clouds: occlude clouds only for terrain above cloud deck

vendor/clouds3d.js

@@ -674,8 +674,9 @@ var FRAG_SRC = [
                 'if (distTex.a > 0.5) {',
                     'float distNorm = (distTex.r * 65536.0 + distTex.g * 256.0 + distTex.b) / 16777215.0;',
                     'float dist = distNorm * max(1.0, uFarPlane);',
-                    'if (dist < tEnter - 0.05) { gl_FragColor = vec4(0.0); return; }',
-                    'tExit = min(tExit, max(tEnter, dist));',
+                    'vec3 pT = uCamPos + rd * dist;',
+                    'float terrainAlt = pT.y + earthDrop(pT.xz);',
+                    'if (terrainAlt > uCloudTop + 0.05) { gl_FragColor = vec4(0.0); return; }',
                 '}',
             '}',
             'tExit = min(tExit, tEnter + 30.0);',
@@ -6855,7 +6856,7 @@ window.clouds3dRender = function(target) {
         gl.activeTexture(gl.TEXTURE7);
         gl.bindTexture(gl.TEXTURE_2D, terrainDistTex || null);
         gl.uniform1i(s.uTerrainDistTex, 7);
-        gl.uniform1f(s.uTerrainDistOk, 0.0); // test: clouds ignore terrain occlusion
+        gl.uniform1f(s.uTerrainDistOk, (terrainDistOk && terrainDistTex) ? 1.0 : 0.0);
         gl.disable(gl.DEPTH_TEST);
         gl.depthMask(false);
         gl.enable(gl.BLEND);