Supporting generic integer attributes

[haralwit]

I'm using meshoptimizer trough gltfpack with GLTF point clouds in a 3DTiles 1.1 format. The optimizer drastically improves the size and unpacking speed, but some key integer fields, most noticebaly Classification and Intensity are stripped. Classification is a uint8 and intensity uint16 which get stripped out during compression. LAS specification
I saw the discussion in #529 about not supporting arbitrary custom data, but is it possible to add support for a generic integer attributes so fields like this can be kept?

[zeux]

Current logic preserves attributes named _ID, _BATCH_ID or _FEATURE_ID_n, as they are known to be integers semantically. I'm assuming the attributes you are using aren't named that? What's their accessor type in the glTF file?

[haralwit]

The attributes are named _CLASSIFICATION and _INTENSITY and the accessor type for these attributes is scalar.

I was wary of the _ID style attributes if some internal logic could assume them to be unique. If they are not unique, a possible workaround might be to remap the attributes to the _ID and _BATCH_ID fields.

The attributes are included using the EXT_structural_metadata extension but this is mostly a carryover from the converter used as the attributes are simple integer attributes.

This is from the glTF file

{
  "EXT_structural_metadata": {
    "schema": {
      "id": "pts_schema",
      "name": "pts_schema",
      "description": "point cloud point attribute schema",
      "version": "1.0.0",
      "classes": {
        "point": {
          "name": "point",
          "description": "Properties of point cloud points",
          "properties": {
            "INTENSITY": {
              "description": "Laser intensity",
              "type": "SCALAR",
              "componentType": "UINT16",
              "required": true
            },
            "CLASSIFICATION": {
              "description": "Point classification",
              "type": "SCALAR",
              "componentType": "UINT16",
              "required": true
            }
          }
        }
      }
    },

...


    "meshes": [
      {
        "name": "PointCloud",
        "primitives": [
          {
            "extensions": {
              "EXT_structural_metadata": {
                "propertyAttributes": [0]
              }
            },
            "attributes": {
              "COLOR_0": 1,
              "POSITION": 0,
              "_CLASSIFICATION": 3,
              "_INTENSITY": 2
            },
            "mode": 0
          }
        ]
      }
    ]
  }
}

[zeux]

You should be able to rename to _FEATURE_ID_CLASSIFICATION (or any other suffix) which carries no semantic meaning.