SceneGraph ​

The nodes in a Sketchfab scene are organized in a hierarchy. This hierarchy is called the SceneGraph. The SceneGraph is a tree structure, where each node can have children and each child can have children, and so on. While the SceneGraph is harder than the NodeMap to work with, it's the best way to understand how a Sketchfab scene is organized.

The nodes in the scenegraph are exactly the same as the nodes from the nodemap. It's the structure that's different. The nodemap is a flat list of nodes. The scenegraph is a tree structure. But they use the exact same nodes.

Reading the scenegraph ​

You can get the scenegraph and print it to the console like so:

api.getSceneGraph(function (err, sceneGraph) {
  console.log(sceneGraph);
});

This is the scenegraph of the scene with the teapot and disc. The original 3D model (created in 3dsMax) contains two objects, called Teapot001 and Cylinder001. These are the hightlighted objects with the ID 3 and 22.

0: 34632ee1cb8f4ded8e8a1759bae0e2a3.fbx (MatrixTransform)
└─2: RootNode (MatrixTransform)
  ├─3: Teapot001 (MatrixTransform)
  │ └─4: Teapot001 (Group)
  │   └─5: Teapot001_Pot_0 (Geometry)
  └─22: Cylinder001 (MatrixTransform)
    └─23: Cylinder001 (Group)
      └─24: Cylinder001_Disk_0 (Geometry)

Root node ​

The file that's uploaded to Sketchfab is an .fbx in this case. When exporting from 3dsMax to .fbx, a RootNode is added. This is a node that contains the entire scene. Sometimes an extra node is added with a random ID. This depends on the file format. Some other file formats seemingly don't have a rootnode. This happens with files uploaded from some scan apps.

Geometry node ​

The fbx exporter splits each object into two nodes: a MatrixTransform and a Geometry. The name of the Geometry node is a combination of the object name and the material name. In my scene, the material of the Teapot is called Pot and the material of the disc is called Disk.

Group nodes ​

A Group node is inserted between the MatrixTransform and the Geometry node. This is a node that can contain multiple nodes. In this case, the Group node contains a single node, the Geometry node. It's a result of the way the fbx exporter works. If an object has a single material, the Group node contains a single Geometry node. If an object has multiple materials, the Group node contains multiple Geometry nodes, one for each material.

Here's an example of a scene with a single object that has multiple materials.