glb2: Implement compatibility mode

This is the other half of go/tbcl/109834, which renames attributes
from (for example) TEXCOORD_n -> _TB_UNITY_TEXCOORD_n.

At import time, rename them back to TEXCOORD_n because that what the
rest of the importer expects. If there happens to already be a
TEXCOORD_n, that accessor is ignored (which is the desired behavior;
we don't want or need to bring in the "compatible" texcoord data)

Change-Id: Id000b015a0ee4b1f702b2f1e6354934ba376cd46

UnitySDK/Assets/TiltBrush/Scripts/Gltf/Gltf1Schema.cs

@@ -63,6 +63,13 @@ public override IEnumerable<GltfMaterialBase> Materials {
     }
   }
 
+  public override IEnumerable<GltfMeshBase> Meshes {
+    get {
+      if (meshes == null) { return new GltfMeshBase[0]; }
+      return meshes.Values.Cast<GltfMeshBase>();
+    }
+  }
+
   // Disposable pattern, with Dispose(void) and Dispose(bool), as recommended by:
   // https://docs.microsoft.com/en-us/dotnet/api/system.idisposable
   protected override void Dispose(bool disposing) {

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

@@ -59,6 +59,13 @@ public override IEnumerable<GltfMaterialBase> Materials {
     }
   }
 
+  public override IEnumerable<GltfMeshBase> Meshes {
+    get {
+      if (meshes == null) { return new GltfMeshBase[0]; }
+      return meshes.Cast<GltfMeshBase>();
+    }
+  }
+
   // Disposable pattern, with Dispose(void) and Dispose(bool), as recommended by:
   // https://docs.microsoft.com/en-us/dotnet/api/system.idisposable
   protected override void Dispose(bool disposing) {

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

@@ -155,6 +155,7 @@ public abstract class GltfRootBase : IDisposable {
   public abstract IEnumerable<GltfImageBase> Images { get; }
   public abstract IEnumerable<GltfTextureBase> Textures { get; }
   public abstract IEnumerable<GltfMaterialBase> Materials { get; }
+  public abstract IEnumerable<GltfMeshBase> Meshes { get; }
 
   public abstract void Dereference(IUriLoader uriLoader = null, PolyFormat gltfFormat = null);
 

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

@@ -42,6 +42,10 @@ public static class ImportGltf {
   /// </summary>
   private const float MIN_VALID_NORMAL_SQRMAGNITUDE = float.Epsilon;  // Only fix normals that are exactly the 0 vector.
 
+  // Attributes intended only for the Tilt Brush Toolkit are prefixed with this, eg
+  // "_TB_UNITY_TEXCOORD_0". This distinguishes them a true TEXCOORD_0 that matches gltf's semantics
+  private const string kTookitAttributePrefix = "_TB_UNITY_";
+
   private static readonly JsonSerializer kSerializer = new JsonSerializer {
     ContractResolver = new GltfJsonContractResolver()
   };
@@ -166,6 +170,19 @@ public static ImportState BeginImport(
       var root = DeserializeGltfRoot(gltfVersion, reader);
       root.Dereference(uriLoader);
 
+      // Convert attribute names to the ones we expect.
+      // This may eg overwrite TEXCOORD_0 with _TB_UNITY_TEXCOORD_0 (which will contain more data)
+      foreach (var mesh in root.Meshes) {
+        foreach (var prim in mesh.Primitives) {
+          string[] attrs = prim.GetAttributeNames()
+              .Where(n => n.StartsWith(kTookitAttributePrefix)).ToArray();
+          foreach (var tbAttr in attrs) {
+            string renamed = tbAttr.Substring(kTookitAttributePrefix.Length);
+            prim.ReplaceAttribute(tbAttr, renamed);
+          }
+        }
+      }
+
       // Extract Google-specific information.
       if (root.asset != null) {
         // Get the generator information so we can tell which app and version generated this file.