atmosphere: add initial volumetric atmosphere shell

config.js

@@ -305,6 +305,15 @@ 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)
+        distanceFog: false,       // legacy aerial perspective is off
+        model: {
+            enabled: true,
+            heightKm: 35.0,       // full atmosphere shell around the planet
+            scaleHeightKm: 8.5,   // density falls exponentially with altitude
+            density: 0.12,        // base optical density
+            grayTau: 1.5,         // high optical depth becomes grey fog
+            gain: 2.0             // scattering brightness
+        },
         hazeResidual: 0.30,       // surface colour surviving at the full fog distance
         cloudBrightness: 1.0,     // cloud albedo (theme hue, brightness from the model)
         cloudHaze: 0.22,          // share of the ground haze applied to the deck

vendor/clouds3d.js

@@ -85,7 +85,15 @@ var FRAG_SRC = [
     'uniform float uSkyOn;',       // 0 until the star texture is built
     'uniform float uStarLon;',     // sidereal rotation offset (radians)
     'uniform vec3 uPole;',         // north celestial pole in world coords
-    'uniform int uMode;',          // 0 = sky+stars, 1 = clouds (transparent over mesh)
+    'uniform int uMode;',          // 0 = sky+stars, 1 = clouds, 2 = atmosphere shell
+    'uniform float uAtmoModelOn;',
+    'uniform float uAtmoHeightUnits;',
+    'uniform float uAtmoScaleHeightUnits;',
+    'uniform float uAtmoDensity;',
+    'uniform float uAtmoGrayTau;',
+    'uniform float uAtmoGain;',
+    'uniform vec3 uAtmoBlueColor;',
+    'uniform vec3 uAtmoGrayColor;',
     'uniform vec3 uSunColor;',     // direct beam after atmospheric extinction
     'uniform float uSunInt;',
     'uniform vec3 uMoonColor;',
@@ -227,6 +235,64 @@ var FRAG_SRC = [
         'vec3 ro = nearH.xyz / nearH.w;',
         'vec3 rd = normalize(farH.xyz / farH.w - ro);',
 
