# Blender USD Importer Design

classDiagram
      USDPrimReader <|-- USDXformReader
      USDPrimReader: -Object* object_
      USDPrimReader: -UsdPrim prim_
      USDPrimReader: -USDPrimReader* parent_reader_
      USDPrimReader: +object() Object*
      USDPrimReader: +object(Object)
      USDPrimReader: +prim() UsdPrim
      USDPrimReader: +parent() USDPrimReader
      USDPrimReader: +parent(USDPrimReader*) 
      USDPrimReader: +valid() bool
      USDPrimReader: +create_object(bmain, time)
      USDPrimReader: +read_object_data(bmain, time)
      <<abstract>> USDPrimReader
      
      USDStageReader "0" --> "*" USDPrimReader : contains
      class USDStageReader{
          -UsdStageRefPtr stage_
          -vector<USDPrimReader*> readers_
          -map<SdfPath, vector<USDPrimReader*>> proto_readers_
          +valid() bool
          +collect_readers(bmain, params, settings)
          +readers()
          +proto_readers()
      }
      class USDXformReader{
          +create_object(bmain, time)
          +read_object_data(bmain, time)
          +read_matrix(mat[4][4], time, scale, is_constant)
      }
      class USDMaterialReader {
          -USDImportParams params_
          -Bmain *bmain_
          +add_material(usd_material) Material*
      }
      USDXformReader <|-- USDGeomReader
      class USDGeomReader{
          +create_object(bmain, time)
          +read_object_data(bmain, time)
          +read_mesh(existing_mesh, time, read_flag, vel_scale, err_str)
          +topology_changed() bool
          +add_cache_modifier()
          +add_subdiv_modifier()
      }
      USDXformReader <|-- USDLightReader
      class USDLightReader{
          +create_object(bmain, time)
          +read_object_data(bmain, time)
      }
      USDXformReader <|-- USDCameraReader
      class USDCameraReader{
          +create_object(bmain, time)
          +read_object_data(bmain, time)
      }
      USDXformReader <|-- USDVolumeReader
      class USDVolumeReader{
          -UsdVolVolume volume_
          +create_object(bmain, time)
          +read_object_data(bmain, time)
      }
      USDXformReader <|-- USDInstanceReader
      class USDInstanceReader{
          +valid()
          +create_object(bmain, time)
          +read_object_data(bmain, time)
          +set_instance_collection(collection)
          +proto_path() SdfPath
      }
      USDGeomReader <|-- USDMeshReader
      USDMaterialReader <-- USDMeshReader
      class USDMeshReader {
          +create_object(bmain, time)
          +read_object_data(bmain, time)
          +read_mesh(existing_mesh, time, read_flag, vel_scale, err_str)
          +topology_changed() bool
      }
      USDGeomReader <|-- USDCurvesReader
      class USDCurvesReader {
          +valid() bool
          +create_object(bmain, time)
          +read_object_data(bmain, time)
          +read_mesh(existing_mesh, time, read_flag, vel_scale, err_str)
      }
      USDGeomReader <|-- USDNurbsReader
      class USDNurbsReader {
          +valid() bool
          +create_object(bmain, time)
          +read_object_data(bmain, time)
          +read_mesh(existing_mesh, time, read_flag, vel_scale, err_str)
      }

      

## Reader Class Descriptions

### USDStageReader
Implements logic for iterating over the USD stage and creating the USDPrimReader instances for convering USD prims to the corresponding Blender object types. If the Instancing import option is enabled, this class also provides functionality for traversing the stage's "master" prims to generate readers for converting scenegraph instance prototypes.

### USDPrimReader
Abstract base class for all readers that create Blender objects from USD schemas. It wraps a single UsdPrim.

### USDXformReader
Derives from USDPrimReader. Base class for all readers that create Blender objects that can compute a transform and create a transform cache constraint. By default, this reader creates a Blender Empty object. It typically wraps UsdGeomXformable schema prims. However, it may also wrap UsdGeomScope prims, in which case the matrix computation returns the identity. The reason for this exception was a design decision to represent USD scopes as Blender empties for organizing the scene, since Blender doesn't otherwise have an object type directly analogous to UsdGeomScope.

### USDGeomReader
Derives from USDXformReader. Base class for readers that can create a mesh and apply cache and subdiv modifiers.

### USDMeshReader
Derives from USDGeomReader. Wraps the UsdGeomMesh schema. Creates a Blender Mesh object.

### USDCurvesReader
Derives from USDGeomReader. Wraps the UsdGeomBasisCurves schema. Creates a Blender Curve object.

### USDNurbsReader
Derives from USDGeomReader. Wraps the UsdGeomNurbsCurves schema. Creates a Blender Curve object.

### USDInstanceReader
Derives from USDXformReader. Wraps a USD prim that is an instance (typically a UsdGeomXformable). Creates a Blender object that stores a collection instance.

### USDCameraReader
Derives from USDXformReader. Wraps the UsdGeomCamera schema. Creates a Blender Camera object.

### USDLightReader
Derives from USDXformReader. Wraps the UsdLuxLight schema. Creates a Blender Light object.

### USDVolumeReader
Derives from USDXformReader. Wraps the UsdVolVolumes schema. Creates a Blender Volume object.

### USDMaterialReader
Stnadalone utility class for converting UsdShadeMaterial prims to Blender Materials. Currently used in USDMeshReader and contains experimental code for converting USD Preview Surface shaders to Blender Principled BSDF shader networks.