Adds 3DTILES_content_gltf extension

[lilleyse]

Fixes #385

Direct Link	Draft Version	Changes
3DTILES_content_gltf	0.0.0	Initial draft

This extension allows a tileset to use glTF 2.0 and GLB models directly as tile content.

Using glTF directly simplifies runtime implementations and content pipelines that already support glTF but don't support 3D Tiles native formats. glTF models may be extended with instancing, feature metadata, and compression extensions to achieve near feature parity with the existing 3D Tiles formats.

The example tileset can be loaded on the feature-metadata branch in CesiumJS.

[lilleyse]

Format Comparison

The sections below describe the differences between native 3D Tiles formats and glTF.

Batched 3D Model (b3dm)

Since b3dm is a thin wrapper around binary glTF there aren't too many differences when using glTF directly.

• RTC_CENTER may be represented as a glTF node translation.
• Batch Table and Batch IDs may be stored with EXT_feature_metadata.

Instanced 3D Model (i3dm)

i3dm instances a glTF model (embedded or external) and provides per-instance transforms and feature IDs.

• EXT_mesh_gpu_instancing may be used to instance glTF nodes.
• RTC_CENTER may be represented as a glTF node translation.
• Batch Table and Batch IDs may be stored with EXT_feature_metadata. Per-instance properties are supported.
• No glTF support for compressed per-instance translations and rotations, uniform scale, multi-instance properties, and EAST_NORTH_UP.
• Longer term a 3D Tiles vector tile format may instance models as point features.

Point Cloud (pnts)

glTF natively supports point clouds with the primitive mode 0 (POINT). Concepts in the pnts format map well to glTF.

• Feature table properties like like POSITION, COLOR, and NORMAL may be stored as glTF attributes.
• EXT_meshopt_compression and KHR_mesh_quantization may be used for point cloud compression. POSITION_QUANTIZED, NORMAL_OCT16P, RGB565, and 3DTILES_draco_point_compression are not directly supported in glTF.
• RTC_CENTER may be represented as a glTF node translation.
• Batch Table and Batch IDs may be stored with EXT_feature_metadata. The extension supports both per-point properties and multi-point properties in the same glTF.
• No glTF support for CONSTANT_RGBA.

Composite (cmpt)

All inner contents may be bundled into the same glTF as separate nodes, meshes, or primitives, up to the tileset author's discretion. EXT_feature_metadata supports multiple feature collections.

[pjcozzi]

@lilleyse please LMK when this is ready. for me to review. Should anyone else review first?

[lilleyse]

A discussion about optional vs. required in a 3d-tiles-validator PR:

3DTILES_content_gltf says it needs to go in extensionsRequired but now I'm thinking about the interaction with 3DTILES_layers and how someone might create a tileset with both b3dm and glTF layers so that the b3dm layer can be a fallback if the client doesn't support 3DTILES_content_gltf. That's a possible use case for allowing 3DTILES_content_gltf to be optional.

[sanjeetsuhag]

Spec looks good. I noticed some problems with the example in the 3d-tiles-next-experimental branch. This issue was fixed.

[sanjeetsuhag]

Can this be removed?

[lilleyse]

I think we should keep it until these extensions get merged into master

[sanjeetsuhag]

Looks good for v.0.0.0. Thanks @lilleyse