weather: add dynamic procedural snow cover

config.js

@@ -149,12 +149,25 @@ window.CONFIG = {
             dryMinutes: 30,       // roads dry out after this many dry minutes
             roadGain: 0.8,        // extra road-ribbon glow when wet
             lightGain: 0.35       // extra halo on vehicle lights when wet
+        },
+        snowCover: {
+            enabled: true,
+            meltMinutes: 90,      // snow melts after this many warm minutes
+            baseLineM: 900,       // snowline at about +5 C
+            tempLineMPerC: 80,    // snowline moves down as temperature drops
+            minLineM: 0,          // full snowline in deep cold
+            maxLineM: 2500,       // no snow above this line when warm
+            transitionM: 120,     // soft transition band around the snowline
+            slopeMax: 0.72,       // steep slopes keep less snow
+            gain: 0.85            // maximum whiteness of the snow albedo
         }
         // weatherEffects: { enabled: false }
         // weatherEffects: { rain: { maxAlpha: 0.55, speed: 1.4 }, snow: { enabled: false } }
         // weatherEffects: { lightning: { gain: 2.0, minDelayS: 3, maxDelayS: 12 } }
         // weatherEffects: { wetRoads: { enabled: false } }
         // weatherEffects: { wetRoads: { dryMinutes: 10, roadGain: 1.4, lightGain: 0.7 } }
+        // weatherEffects: { snowCover: { baseLineM: 600, tempLineMPerC: 120, gain: 1.0 } }
+        // weatherEffects: { snowCover: { enabled: false } }
     },
 
     // ------------------------------------------------------------------

vendor/clouds3d.js

@@ -896,6 +896,11 @@ var TERRAIN_FRAG_SRC = [
     'uniform float uCloudShadowAmbient;', // how much of the skylight a cloud also blocks
     'uniform sampler2D uElev;',        // height field, same grid the mesh was built from
     'uniform float uElevScale;',       // metres -> world units (incl. exaggeration)
+    'uniform float uSnowCover;',       // accumulated snow cover 0..1
+    'uniform float uSnowLineM;',       // dynamic snowline in metres
+    'uniform float uSnowTransitionM;', // soft transition band around the snowline
+    'uniform float uSnowSlopeMax;',    // slope cutoff for snow retention
+    'uniform float uSnowGain;',        // maximum whiteness
     'uniform float uTerrainShadowOn;',
     'uniform float uTerrainShadowFloor;', // brightness a fully shadowed slope keeps
     'uniform vec3 uAirColor;',     // Rayleigh airlight (sun-tinted, blue-biased)
@@ -969,6 +974,15 @@ var TERRAIN_FRAG_SRC = [
         // ramp: the sun altitude drives colour and strength continuously.
         'float outsideMask = clamp(outside, 0.0, 1.0);',
         'vec3 alb = mix(texture2D(uGround, gUv).rgb, vColor, outsideMask) * uAlbedoTint;',
+        'if (uSnowCover > 0.001) {',
+            'float elevM = (vY + earthDrop(vWorld.xz)) / max(0.000001, uElevScale);',
+            'float slope = 1.0 - clamp(n.y, 0.0, 1.0);',
+            'float snowMask = smoothstep(uSnowLineM + uSnowTransitionM, uSnowLineM - uSnowTransitionM, elevM);',
+            'snowMask *= smoothstep(uSnowSlopeMax, uSnowSlopeMax * 0.45, slope);',
+            'snowMask *= 0.82 + 0.18 * hash3(floor(vWorld * 80.0));',
+            'snowMask *= uSnowCover * uSnowGain;',
+            'alb = mix(alb, vec3(0.92, 0.95, 1.0), clamp(snowMask, 0.0, 1.0));',
+        '}',
             'float ndl = max(0.0, dot(n, uSunDir));',
             // Cloud shadow: march from the surface point towards the sun
             // through the cloud layer and accumulate optical depth. Same
@@ -1237,6 +1251,11 @@ function init(canvas) {
     var terrainUelev = terrainProg ? gl.getUniformLocation(terrainProg, 'uElev') : null;
     var terrainUterrainShadow = terrainProg ? gl.getUniformLocation(terrainProg, 'uTerrainShadowOn') : null;
     var terrainUterrainShadowFloor = terrainProg ? gl.getUniformLocation(terrainProg, 'uTerrainShadowFloor') : null;
+    var terrainUsnowCover = terrainProg ? gl.getUniformLocation(terrainProg, 'uSnowCover') : null;
+    var terrainUsnowLineM = terrainProg ? gl.getUniformLocation(terrainProg, 'uSnowLineM') : null;
+    var terrainUsnowTransitionM = terrainProg ? gl.getUniformLocation(terrainProg, 'uSnowTransitionM') : null;
+    var terrainUsnowSlopeMax = terrainProg ? gl.getUniformLocation(terrainProg, 'uSnowSlopeMax') : null;
+    var terrainUsnowGain = terrainProg ? gl.getUniformLocation(terrainProg, 'uSnowGain') : null;
 
