use ray-sphere intersection for cloud shadows

Replace the fixed UV offset with a ray-sphere intersection that traces
from each surface point toward the sun to find the exact cloud layer
hit point. Increases shadow strength from 60% to 90%.

Author: sandrwich

src/main.c

@@ -62,10 +62,16 @@ const char *fs3D = "#version 330\n"
                    "        \n"
                    "        float cShadow = 1.0;\n"
                    "        if (showClouds == 1) {\n"
-                   "            vec2 cUV = fragTexCoord + vec2(sunDir.x, -sunDir.y) * 0.005;\n"
+                   "            float cloudR = earthRadius * (1.0 + 25.0 / 6371.0);\n"
+                   "            float b = 2.0 * dot(fragPos, sunDir);\n"
+                   "            float c = earthRadius * earthRadius - cloudR * cloudR;\n"
+                   "            float t = (-b + sqrt(b * b - 4.0 * c)) * 0.5;\n"
+                   "            vec3 cn = normalize(fragPos + t * sunDir);\n"
+                   "            vec2 cUV = vec2(atan(-cn.z, cn.x) / 6.28318530718 + 0.5, cn.y);\n"
+                   "            cUV.y = acos(clamp(cUV.y, -1.0, 1.0)) / 3.14159265359;\n"
                    "            cUV.x = fract(cUV.x + cloudUVOffset);\n"
                    "            float cAlpha = texture(texture2, cUV).a;\n"
-                   "            cShadow = mix(1.0, 0.4, cAlpha * smoothstep(-0.1, 0.2, intensity));\n"
+                   "            cShadow = mix(1.0, 0.1, cAlpha * smoothstep(-0.1, 0.2, intensity));\n"
                    "        }\n"
                    "        \n"
                    "        scatteredDay = (scatteredDay * cShadow) + specular;\n"