FB_geometry_metadata

Contributors

• Pär Winzell, Facebook, @zellski
• Andrew Prasetyo Jo, Facebook

Status

Draft

Dependencies

Written against the glTF 2.0 spec.

Overview

This extension annotates glTF scene objects with a summary of the cumulative geometric complexity and scene-space extents of the scene’s associated scene graph. Each recursively referenced mesh is enumerated for total vertex and primitive count, and each individual vertex is transformed into scene-space for a perfectly computed bounding box.

Some proven real-world applications for this information:

• When a viewer needs to make a quick, high-level decision which scene to display. This is perhaps most meaningful e.g. when scenes are used to represent different variants (e.g. LoD levels) of a model.
• When server-side systems need to make decisions based on geometric complexity, and don’t want to have to include their own complex matrix/vector code.

Specifying Geometry Data

The summary data is defined as an optional extension to a glTF scene, by adding an extensions property and defining a FB_geometry_metadata property with the scene metadata inside it:

"extensions": {
    "FB_geometry_metadata" : {
        "vertexCount": 28727,
        "primitiveCount": 15451,
        "sceneBounds": {
            "min": [
                -3.0651053919852,
                0,
                -2.6551087079312
            ],
            "max": [
                3.0651053919852,
                9.8512265772065,
                2.6538096091834
            ]
        }
    }
}

Limitations

All these values should be computed with disregard for animation: all morph targets are inactive, all node transforms are in their static form and unaffected by any animation. Consequently, an application that parses the model should interpret the values the same way.

Computing the data

Conceptually, computing and gathering this information is very straight-forward. On a hypothetical glTF node object:

  public function computeGeometryMetadata(
    GltfMat44f $world_transform,
    inout A3DGltfBounds $bounds,
    inout int $vertex_count,
    inout int $primitive_count,
  ): void {
    // compute world transform forward
    $world_transform = A3DGltfMath::multiplyMatrices(
      $world_transform,
      $this->computeLocalTransform(),
    );

    // if this node references a mesh, traverse it
    $this->mesh?->computeGeometryMetadata(
      $world_transform,
      inout $bounds,
      inout $vertex_count,
      inout $primitive_count,
    );

    // depth-first scene graph traversal
    foreach ($this->getChildren() as $child) {
      $child->computeGeometryMetadata(
        $world_transform,
        inout $bounds,
        inout $vertex_count,
        inout $primitive_count,
      );
    }
  }

and on the mesh primitive, simply:

  public function computeGeometryMetadata(
    GltfMat44f $world_transform,
    inout A3DGltfBounds $bounds,
    inout int $vertex_count,
    inout int $primitive_count,
  ): void {
    $vertex_count += $this->getVertexCount();
    $primitive_count += $this->getPrimitiveCount();

    // this is very expensive, and will one day move to native code
    foreach ($this->positions as $position) {
      $bounds->expandByVec(
        A3DGltfMath::transformVec3($world_transform, $position),
      );
    }
  }