perf: distance-adaptive cloud raymarch

vendor/clouds3d.js

@@ -719,8 +719,8 @@ var FRAG_SRC = [
             'float maxMarch = max(1.0, uCloudMaxMarch);',
             'tExit = min(tExit, tEnter + maxMarch);',
             'if (tExit <= tEnter) { gl_FragColor = vec4(0.0); return; }',
-            'float stepSize = (tExit - tEnter) / 32.0;',
+            'float marchCurve = 2.5;',
-            'float t = tEnter + stepSize * 0.5;',
+            'float marchDenom = 1.0 - exp(-marchCurve);',
             'float coverage = clamp(uDensity, 0.0, 1.0);',
             'float baseThresh = mix(0.62, 0.20, coverage);',
             'float densityGain = mix(0.45, 1.0, coverage);',
@@ -730,6 +730,12 @@ var FRAG_SRC = [
             'float trans = 1.0;',
             'vec3 cloudSum = vec3(0.0);',
             'for (int i = 0; i < 32; i++) {',
+                'float u = (float(i) + 0.5) / 32.0;',
+                'float uNext = (float(i) + 1.5) / 32.0;',
+                'float mapU = (1.0 - exp(-marchCurve * u)) / marchDenom;',
+                'float mapUNext = (1.0 - exp(-marchCurve * uNext)) / marchDenom;',
+                'float t = tEnter + (tExit - tEnter) * mapU;',
+                'float stepSize = (tExit - tEnter) * (mapUNext - mapU);',
                 'vec3 p = uCamPos + rd * t;',
                 'p += rd * (ign(gl_FragCoord.xy + vec2(float(i) * 7.13, float(i) * 3.71)) - 0.5) * stepSize * 0.25;',
                 'float alt = p.y + earthDrop(p.xz);',
@@ -751,7 +757,6 @@ var FRAG_SRC = [
                     'trans *= (1.0 - alpha);',
                     'if (trans < 0.04) break;',
                 '}',
-                't += stepSize;',
             '}',
             'float cloudAlpha = 1.0 - trans;',
             'vec3 cloudColor = (cloudAlpha > 0.001) ? cloudSum / cloudAlpha : vec3(0.0);',