clouds: tune cloud coverage by weather density

vendor/clouds3d.js

@@ -684,7 +684,9 @@ var FRAG_SRC = [
             'if (tExit <= tEnter) { gl_FragColor = vec4(0.0); return; }',
             'float stepSize = (tExit - tEnter) / 24.0;',
             'float t = tEnter + stepSize * 0.5;',
-            'float baseThresh = mix(0.55, 0.25, uDensity);',
+            'float coverage = clamp(uDensity, 0.0, 1.0);',
+            'float baseThresh = mix(0.72, 0.22, coverage);',
+            'float densityGain = mix(0.25, 1.0, coverage);',
             'float sunUp = smoothstep(-0.20, 0.05, uSunAlt);',
             'float cosT = dot(rd, uSunDir);',
             'float phase = 0.30 + 0.70 * pow(max(0.0, cosT), 2.5);',
@@ -705,7 +707,7 @@ var FRAG_SRC = [
                     'vec3 ambC = albC * max(vec3(uSkyAmb), uStarAmb * vec3(0.7, 0.85, 1.0)) * 1.7;',
                     'vec3 moonC = albC * uMoonColor * (0.35 + 0.65 * max(0.0, dot(rd, uMoonDir))) * uMoonInt * 3.0;',
                     'vec3 cloudCol = direct + ambC + moonC;',
-                    'float alpha = clamp(d * stepSize * 3.50, 0.0, 1.0);',
+                    'float alpha = clamp(d * stepSize * 3.50 * densityGain, 0.0, 1.0);',
                     'cloudSum += cloudCol * alpha * trans;',
                     'trans *= (1.0 - alpha);',
                     'if (trans < 0.04) break;',