External tileset traversal fix

CHANGES.md

@@ -7,6 +7,7 @@ Change Log
 * Added `cartographicLimitRectangle` to `Globe`. Use this to limit terrain and imagery to a specific `Rectangle` area. [#6987](https://github.com/AnalyticalGraphicsInc/cesium/pull/6987)
 
 ##### Fixes :wrench:
+* Fixed an issue in the 3D Tiles traversal where external tilesets would not always traverse to their root tile. [#7035](https://github.com/AnalyticalGraphicsInc/cesium/pull/7035)
 * Fixed an issue in the 3D Tiles traversal where empty tiles would be selected instead of their nearest loaded ancestors. [#7011](https://github.com/AnalyticalGraphicsInc/cesium/pull/7011)
 
 ### 1.49 - 2018-09-04

Source/Scene/Cesium3DTilesetTraversal.js

@@ -215,7 +215,7 @@ define([
             return tile._distanceToCamera;
         }
         var parent = tile.parent;
-        var useParentScreenSpaceError = defined(parent) && (!skipLevelOfDetail(tileset) || (tile._screenSpaceError === 0.0));
+        var useParentScreenSpaceError = defined(parent) && (!skipLevelOfDetail(tileset) || (tile._screenSpaceError === 0.0) || parent.hasTilesetContent);
         var screenSpaceError = useParentScreenSpaceError ? parent._screenSpaceError : tile._screenSpaceError;
         var rootScreenSpaceError = tileset.root._screenSpaceError;
         return rootScreenSpaceError - screenSpaceError; // Map higher SSE to lower values (e.g. root tile is highest priority)
@@ -307,7 +307,7 @@ define([
 
         // Use parent's geometric error with child's box to see if the tile already meet the SSE
         var parent = tile.parent;
-        if (defined(parent) && (parent.refine === Cesium3DTileRefine.ADD) && getScreenSpaceError(tileset, parent.geometricError, tile, frameState) <= tileset._maximumScreenSpaceError) {
+        if (defined(parent) && !parent.hasTilesetContent && (parent.refine === Cesium3DTileRefine.ADD) && getScreenSpaceError(tileset, parent.geometricError, tile, frameState) <= tileset._maximumScreenSpaceError) {
             tile._visible = false;
             return;
         }
@@ -438,6 +438,18 @@ define([
         return tile._screenSpaceError > baseScreenSpaceError;
     }
 
+    function canTraverse(tileset, tile) {
+        if (tile.children.length === 0) {
+            return false;
+        }
+        if (tile.hasTilesetContent) {
+            // Traverse external tileset to visit its root tile
+            // Don't traverse if the subtree is expired because it will be destroyed
+            return !tile.contentExpired;
+        }
+        return tile._screenSpaceError > tileset._maximumScreenSpaceError;
+    }
+
     function executeTraversal(tileset, root, baseScreenSpaceError, maximumScreenSpaceError, frameState) {
         // Depth-first traversal that traverses all visible tiles and marks tiles for selection.
         // If skipLevelOfDetail is off then a tile does not refine until all children are loaded.
@@ -455,19 +467,11 @@ define([
             var baseTraversal = inBaseTraversal(tileset, tile, baseScreenSpaceError);
             var add = tile.refine === Cesium3DTileRefine.ADD;
             var replace = tile.refine === Cesium3DTileRefine.REPLACE;
-            var children = tile.children;
-            var childrenLength = children.length;
             var parent = tile.parent;
             var parentRefines = !defined(parent) || parent._refines;
-            var traverse = (childrenLength > 0) && (tile._screenSpaceError > maximumScreenSpaceError);
             var refines = false;
 
-            if (tile.hasTilesetContent && tile.contentExpired) {
-                // Don't traverse expired subtree because it will be destroyed
-                traverse = false;
-            }
-
-            if (traverse) {
+            if (canTraverse(tileset, tile)) {
                 refines = updateAndPushChildren(tileset, tile, stack, frameState) && parentRefines;
             }
 
@@ -514,7 +518,6 @@ define([
     function executeEmptyTraversal(tileset, root, frameState) {
         // Depth-first traversal that checks if all nearest descendants with content are loaded. Ignores visibility.
         var allDescendantsLoaded = true;
-        var maximumScreenSpaceError = tileset._maximumScreenSpaceError;
         var stack = emptyTraversal.stack;
         stack.push(root);
 
@@ -526,7 +529,7 @@ define([
             var childrenLength = children.length;
 
             // Only traverse if the tile is empty - traversal stop at descendants with content
-            var traverse = hasEmptyContent(tile) && (childrenLength > 0) && (tile._screenSpaceError > maximumScreenSpaceError);
+            var traverse = hasEmptyContent(tile) && canTraverse(tileset, tile);
 
             // Traversal stops but the tile does not have content yet.
             // There will be holes if the parent tries to refine to its children, so don't refine.
@@ -573,7 +576,6 @@ define([
      * selected tiles must be no deeper than 15. This is still very unlikely.
      */
     function traverseAndSelect(tileset, root, frameState) {
-        var maximumScreenSpaceError = tileset._maximumScreenSpaceError;
         var stack = selectionTraversal.stack;
         var ancestorStack = selectionTraversal.ancestorStack;
         var lastAncestor;
@@ -606,7 +608,7 @@ define([
             var shouldSelect = tile._shouldSelect;
             var children = tile.children;
             var childrenLength = children.length;
-            var traverse = (childrenLength > 0) && (tile._screenSpaceError > maximumScreenSpaceError);
+            var traverse = canTraverse(tileset, tile);
 
             if (shouldSelect) {
                 if (add) {

Specs/Data/Cesium3DTiles/Tilesets/TilesetOfTilesets/tileset.json

@@ -33,7 +33,7 @@
         88
       ]
     },
-    "geometricError": 240,
+    "geometricError": 70,
     "refine": "ADD",
     "content": {
       "uri": "tileset2.json"

Specs/Data/Cesium3DTiles/Tilesets/TilesetOfTilesets/tileset2.json

@@ -31,7 +31,7 @@
             20
           ]
         },
-        "geometricError": 70,
+        "geometricError": 0,
         "content": {
           "uri": "tileset3/tileset3.json"
         }

Specs/Scene/Cesium3DTilesetSpec.js

@@ -1079,8 +1079,7 @@ defineSuite([
         //
         return Cesium3DTilesTester.loadTileset(scene, tilesetReplacement1Url).then(function(tileset) {
             tileset.root.geometricError = 90;
-            viewRootOnly();
-            scene.camera.zoomIn(20);
+            setZoom(80);
             scene.renderForSpecs();
 
             var statistics = tileset._statistics;
@@ -1364,6 +1363,15 @@ defineSuite([
         });
     });
 
+    it('always visits external tileset root', function() {
+        viewRootOnly();
+        return Cesium3DTilesTester.loadTileset(scene, tilesetOfTilesetsUrl).then(function(tileset) {
+            var statistics = tileset._statistics;
+            expect(statistics.visited).toEqual(2); // Visits external tileset tile, and external tileset root
+            expect(statistics.numberOfCommands).toEqual(1); // Renders external tileset root
+        });
+    });
+
     it('set tile color', function() {
         return Cesium3DTilesTester.loadTileset(scene, noBatchIdsUrl).then(function(tileset) {
             // Get initial color