Best-effort at supporting spec/glossiness gltf

Create a best-effort metal/rough material when provided with
a spec/glossiness material.

Change-Id: Id000b0153f7be0fa8cbdf634cd22e3c4a6c8baf1

UnitySDK/Assets/TiltBrush/Scripts/Gltf/Gltf2Schema.cs

@@ -137,6 +137,7 @@ public override void Dereference(IUriLoader uriLoader = null) {
         DereferenceTextureInfo(mat.pbrMetallicRoughness.baseColorTexture);
         DereferenceTextureInfo(mat.pbrMetallicRoughness.metallicRoughnessTexture);
       }
+      DereferenceTextureInfo(mat.extensions?.KHR_materials_pbrSpecularGlossiness?.diffuseTexture);
     }
     for (int i = 0; i < nodes.Count; i++) {
       nodes[i].gltfIndex = i;
@@ -258,6 +259,7 @@ public override IEnumerable<GltfTextureBase> ReferencedTextures {
   [Serializable]
   public class Extensions {
     public GOOGLE_tilt_brush_material GOOGLE_tilt_brush_material;
+    public KHR_materials_pbrSpecularGlossiness KHR_materials_pbrSpecularGlossiness;
   }
 
   [Serializable]
@@ -269,6 +271,14 @@ public class PbrMetallicRoughness {
     public TextureInfo metallicRoughnessTexture;
   }
 
+  [Serializable]
+  public class KHR_materials_pbrSpecularGlossiness {
+    public Color diffuseFactor = Color.white;
+    public TextureInfo diffuseTexture;
+    // public Vector3 specularFactor; not supported
+    public float glossinessFactor = 1.0f;
+  }
+
   [Serializable]
   public class TextureInfo {
     public int index;  // indexes into root.textures[]

UnitySDK/Assets/TiltBrush/Scripts/Gltf/GltfMaterialConverter.cs

@@ -273,8 +273,20 @@ private static Material LookUpGlobalMaterial(GltfMaterialBase gltfMaterial) {
     }
 
     if (gltfMat.pbrMetallicRoughness == null) {
-      Debug.LogWarningFormat("Material #{0} has no PBR info.", gltfMat.gltfIndex);
-      return null;
+      var specGloss = gltfMat.extensions?.KHR_materials_pbrSpecularGlossiness;
+      if (specGloss != null) {
+        // Try and make the best of pbrSpecularGlossiness.
+        // Maybe it would be better to support "extensionsRequired" and raise an error
+        // if the asset requires pbrSpecularGlossiness.
+        gltfMat.pbrMetallicRoughness = new Gltf2Material.PbrMetallicRoughness {
+            baseColorFactor = specGloss.diffuseFactor,
+            baseColorTexture = specGloss.diffuseTexture,
+            roughnessFactor = 1f - specGloss.glossinessFactor
+        };
+      } else {
+        Debug.LogWarningFormat("Material #{0} has no PBR info.", gltfMat.gltfIndex);
+        return null;
+      }
     }
 
     return CreateNewPbrMaterial(baseMaterial, gltfMat.name, gltfMat.pbrMetallicRoughness);