Fix flickering for tileset with thin walls

CHANGES.md

@@ -16,6 +16,7 @@ Change Log
 * Added `customTags` property to the UrlTemplateImageryProvider to allow custom keywords in the template URL. [#5696](https://github.com/AnalyticalGraphicsInc/cesium/pull/5696)
 * Improved CZML Reference Properties example [#5754](https://github.com/AnalyticalGraphicsInc/cesium/pull/5754)
 * Fixed bug with placemarks in imported KML: placemarks with no specified icon would be displayed with default icon. [#5819](https://github.com/AnalyticalGraphicsInc/cesium/issues/5819)
+* Fixed flickering artifacts on tilesets with thin walls. [#5940](https://github.com/AnalyticalGraphicsInc/cesium/pull/5940)
 * Fixed bright fog when terrain lighting is enabled and added `Fog.minimumBrightness` to affect how bright the fog will be when in complete darkness. [#5934](https://github.com/AnalyticalGraphicsInc/cesium/pull/5934)
 
 ### 1.38 - 2017-10-02

Source/Scene/Batched3DModel3DTileContent.js

@@ -452,7 +452,8 @@ define([
         // If any commands were pushed, add derived commands
         var commandEnd = frameState.commandList.length;
         if ((commandStart < commandEnd) && frameState.passes.render) {
-            this._batchTable.addDerivedCommands(frameState, commandStart);
+            var finalResolution = this._tile._finalResolution;
+            this._batchTable.addDerivedCommands(frameState, commandStart, finalResolution);
         }
    };
 

Source/Scene/Cesium3DTileBatchTable.js

@@ -1209,7 +1209,7 @@ define([
         OPAQUE_AND_TRANSLUCENT : 2
     };
 
-    Cesium3DTileBatchTable.prototype.addDerivedCommands = function(frameState, commandStart) {
+    Cesium3DTileBatchTable.prototype.addDerivedCommands = function(frameState, commandStart, finalResolution) {
         var commandList = frameState.commandList;
         var commandEnd = commandList.length;
         var tile = this._content._tile;
@@ -1236,7 +1236,7 @@ define([
             }
 
             if (bivariateVisibilityTest) {
-                if (command.pass !== Pass.TRANSLUCENT) {
+                if (command.pass !== Pass.TRANSLUCENT && !finalResolution) {
                     if (!defined(derivedCommands.zback)) {
                         derivedCommands.zback = deriveZBackfaceCommand(derivedCommands.originalCommand);
                     }
@@ -1329,6 +1329,20 @@ define([
         var rs = clone(derivedCommand.renderState, true);
         rs.cull.enabled = true;
         rs.cull.face = CullFace.FRONT;
+        // Back faces do not need to write color.
+        rs.colorMask = {
+            red : false,
+            green : false,
+            blue : false,
+            alpha : false
+        };
+        // Push back face depth away from the camera so it is less likely that back faces and front faces of the same tile
+        // intersect and overlap. This helps avoid flickering for very thin double-sided walls.
+        rs.polygonOffset = {
+            enabled : true,
+            factor : 5.0,
+            units : 5.0
+        };
         derivedCommand.renderState = RenderState.fromCache(rs);
         derivedCommand.castShadows = false;
         derivedCommand.receiveShadows = false;

Source/Scene/Instanced3DModel3DTileContent.js

@@ -517,7 +517,7 @@ define([
         // If any commands were pushed, add derived commands
         var commandEnd = frameState.commandList.length;
         if ((commandStart < commandEnd) && frameState.passes.render) {
-            this._batchTable.addDerivedCommands(frameState, commandStart);
+            this._batchTable.addDerivedCommands(frameState, commandStart, false);
         }
     };
 

Specs/Scene/Cesium3DTilesetSpec.js

@@ -2492,8 +2492,9 @@ defineSuite([
 
             scene.renderForSpecs();
 
-            // 2 for root tile, 2 for child, 1 for stencil clear
-            expect(statistics.numberOfCommands).toEqual(5);
+            // 2 for root tile, 1 for child, 1 for stencil clear
+            // Tiles that are marked as finalResolution, including leaves, do not create back face commands
+            expect(statistics.numberOfCommands).toEqual(4);
             expect(root.selected).toBe(true);
             expect(root._finalResolution).toBe(false);
             expect(root.children[0].children[0].children[3].selected).toBe(true);
@@ -2504,6 +2505,7 @@ defineSuite([
             var rs = commandList[1].renderState;
             expect(rs.cull.enabled).toBe(true);
             expect(rs.cull.face).toBe(CullFace.FRONT);
+            expect(rs.polygonOffset.enabled).toBe(true);
         });
     });