     // 1x1 black texture: bound wherever a real layer does not exist yet
     // (e.g. the basemap before the first tile, or terrain disabled) so the
@@ -1325,7 +1344,10 @@ function init(canvas) {
             uCloudShadowFloor: terrainUcsFloor,
             uCloudShadowAmbient: terrainUcsAmb,
             uElev: terrainUelev, uTerrainShadowOn: terrainUterrainShadow,
-            uTerrainShadowFloor: terrainUterrainShadowFloor
+            uTerrainShadowFloor: terrainUterrainShadowFloor,
+            uSnowCover: terrainUsnowCover, uSnowLineM: terrainUsnowLineM,
+            uSnowTransitionM: terrainUsnowTransitionM, uSnowSlopeMax: terrainUsnowSlopeMax,
+            uSnowGain: terrainUsnowGain
             // vbo / vertCount / wireVbo / wireVertCount are read from the
             // module-level terrainVbo/terrainVertCount/terrainWireVbo/
             // terrainWireVertCount (set by buildTerrainMesh).
@@ -6073,6 +6095,43 @@ function updateRoadWetness(now) {
     return roadWetness;
 }
 
+var SNOW_COVER_CFG = WEATHER_FX_CFG.snowCover || {};
+var SNOW_COVER_ON = SNOW_COVER_CFG.enabled !== false;
+var SNOW_COVER_MELT_MINUTES = (typeof SNOW_COVER_CFG.meltMinutes === 'number') ? Math.max(1, SNOW_COVER_CFG.meltMinutes) : 90;
+var SNOW_COVER_BASE_LINE = (typeof SNOW_COVER_CFG.baseLineM === 'number') ? Math.max(0, SNOW_COVER_CFG.baseLineM) : 900;
+var SNOW_COVER_TEMP_LINE = (typeof SNOW_COVER_CFG.tempLineMPerC === 'number') ? Math.max(0, SNOW_COVER_CFG.tempLineMPerC) : 80;
+var SNOW_COVER_MIN_LINE = (typeof SNOW_COVER_CFG.minLineM === 'number') ? Math.max(0, SNOW_COVER_CFG.minLineM) : 0;
+var SNOW_COVER_MAX_LINE = (typeof SNOW_COVER_CFG.maxLineM === 'number') ? Math.max(SNOW_COVER_MIN_LINE, SNOW_COVER_CFG.maxLineM) : 2500;
+var SNOW_COVER_TRANSITION = (typeof SNOW_COVER_CFG.transitionM === 'number') ? Math.max(1, SNOW_COVER_CFG.transitionM) : 120;
+var SNOW_COVER_SLOPE_MAX = (typeof SNOW_COVER_CFG.slopeMax === 'number') ? Math.max(0.1, Math.min(1.2, SNOW_COVER_CFG.slopeMax)) : 0.72;
+var SNOW_COVER_GAIN = (typeof SNOW_COVER_CFG.gain === 'number') ? Math.max(0, Math.min(1.2, SNOW_COVER_CFG.gain)) : 0.85;
+var snowCoverAmount = 0, lastSnowT = null, snowLineM = SNOW_COVER_MAX_LINE;
+
+function updateSnowCover(now) {
+    if (!SNOW_COVER_ON) {
+        snowCoverAmount = 0;
+        snowLineM = SNOW_COVER_MAX_LINE;
+        lastSnowT = null;
+        return 0;
+    }
+    if (lastSnowT === null) { lastSnowT = now; return snowCoverAmount; }
+    var dt = Math.max(0, Math.min(5, now - lastSnowT));
