3D Tiles styling loose ends

[lilleyse]

3D Tiles styling was added in #558, #559, and #573 but there were a few things that were left out to reduce scope. Note that this only applies to 3D Tiles styling, which is implemented in MDL, and doesn't necessarily apply to metadata picking #154 .

Missing support

• Support for matrix types
  • Property attributes: may need to be packed in multiple primvars (one per row)
  • Property tables: may need to be packed in multiple texels (one per row)
  • Property textures: N/A
• Support for fixed-length arrays
  • SCALAR with count <= 4 - treat as regular scalar / vector types
  • SCALAR with count > 4 - more complicated
  • Vector and matrix types - more complicated
• Support for UINT32, UINT64, INT64 property table properties: Add support for UINT32, UINT64, INT64 property table properties #614
• Support for BOOLEAN: Add support for boolean property table properties #613
• Support for ENUM: Support metadata ENUMs cesium-native#728
• Support for property texture properties that are packed across multiple texture channels
  • May be able to use int_bits_to_float and/or bit shifting math
• Support for schemaUri: Support external metadata schemas cesium-native#727
• Support for class properties that have a default value but aren't included in property attribute/texture/table

Unlikely to be supported

• Support for variable-length arrays
• Support for STRING
• Support for 16 or 32-bit property textures. Unlikely to see this in practice.
• Support for property texture properties with more than four channels

Features

• Metadata discovery - see what properties are available for styling like in Cesium for Unreal
• Metadata encoding options - Cesium for Unreal has a mechanism for coercing values from one type to another, including basic conversions like float64 -> float32 and also string -> numeric conversions. This functionality was recently ported over to cesium-native: Add conversion logic to cesium-native cesium-native#756 and could be used in omniverse.
• Make min/max properties available as material graph nodes. Useful for creating color ramps.
• Make statistics available as material graph nodes. Useful for creating color ramps.
• Make featureCount available as material graph node
• Handle property ID naming collisions

Precision

• Potential narrowing issues when unpacking uint32 features ids that are stored across multiple texture channels into an int. Unfortunately MDL doesn't have a uint type. See unpack_channels in cesium.mdl.
• Property table precision issues - 64-bit floats, 32 and 64-bit integers lose precision when encoded as floats

Performance

• Pack multiple properties in the same property table texture to reduce the number of bound textures.
• Create texture resources on demand