in float x) {
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return clamp(abs(fract(x) - 0.5), 0.01, 0.49);
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}
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vec2 tri2(in vec2 p) {
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float triX = tri(p.x);
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float triY = tri(p.y);
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return vec2(triX + triY, tri(triX + p.y));
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}
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float triNoise2d(in vec2 p) {
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float z = 1.8;
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float z2 = 2.5;
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float rz = 0.0;
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p *= mm2(p.x * 0.06);
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vec2 bp = p;
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mat2 rotation = mm2(frameTimeCounter * 0.06);
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for (int i = 0; i < 3; i++) {
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vec2 dg = tri2(bp * 1.75) * 0.75;
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dg *= rotation;
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p -= dg / z2;
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bp *= 1.3;
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z2 *= 0.45;
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z *= 0.42;
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p *= 1.21 + (rz - 1.0) * 0.02;
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rz += tri(p.x + tri(p.y)) * z;
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p *= -m2;
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}
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return clamp(1.0 / pow(rz * 29.0, 1.6), 0.0, 0.55);
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}
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vec3 aurora(vec3 dir, int samples, float noise, float WmoonVecY, float WsunVecY) {
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vec3 col = vec3(0.0);
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vec3 avgCol = vec3(0.0);
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float hash = 0.05 * noise;
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float fade = dir.y * 2.0 + 0.4;
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float atmosphereGround = 1.0 - exp2(-50.0 * pow(clamp(dir.y+0.025,0.0,1.0),2.0));
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#ifdef LUT
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float mult = 6.0;
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#else
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float mult = 3.0;
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#endif
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for (int i = 0; i < samples; i++) {
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float mI = mult * float(i);
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float of = hash * smoothstep(0.0, 12.0, mI);
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float pt = (0.8 + pow(mI, 1.4) * 0.0016) / fade - of;
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vec3 bpos = pt * dir;
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float rzt = triNoise2d(bpos.zx);
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vec3 col2 = vec3(0.0, 0.0, 0.0);
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col2 = (sin(vec3(AURORA_R, AURORA_G, AURORA_B) + mI * 0.063) * 0.5 + 0.5) * rzt;
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avgCol = mix(avgCol, col2, 0.1);
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col += avgCol * exp2(-mI * 0.05 - 2.5);
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}
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vec3 auroraCol = 14.5*pow(col, vec3(1.45));
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#ifdef AURORA_MOON
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auroraCol *= smoothstep(0.1, 0.0, WmoonVecY);
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#endif
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auroraCol *= smoothstep(0.0, -0.1, WsunVecY);
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return auroraCol * atmosphereGround * auroraAmount * AURORA_BRIGHTNESS;
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}
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