This sample demonstrates usage of the EXT_structural_metadata extension for storing a property table with metadata that is looked up based on different feature IDs.
The sample contains a glTF asset with a single mesh primitive. The mesh primitive consists of 4 quads (each formed by 2 triangles). The vertices in this mesh primitive have the usual POSITION and NORMAL vertex attributes.
The sample uses the EXT_mesh_features extension for storing feature IDs that are associated with vertices of a mesh. Each vertex has two feature IDs. The first feature ID set is the same as for the FeatureIdAttribute example. The second feature ID set simply contains the feature IDs in reverse order for demonstration purposes. So the vertex indices and their feature IDs are listed in this table:
| Vertex Index | Feature ID 0 | Feature ID 1 |
|---|---|---|
| 0 | 0 | 3 |
| 1 | 0 | 3 |
| 2 | 0 | 3 |
| 3 | 0 | 3 |
| 4 | 1 | 2 |
| 5 | 1 | 2 |
| 6 | 1 | 2 |
| 7 | 1 | 2 |
| 8 | 2 | 1 |
| 9 | 2 | 1 |
| 10 | 2 | 1 |
| 11 | 2 | 1 |
| 12 | 3 | 0 |
| 13 | 3 | 0 |
| 14 | 3 | 0 |
| 15 | 3 | 0 |
Additionally, this sample defines metadata that is associated with the feature IDs.
The structure of the metadata is defined with an EXT_structural_metadata schema that contains a single class. The class contains two properties. The first one called example_VEC3_FLOAT32, and is a 3D vector with 32 bit floating-point components. The second one is called example_STRING, and represents a single string.
The metadata entities are the actual instances of this class. They are defined with an EXT_structural_metadata property table. The rows of this table correspond to the feature IDs. The columns of this table correspond to the properties of the class. The data for each column is stored in a standard glTF bufferView.
Based on the type information from the metadata class, the contents of this buffer view is interpreted as four 3D vectors with 32 bit floating point components, and four strings, respectively:
"example_VEC3_FLOAT32" : [
0.0, 0.1, 0.2,
1.0, 1.1, 1.2,
2.0, 2.1, 2.2,
3.0, 3.1, 3.2
],
"example_STRING" : [
"Rain 🌧",
"Thunder ⛈",
"Sun ☀",
"Snow 🌨"
]
This example can be viewed with the common sandcastle code.