Overview

glTF (derivative short form of Graphics Language Transmission Format or GL Transmission Format) is a standard file format for three-dimensional scenes and models.

An OpenStudio model can be ForwardTranslated to a glTF file for external viewing such as in-browser via WebGL. OpenStudio uses the glTF standard 2.0. Internally it uses the tinygltf library to write the glTF file.

The following document aims to explains briefly how glTF and supporting OpenStudio class are laid out.

The following classes form the foundation of the OpenStudio Gltf functionality:

The GltfForwardTranslator class is the orchestrator. The other classes are interface classes between the OpenStudio Model and the gltf Model. Their data members are there to represent the data that will eventually be stored in the gltf Model.

They have one or several constructors that take in OpenStudio Model or ModelObject to populate the data members.
They have a function that will export to a tinygltf::Value::Object
They also have a constructor that does the reverse: populating the data members by reading a tinygltf::Value for inspection for eg

Geometry: PlanarSurfaces <=> glTF Node

The geometry part of the glTF represents the surfaces of the OpenStudio Model.

OpenStudio Model PlanarSurfaces (such as Surface, SubSurface, ShadingSurface, etc) are glTF Nodes. The Surface Vertices are packed in a base64 encoded buffer that the glTF Node will reference.

The class GltfUserData is used to create node extras: we place some attribute on these nodes, such as the surface type, its construction, the Space and SpaceType and ThermalZone it belongs to. External viewers can use that information for displaying attributes or filtering.

A glTF Material is also attached to these gltf Nodes, with the help of the GltfMaterialData class, which interfaces between an OpenStudio RenderingColor and a tinygltf::Material

Scene MetaData

A glTF Scene can also bear extras. We use the class GltfMetaData to do so. GltfMetaData holds:

An array of GltfModelObjectMetaData
a GltfBoundingBox

GltfMetadata has a constructor that takes an OpenStudio Model. For there, it sets a few members like the northAxis, and will dispatch ModelObjects to its supporting classes:

PlanarSurfaceGroups are dispatched to the GltfBoundingBox to calculate the glTF boundingbox
ThermalZone, SpaceType, AirLoopHVAC, BuildingStory, BuildingUnit, DefaultConstructionSet objects are sent to the GltfModelObjectMetaData class to store attributes such as the name, handle, iddObjectType, multiplier, etc

It has a function that exports to a tinygltf::Value::Object and it has a constructor that can do the reverse: read from a tinygltf::Value. It dispatches to its supporting classes which are able to the do the same thing.