clouds: repair curved deck entry and terrain occlusion

vendor/clouds3d.js

@@ -241,6 +241,18 @@ var FRAG_SRC = [
         'float sq = sqrt(disc);',
         'return vec2((-b - sq) / (2.0 * a), (-b + sq) / (2.0 * a));',
     '}',
+    'float minPositiveRoot(vec2 r) {',
+        'float m = 1e9;',
+        'if (r.x > 0.0) m = min(m, r.x);',
+        'if (r.y > 0.0) m = min(m, r.y);',
+        'return m;',
+    '}',
+    'float maxPositiveRoot(vec2 r) {',
+        'float m = -1e9;',
+        'if (r.x > 0.0) m = max(m, r.x);',
+        'if (r.y > 0.0) m = max(m, r.y);',
+        'return m;',
+    '}',
     // Two modes driven by uMode:
     //   0 = SKY:    full-screen pass that paints the sky gradient and
     //                stars into the framebuffer (opaque, alpha = 1).
@@ -440,18 +452,10 @@ var FRAG_SRC = [
                 'float terrainY = terrElev(hp.xz) * uElevScale - earthDrop(hp.xz);',
                 'float dh = terrainY - hp.y;',
                 'if (abs(dh) < 0.02) break;',
-                'tTerrain += dh / -rd.y;',
+                'tTerrain -= dh / -rd.y;',
                 'if (tTerrain < 0.1 || tTerrain > 90.0) { tTerrain = -1.0; break; }',
             '}',
         '}',
-        // Cloud fully behind the terrain -> discard (framebuffer keeps
-        // the terrain color from the mesh pass).
-        'if (tTerrain > 0.0) {',
-            'vec2 topRoots = curvedLayerRoots(uCloudTop, ro, rd);',
-            'float tCloudTop = max(topRoots.x, topRoots.y);',
-            'if (tCloudTop > tTerrain) discard;',
-        '}',
-
         // Cloud march. The output is the cloud's effective colour and
         // coverage (alpha); the render pass uses alpha-blend so the
         // terrain mesh from the previous pass shows through transparent
@@ -466,15 +470,23 @@ var FRAG_SRC = [
         // have.
         'vec2 bot = curvedLayerRoots(uCloudBot, ro, rd);',
         'vec2 top = curvedLayerRoots(uCloudTop, ro, rd);',
-        'float tEnter = 1e9;',
-        'if (bot.x > 0.0) tEnter = min(tEnter, bot.x);',
-        'if (bot.y > 0.0) tEnter = min(tEnter, bot.y);',
-        'if (top.x > 0.0) tEnter = min(tEnter, top.x);',
-        'if (top.y > 0.0) tEnter = min(tEnter, top.y);',
-        'float tExit = max(max(bot.x, bot.y), max(top.x, top.y));',
         'float roPy = ro.y + earthDrop(ro.xz);',
-        'if (roPy >= uCloudBot && roPy <= uCloudTop) tEnter = 0.0;',
-        'if (tEnter > 1e8) { tEnter = 0.0; tExit = 14.0; }',
+        'float tEnter = 1e9;',
+        'float tExit = -1e9;',
+        'if (roPy >= uCloudBot && roPy <= uCloudTop) {',
+            'tEnter = 0.0;',
+            'tExit = max(maxPositiveRoot(bot), maxPositiveRoot(top));',
+        '} else if (roPy > uCloudTop) {',
+            'tEnter = minPositiveRoot(top);',
+            'tExit = maxPositiveRoot(bot);',
+            'if (tExit < 0.0) tExit = maxPositiveRoot(top);',
+        '} else {',
+            'tEnter = minPositiveRoot(bot);',
+            'tExit = maxPositiveRoot(top);',
+            'if (tExit < 0.0) tExit = maxPositiveRoot(bot);',
+        '}',
+        'if (tEnter > 1e8 || tExit <= tEnter) { tEnter = 0.0; tExit = 14.0; }',
+        'if (tTerrain > 0.0 && tEnter > tTerrain) discard;',
         'if (tTerrain > 0.0) tExit = min(tExit, tTerrain);',
         // A grazing ray would sample a path of well over a thousand km with
         // only 24 steps - coarse blobs along the horizon. Past this length