Fix model crash by adding default pbr material

CHANGES.md

@@ -1,7 +1,7 @@
 Change Log
 ==========
 
-### 1.49 - 2018-09-03
+### 1.49 - 2018-09-04
 
 ##### Breaking Changes :mega:
 * Removed `ClippingPlaneCollection.clone` [#6872](https://github.com/AnalyticalGraphicsInc/cesium/pull/6872)
@@ -33,6 +33,7 @@ Change Log
 * Improved `Plane` entities so they are better aligned with the globe surface [#6887](https://github.com/AnalyticalGraphicsInc/cesium/pull/6887)
 * Fixed crash when rendering translucent objects when all shadow maps in the scene set `fromLightSource` to false. [#6883](https://github.com/AnalyticalGraphicsInc/cesium/pull/6883)
 * Fixed night shading in 2D and Columbus view. [#4122](https://github.com/AnalyticalGraphicsInc/cesium/issues/4122)
+* Fixed model loading failure when a glTF 2.0 primitive does not have a material. [6906](https://github.com/AnalyticalGraphicsInc/cesium/pull/6906)
 * Fixed a crash when setting show to `false` on a polyline clamped to the ground. [#6912](https://github.com/AnalyticalGraphicsInc/cesium/issues/6912)
 * Fixed a bug where `Cesium3DTileset` wasn't using the correct tilesetVersion [#6933](https://github.com/AnalyticalGraphicsInc/cesium/pull/6933)
 * Fixed crash that happened when calling `scene.pick` after setting a new terrain provider [#6918](https://github.com/AnalyticalGraphicsInc/cesium/pull/6918)

Source/ThirdParty/GltfPipeline/addDefaults.js

@@ -242,6 +242,15 @@ define([
         });
     }
 
+    var defaultPbrMaterial = {
+        emissiveFactor: [0.0, 0.0, 0.0],
+        alphaMode: 'OPAQUE',
+        doubleSided: false,
+        extras : {
+            _pipeline: {}
+        }
+    };
+
     var defaultMaterial = {
         values : {
             emission : [
@@ -335,6 +344,24 @@ define([
         }
     };
 
+    function addDefaultPbrMaterial(gltf) {
+        var materials = gltf.materials;
+        var meshes = gltf.meshes;
+
+        var meshesLength = meshes.length;
+        for (var meshId = 0; meshId < meshesLength; meshId++) {
+            var mesh = meshes[meshId];
+            var primitives = mesh.primitives;
+            var primitivesLength = primitives.length;
+            for (var j = 0; j < primitivesLength; j++) {
+                var primitive = primitives[j];
+                if (!defined(primitive.material)) {
+                    primitive.material = addToArray(materials, clone(defaultPbrMaterial, true));
+                }
+            }
+        }
+    }
+
     function addDefaultMaterial(gltf) {
         var materials = gltf.materials;
         var meshes = gltf.meshes;
@@ -349,10 +376,7 @@ define([
             for (var j = 0; j < primitivesLength; j++) {
                 var primitive = primitives[j];
                 if (!defined(primitive.material)) {
-                    if (!defined(defaultMaterialId)) {
-                        defaultMaterialId = addToArray(materials, clone(defaultMaterial, true));
-                    }
-                    primitive.material = defaultMaterialId;
+                    primitive.material = addToArray(materials, clone(defaultMaterial, true));
                 }
             }
         }
@@ -512,6 +536,8 @@ define([
         if (gltf.asset.extras.gltf_pipeline_upgrade_10to20) {
             addDefaultMaterial(gltf);
             addDefaultTechnique(gltf);
+        } else {
+            addDefaultPbrMaterial(gltf);
         }
 
         addDefaultByteOffsets(gltf);