atmosphere: add physical Rayleigh sky scattering

config.js

@@ -313,6 +313,8 @@ window.CONFIG = {
         albedoGain: 1.0,          // theme share on surface colours
         skyGain: 0.45,            // diffuse skylight share on surfaces
         airGain: 0.55,            // Rayleigh airlight strength (blue-biased)
+        physicalSky: true,        // Rayleigh sky scattering instead of pure theme gradient
+        rayleighGain: 8.0,        // exposure for the physical sky term
         distanceFog: false,       // legacy aerial perspective is off
         model: {
             enabled: false        // volumetric atmosphere/fog removed

vendor/clouds3d.js

@@ -105,6 +105,10 @@ var FRAG_SRC = [
     'uniform float uStarAmb;',     // diffuse starlight + airglow
     'uniform float uPhysical;',
     'uniform vec3 uAirColor;',     // Rayleigh airlight at the horizon
+    'uniform vec3 uRayleighBeta;', // vertical Rayleigh optical depth RGB
+    'uniform float uSunAirMass;',  // relative air mass of the sun beam
+    'uniform float uRayleighGain;',
+    'uniform float uPhysicalSkyOn;',
     'uniform vec3 uCloudAlbedo;',  // theme-tinted but brightness-normalised
     'uniform float uAirDensity;',  // slant optical depth per world unit
     'uniform float uCloudHaze;',   // clouds sit above most of the haze: much weaker
@@ -336,6 +340,21 @@ var FRAG_SRC = [
             // distance fog saturates to, so no seam at the silhouette.
             'float skyT = clamp(rd.y * 0.7 + 0.5, 0.5, 1.0);',
             'vec3 col = mix(uBg, uSky, skyT);',
+            'if (uPhysicalSkyOn > 0.5) {',
+                'float cosT = dot(rd, uSunDir);',
+                'float phase = 0.5 + 0.5 * cosT * cosT;',
+                'float sunUp = smoothstep(-0.25, 0.05, uSunAlt);',
+                'vec3 sunTop = uSunColor * uSunInt * sunUp;',
+                'float viewY = max(rd.y, 0.0);',
+                'float mView = (viewY > 0.02) ? min(38.0, 1.0 / viewY) : 38.0;',
+                'vec3 tauView = uRayleighBeta * mView;',
+                'vec3 single = phase * sunTop * (1.0 - exp(-tauView));',
+                'vec3 multi = phase * sunTop * tauView * tauView * 0.12;',
+                'vec3 ambient = max(vec3(uSkyAmb), uStarAmb * vec3(0.7, 0.85, 1.0)) *',
+                    '(1.0 - exp(-tauView)) * vec3(0.35, 0.55, 1.0);',
+                'vec3 phys = (single + multi + ambient) * uRayleighGain;',
+                'col = mix(col, phys, 0.82);',
+            '}',
             'float nightFactor = 1.0 - smoothstep(-0.15, 0.05, uSunAlt);',
             'float skyMask = smoothstep(-0.20, 0.40, rd.y);',
             // Stars must not sit on the moon: the disc is opaque, so every
@@ -825,6 +844,10 @@ function init(canvas) {
     var uStarAmb = gl.getUniformLocation(prog, 'uStarAmb');
     var uPhysical = gl.getUniformLocation(prog, 'uPhysical');
     var uAirColor = gl.getUniformLocation(prog, 'uAirColor');
+    var uRayleighBeta = gl.getUniformLocation(prog, 'uRayleighBeta');
+    var uSunAirMass = gl.getUniformLocation(prog, 'uSunAirMass');
+    var uRayleighGain = gl.getUniformLocation(prog, 'uRayleighGain');
+    var uPhysicalSkyOn = gl.getUniformLocation(prog, 'uPhysicalSkyOn');
     var uCloudAlbedo = gl.getUniformLocation(prog, 'uCloudAlbedo');
     var uAirDensity = gl.getUniformLocation(prog, 'uAirDensity');
     var uCloudHaze = gl.getUniformLocation(prog, 'uCloudHaze');
@@ -921,7 +944,9 @@ function init(canvas) {
         uAtmoBlueColor: uAtmoBlueColor, uAtmoGrayColor: uAtmoGrayColor,
         uSunColor: uSunColor, uSunInt: uSunInt, uMoonColor: uMoonColor,
         uMoonInt: uMoonInt, uSkyAmb: uSkyAmb, uStarAmb: uStarAmb,
-        uPhysical: uPhysical, uAirColor: uAirColor, uCloudAlbedo: uCloudAlbedo,
+        uPhysical: uPhysical, uAirColor: uAirColor,
+        uRayleighBeta: uRayleighBeta, uSunAirMass: uSunAirMass,
+        uRayleighGain: uRayleighGain, uPhysicalSkyOn: uPhysicalSkyOn, uCloudAlbedo: uCloudAlbedo,
         uAirDensity: uAirDensity, uCloudHaze: uCloudHaze,
         uSkyTex: uSkyTex, uSkyOn: uSkyOn, uStarLon: uStarLon, uPole: uPole,
         uMoonTex: uMoonTex, uMoonTexOn: uMoonTexOn, uMoonR: uMoonR,
@@ -2337,6 +2362,9 @@ var MOON_TEX_SRC = (typeof window.__moonTex === 'string' && window.__moonTex)
 var ATMO_NIGHT_FLOOR = (typeof ATMO.nightFloor === 'number') ? Math.max(0.01, ATMO.nightFloor) : 0.05;
 var ATMO_ALBEDO_GAIN = (typeof ATMO.albedoGain === 'number') ? Math.max(0.1, ATMO.albedoGain) : 1.0;
 var ATMO_SKY_GAIN = (typeof ATMO.skyGain === 'number') ? Math.max(0.05, ATMO.skyGain) : 0.55;
+var ATMO_PHYSICAL_SKY = ATMO.physicalSky !== false;
+var ATMO_RAYLEIGH_GAIN = (typeof ATMO.rayleighGain === 'number')
+    ? Math.max(0.1, Math.min(30.0, ATMO.rayleighGain)) : 8.0;
 
 // ---- Artificial light switching (CONFIG.lights) -------------------------
 // Every individual light owns a sun-altitude threshold inside the window
@@ -5457,6 +5485,10 @@ window.clouds3dRender = function(target) {
         _amb *= _exp;
     }
     gl.uniform3f(s.uAirColor, _air[0], _air[1], _air[2]);
+    gl.uniform3f(s.uRayleighBeta, TAU_RAYLEIGH[0], TAU_RAYLEIGH[1], TAU_RAYLEIGH[2]);
+    gl.uniform1f(s.uSunAirMass, airMass(Math.max(sunAlt, 0.0)));
+    gl.uniform1f(s.uRayleighGain, ATMO_RAYLEIGH_GAIN);
+    gl.uniform1f(s.uPhysicalSkyOn, ATMO_PHYSICAL_SKY ? 1.0 : 0.0);
     gl.uniform3f(s.uSunColor, _beam[0], _beam[1], _beam[2]);
     gl.uniform1f(s.uSunInt, _beamL);
     gl.uniform3f(s.uMoonColor, _mBeam[0], _mBeam[1], _mBeam[2]);