GLTFLoader : Failure of combination Draco compression and multiple primitives

I’m sorry for sentences that are difficult to understand by automatic translation.

Hi, I am developing glTF Importer/Exporter in C++.

With Draco compression and some combinations, I found two problems in GLTFLoader.js.

I created a sample glb file for confirmation (Using my exporter). I confirmed it with three.js(r97) and glTF Viewer ( https://gltf-viewer.donmccurdy.com/ ).

Case 1 : Draco compression + Share vertices data

Sample glb file : https://ft-lab.github.io/gltf/apple/apple_cut_vert_share_draco.glb

The following conditions are specified.

• Draco compression
• Multiple primitives in one mesh
• Share vertices data (position/normal/uv) of each primitive with a BufferView

However, in this case, Indices + all the vertices data are stored in Draco compressed BuferView, so waste increases ,

Result : accessors is null. I confirmed it with glTF Viewer. Reference : https://ft-lab.github.io/gltf/apple/apple_cut_vert_share.glb (Draco compression unused glb file)

Cause place : js/loaders/GLTFLoader.js

When isMultiPass = true in GLTFParser.prototype.loadGeometries, The number of array elements of primitives is 1. In the case of Draco compression, I think that an error occurs because isMultiPass = true processing is not included.

Case 2 : Draco compression + Morph Targets

Sample glb file : https://ft-lab.github.io/gltf/MorphTargets/multiplePrimitives_draco.glb

The following conditions are specified.

• Draco compression
• Multiple primitives in one mesh
• Morph Targets

Result : If the weight value of Morph Targets is not 0.0, some Primitives disappear. Reference : https://ft-lab.github.io/gltf/MorphTargets/multiplePrimitives.glb (Draco compression unused glb file)

Cause place : js/loaders/GLTFLoader.js

Failure with drawElements. In GLTFParser.prototype.loadGeometries, In “if ( primitive.extensions && primitive.extensions[ EXTENSIONS.KHR_DRACO_MESH_COMPRESSION ] )”, I think that asynchronous processing in “var geometryPromise … then”, the primitive is not passed correctly.

It worked fine as follows.

In decodePrimitive function,

resolve( geometry );

–>

resolve( {'geometry' : geometry, 'primitive' : primitive} );

In GLTFParser.prototype.loadGeometries function,

var geometryPromise = extensions[ EXTENSIONS.KHR_DRACO_MESH_COMPRESSION ]
    .decodePrimitive( primitive, parser )
    .then( function ( geometry ) {
      addPrimitiveAttributes( geometry, primitive, accessors );
      return geometry;
  } );

–>

var geometryPromise = extensions[ EXTENSIONS.KHR_DRACO_MESH_COMPRESSION ]
  .decodePrimitive( primitive, parser )
  .then( function ( data ) {
      addPrimitiveAttributes( data['geometry'], data['primitive'], accessors );
      return data['geometry'];
  } );

Three.js version

• Dev
• r97
• …

Browser

• All of them
• Chrome
• Firefox
• Internet Explorer

OS

• All of them
• Windows
• macOS
• Linux
• Android
• iOS

Hardware Requirements (graphics card, VR Device, …)

[donmccurdy]

You’re right, it was a bug. I’ll have this fixed soon. 😃

[ft-lab]

I did not upload the sample here, but I’ll write it for reference. When using Draco Since I store the triangle indices + vertices data with one Draco’s bitstream, I think that waste may increase when compressing Draco.

When I verified with the specification of Draco, when sharing vertices data with multiple primitives, All the vertices data had to be copied to Draco’s bitstream.

I’m sorry if I misunderstood the specification.