+    lastSnowT = now;
+    var wx = window.__currentWeather || {};
+    var tempC = Number(wx.tempC || 0);
+    var snow = Number(wx.snowMm || 0);
+    var precip = Number(wx.precipMm || 0);
+    var coldSnow = (tempC < 2.0 ? 1.0 : 0.0);
+    var input = Math.min(1.0, (snow + (tempC < 1.0 ? precip * 0.55 : 0.0)) * 0.85) * coldSnow;
+    var melt = dt / (SNOW_COVER_MELT_MINUTES * 60.0);
+    if (tempC > 2.0) melt *= 1.0 + Math.min(4.0, tempC - 2.0) * 0.35;
+    snowCoverAmount = Math.max(0, Math.min(1, snowCoverAmount + input * dt * 0.22 - melt));
+    var line = SNOW_COVER_BASE_LINE - (5.0 - tempC) * SNOW_COVER_TEMP_LINE;
+    line = Math.max(SNOW_COVER_MIN_LINE, Math.min(SNOW_COVER_MAX_LINE, line));
+    snowLineM = line;
+    return snowCoverAmount;
+}
+
 function updateLightning(now) {
     if (!WEATHER_LIGHTNING_ON) {
         lightningFlash = 0;
@@ -6733,6 +6792,11 @@ window.clouds3dRender = function(target) {
         gl.uniform3f(terr.uMoonColor, _mBeam[0], _mBeam[1], _mBeam[2]);
         gl.uniform1f(terr.uMoonInt, _moonI);
         gl.uniform1f(terr.uSkyAmb, _amb * ATMO_SKY_GAIN);
+        gl.uniform1f(terr.uSnowCover, _snowCover);
+        gl.uniform1f(terr.uSnowLineM, snowLineM);
+        gl.uniform1f(terr.uSnowTransitionM, SNOW_COVER_TRANSITION);
+        gl.uniform1f(terr.uSnowSlopeMax, SNOW_COVER_SLOPE_MAX);
+        gl.uniform1f(terr.uSnowGain, SNOW_COVER_GAIN);
         gl.uniform1f(terr.uStarAmb, _star * ATMO_STAR_GROUND);
         gl.uniform3f(terr.uAlbedoTint, _pl[0] / _pm * ATMO_ALBEDO_GAIN,
             _pl[1] / _pm * ATMO_ALBEDO_GAIN, _pl[2] / _pm * ATMO_ALBEDO_GAIN);
@@ -6827,6 +6891,7 @@ window.clouds3dRender = function(target) {
     // aircraft): the mean over the per-light thresholds.
     var carNight = lightFactor(window.__currentSunAlt || 0);
     var _roadWet = updateRoadWetness(now);
+    var _snowCover = updateSnowCover(now);
     if (carProg && (carVboM || carVboB) && carNight > 0.01) {
         var carPxScale = Math.max(0.45, Math.min(1.25, gl.drawingBufferHeight / 1080));
         var tCurve = Math.max(0, Math.min(1, (carNight - 0.10) / 0.40));
@@ -7357,6 +7422,8 @@ window.clouds3dRender = function(target) {
             snow: +_weather[1].toFixed(2),
             lightning: +lightningFlash.toFixed(2),
             wet: +roadWetness.toFixed(2),
+            snowCover: +snowCoverAmount.toFixed(2),
+            snowLineM: Math.round(snowLineM),
             code: Number((window.__currentWeather || {}).code || 0)
         };
         var _rainI = _weather[0];