Fix tile refinement when leaf is empty

CHANGES.md

@@ -1,5 +1,11 @@
 # Change Log
 
+### 1.72 - 2020-08-03
+
+##### Fixes :wrench:
+
+- Fixed 3D Tileset replacement refinement when leaf is empty. [#8996](https://github.com/CesiumGS/cesium/8996)
+
 ### 1.71 - 2020-07-01
 
 ##### Breaking Changes :mega:

Source/Scene/Cesium3DTilesetTraversal.js

@@ -652,11 +652,14 @@ function executeEmptyTraversal(tileset, root, frameState) {
     var childrenLength = children.length;
 
     // Only traverse if the tile is empty - traversal stop at descendants with content
-    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.
-    if (!traverse && !tile.contentAvailable) {
+    var emptyContent = hasEmptyContent(tile);
+    var traverse = emptyContent && canTraverse(tileset, tile);
+    var emptyLeaf = emptyContent && tile.children.length === 0;
+
+    // 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
+    // One exception: a parent may refine even if one of its descendants is an empty leaf
+    if (!traverse && !tile.contentAvailable && !emptyLeaf) {
       allDescendantsLoaded = false;
     }
 

Specs/Scene/Cesium3DTilesetSpec.js

@@ -1411,6 +1411,32 @@ describe(
       });
     });
 
+    it("replacement refinement - refines if descendant is empty leaf tile", function () {
+      // Check that the root is refinable once its children with content are loaded
+      //
+      //          C
+      //     C  C  C  E
+      //
+      viewAllTiles();
+      var originalLoadJson = Cesium3DTileset.loadJson;
+      spyOn(Cesium3DTileset, "loadJson").and.callFake(function (tilesetUrl) {
+        return originalLoadJson(tilesetUrl).then(function (tilesetJson) {
+          tilesetJson.root.refine = "REPLACE";
+          tilesetJson.root.children[3].content = undefined;
+          return tilesetJson;
+        });
+      });
+
+      return Cesium3DTilesTester.loadTileset(scene, tilesetUrl).then(function (
+        tileset
+      ) {
+        tileset.skipLevelOfDetail = false;
+        var statistics = tileset._statistics;
+        scene.renderForSpecs();
+        expect(statistics.numberOfCommands).toEqual(3);
+      });
+    });
+
     it("replacement and additive refinement", function () {
       //          A
       //      A       R (not rendered)