+        'if (uMode == 2) {',
+            'if (uAtmoModelOn < 0.5) { gl_FragColor = vec4(0.0); return; }',
+            'float R = max(1.0, uEarthRadius);',
+            'float H = max(0.1, uAtmoHeightUnits);',
+            'float Hs = max(0.01, uAtmoScaleHeightUnits);',
+            'vec3 C = vec3(ro.x, -R, ro.z);',
+            'vec3 oc = ro - C;',
+            'float b = dot(oc, rd);',
+            'float rOut = R + H;',
+            'float cOut = dot(oc, oc) - rOut * rOut;',
+            'float discOut = b * b - cOut;',
+            'float tStart = 0.0;',
+            'float tEnd = 100.0;',
+            'if (discOut > 0.0) {',
+                'float sq = sqrt(discOut);',
+                'float t1 = -b - sq;',
+                'float t2 = -b + sq;',
+                'if (dot(oc, oc) > rOut * rOut) tStart = max(0.0, t1);',
+                'tEnd = max(0.0, t2);',
+            '}',
+            'float cIn = dot(oc, oc) - R * R;',
+            'float discIn = b * b - cIn;',
+            'float tInner = 100000.0;',
+            'if (discIn > 0.0) {',
+                'float sq = sqrt(discIn);',
+                'float t1 = -b - sq;',
+                'float t2 = -b + sq;',
+                'if (t1 > 0.0) tInner = t1;',
+                'else if (t2 > 0.0) tInner = t2;',
+            '}',
+            'if (tInner < 100000.0) tEnd = min(tEnd, tInner);',
+            'if (tEnd <= tStart) { gl_FragColor = vec4(0.0); return; }',
+            'float step = (tEnd - tStart) / 32.0;',
+            'float t = tStart + step * 0.5;',
+            'float tau = 0.0;',
+            'vec3 scatter = vec3(0.0);',
+            'float cosTA = dot(rd, uSunDir);',
+            'float cosTM = dot(rd, uMoonDir);',
+            'float sunPhase = 0.25 + 0.75 * pow(max(0.0, cosTA), 2.0);',
+            'float moonPhase = 0.25 + 0.75 * pow(max(0.0, cosTM), 2.0);',
+            'for (int i = 0; i < 32; i++) {',
+                'vec3 p = ro + rd * t;',
+                'float alt = max(0.0, length(p - C) - R);',
+                'float dens = exp(-alt / Hs);',
+                'float dtau = uAtmoDensity * dens * step;',
+                'float transBefore = exp(-tau);',
+                'float grayMix = clamp(tau / max(0.01, uAtmoGrayTau), 0.0, 1.0);',
+                'vec3 atmoCol = mix(uAtmoBlueColor, uAtmoGrayColor, grayMix);',
+                'vec3 light = uSunColor * uSunInt * sunPhase + uMoonColor * uMoonInt * moonPhase * 2.0 + max(vec3(uSkyAmb), uStarAmb * vec3(0.7, 0.85, 1.0)) * 0.65;',
+                'scatter += atmoCol * light * dtau * transBefore;',
+                'tau += dtau;',
+                't += step;',
+            '}',
+            'float alpha = clamp(1.0 - exp(-tau), 0.0, 1.0);',
+            'vec3 col = scatter / max(alpha, 1e-4) * uAtmoGain;',
+            'gl_FragColor = vec4(col, alpha);',
+            'return;',
+        '}',
         'if (uMode == 0) {',
             // skyT floor at 0.5: at and below the horizon the sky keeps
             // the exact horizon colour - the same value the terrain's
@@ -853,6 +919,11 @@ function init(canvas) {
         uCloudTop: uCloudTop, uCloudBot: uCloudBot,
         uElev: uElev, uElevOn: uElevOn, uElevScale: uElevScale,
         uSunDir: uSunDir, uMoonDir: uMoonDir, uMode: uMode,
+        uAtmoModelOn: uAtmoModelOn, uAtmoHeightUnits: uAtmoHeightUnits,
+        uAtmoScaleHeightUnits: uAtmoScaleHeightUnits, uAtmoDensity: uAtmoDensity,
+        uAtmoGrayTau: uAtmoGrayTau, uAtmoGain: uAtmoGain,
+        uEarthRadius: uEarthRadius,
+        uAtmoBlueColor: uAtmoBlueColor, uAtmoGrayColor: uAtmoGrayColor,
         uSunColor: uSunColor, uSunInt: uSunInt, uMoonColor: uMoonColor,
         uMoonInt: uMoonInt, uSkyAmb: uSkyAmb, uStarAmb: uStarAmb,
         uPhysical: uPhysical, uAirColor: uAirColor, uCloudAlbedo: uCloudAlbedo,
@@ -2041,6 +2112,23 @@ var RAYLEIGH = [0.192, 0.451, 1.0];
 var ATMO_AIR_GAIN = (typeof ATMO.airGain === 'number') ? Math.max(0.05, ATMO.airGain) : 0.55;
 var ATMO_HAZE_RESIDUAL = (typeof ATMO.hazeResidual === 'number')
     ? Math.max(0.02, Math.min(0.95, ATMO.hazeResidual)) : 0.30;
+var ATMO_DISTANCE_FOG = ATMO.distanceFog !== false;
+var ATMO_MODEL = ATMO.model || {};
+var ATMO_MODEL_ON = ATMO_MODEL.enabled !== false;
+var ATMO_MODEL_HEIGHT_KM = (typeof ATMO_MODEL.heightKm === 'number')
+    ? Math.max(1.0, Math.min(80.0, ATMO_MODEL.heightKm)) : 35.0;
+var ATMO_MODEL_HEIGHT_UNITS = ATMO_MODEL_HEIGHT_KM / KM_PER_TILE;
+var ATMO_MODEL_SCALE_HEIGHT_KM = (typeof ATMO_MODEL.scaleHeightKm === 'number')
+    ? Math.max(1.0, Math.min(30.0, ATMO_MODEL.scaleHeightKm)) : 8.5;
+var ATMO_MODEL_SCALE_HEIGHT_UNITS = ATMO_MODEL_SCALE_HEIGHT_KM / KM_PER_TILE;
+var ATMO_MODEL_DENSITY = (typeof ATMO_MODEL.density === 'number')
+    ? Math.max(0.01, Math.min(5.0, ATMO_MODEL.density)) : 0.12;
+var ATMO_MODEL_GRAY_TAU = (typeof ATMO_MODEL.grayTau === 'number')
+    ? Math.max(0.1, Math.min(10.0, ATMO_MODEL.grayTau)) : 1.5;
+var ATMO_MODEL_GAIN = (typeof ATMO_MODEL.gain === 'number')
+    ? Math.max(0.1, Math.min(5.0, ATMO_MODEL.gain)) : 2.0;
+var ATMO_MODEL_BLUE_COLOR = new Float32Array([0.22, 0.42, 0.95]);
+var ATMO_MODEL_GRAY_COLOR = new Float32Array([0.72, 0.76, 0.82]);
 // Integrated starlight + airglow. Physically this is about a millionth of
 // daylight, so it is a stylised floor (CONFIG.atmosphere.starGain) that
 // keeps clouds, contrails and the ground from going fully black on a
@@ -5112,6 +5200,15 @@ window.clouds3dRender = function(target) {
     gl.uniform3f(s.uMoonColor, _mBeam[0], _mBeam[1], _mBeam[2]);
     gl.uniform1f(s.uMoonInt, _moonI);
     gl.uniform1f(s.uSkyAmb, _amb * ATMO_SKY_GAIN);
+    gl.uniform1f(s.uAtmoModelOn, ATMO_MODEL_ON ? 1.0 : 0.0);
+    gl.uniform1f(s.uAtmoHeightUnits, ATMO_MODEL_HEIGHT_UNITS);
+    gl.uniform1f(s.uAtmoScaleHeightUnits, ATMO_MODEL_SCALE_HEIGHT_UNITS);
+    gl.uniform1f(s.uAtmoDensity, ATMO_MODEL_DENSITY);
+    gl.uniform1f(s.uAtmoGrayTau, ATMO_MODEL_GRAY_TAU);
+    gl.uniform1f(s.uAtmoGain, ATMO_MODEL_GAIN);
+    gl.uniform1f(s.uEarthRadius, EARTH_RADIUS_UNITS);
+    gl.uniform3fv(s.uAtmoBlueColor, ATMO_MODEL_BLUE_COLOR);
+    gl.uniform3fv(s.uAtmoGrayColor, ATMO_MODEL_GRAY_COLOR);
     gl.uniform1f(s.uStarAmb, _star);
     // Theme cloud hue kept, brightness normalised: the light model, not the
     // palette, decides how bright a cloud is.
@@ -5937,6 +6034,12 @@ window.clouds3dRender = function(target) {
     gl.blendFunc(gl.SRC_ALPHA, gl.ONE_MINUS_SRC_ALPHA);
     gl.uniform1i(s.uMode, 1);
     gl.drawArrays(gl.TRIANGLES, 0, 3);
+
+    if (ATMO_MODEL_ON) {
+        gl.uniform1i(s.uMode, 2);
+        gl.drawArrays(gl.TRIANGLES, 0, 3);
+    }
+
     // Restore default state for the next frame.
     gl.disable(gl.BLEND);
     gl.enable(gl.DEPTH_TEST);