Universal Scene Description

Exportación a archivos USD

Universal Scene Description (USD) files can contain complex layering, overriding, and references to other files. Blender’s USD Exporter takes a much simpler approach. When exporting, all visible, supported objects in the scene are exported, optionally limited by their selection state. Blender does not (yet) support exporting invisible objects, USD layers, variants, skeletal animation, etc.

Los siguientes tipos de objetos podrán ser exportados a USD:

  • Mallas (de diferentes tipos, ver debajo).

  • Cámaras (por el momento sólo cámaras en perspectiva, no las ortogonales).

  • Luces (todos los tipos, excepto las de área).

  • Pelo (exportado como curvas y limitado a las hebras primarias).

  • Volúmenes (tanto estáticos como animados).

When exporting an animation, the final, evaluated mesh is written to USD. This means that the following meshes can be exported:

  • Static meshes.

  • Deforming meshes; here the topology of the mesh does not change, but the locations of the vertices change over time. Examples are animated characters or bouncing (but not cracking) objects.

  • Arbitrarily animated meshes; here the topology does change. An example is the result of a fluid simulation, where splashes of fluid can break off the main body.

  • Metaballs are exported as animated meshes.

../../_images/files_import-export_usd_example.png

Shot from Spring exported to USD and opened in USDView.

Opciones de exportación

The following options are available when exporting to USD:

Sólo lo seleccionado

When checked, only selected objects are exported. Instanced objects, for example collections that are instanced in the scene, are considered “selected” when their instancer is selected.

Sólo objetos visibles

Only exports objects that are not hidden. Invisible parents of exported objects are exported as empty transforms.

Animación

When checked, the entire scene frame range is exported. When unchecked, only the current scene frame is exported.

Pelo

When checked, parent hair strands are exported as a curve system. Hair strand colors are not exported.

Mapas UV

When checked, includes UV coordinates for exported meshes. The name of the UV map in USD is the same as the name in Blender. In USD the default name is st whereas in Blender the default name is UVMap. To export to the standard UV map name st, rename the UV map in Blender to st.

Normales

When checked, includes normals for exported meshes. This includes custom loop normals.

Materiales

Exports material information of the object. By default the exporter approximates the BSDF Principista node tree by converting it to USD’s Preview Surface format. If To USD Preview Surface is disabled, the material is set to the viewport materials of meshes.

Additional material properties are set in the Material grouping of options.

When a mesh has multiple materials assigned, a geometry subset is created for each material. The first material (if any) is always applied to the mesh itself as well (regardless of the existence of geometry subsets), because the Hydra viewport does not support materials on subsets. See USD issue #542 for more information.

Usar opciones de

Determines the whether to use Viewport or Render visibility of collection, modifiers, or any other property that can be set for both the Viewport and Render.

Materiales

Additional options when Materials are enabled for export.

To USD Preview Surface

When exporting materials, approximate a BSDF Principista node tree to by converting it to USD’s Preview Surface format. If disabled, the material is set to the viewport materials of meshes.

Advertencia

Not all nodes are supported; currently only Diffuse, Principle, Image Textures, and UVMap nodes are support.

Exportar texturas

Export textures referenced by shader nodes to a «textures» folder which in the same directory as the USD file.

Sobrescribir texturas

Allow overwriting existing texture files when exporting textures.

Archivos referenciados

Rutas relativas

Use relative paths to reference external files (i.e. textures, volumes) in the exported USD file, otherwise use absolute paths.

Experimental

Instanciado

As this is an experimental option. When unchecked, duplicated objects are exported as real objects, so a particle system with 100 particles that is displayed with 100 meshes will have 100 individual meshes in the exported file. When checked, duplicated objects are exported as a reference to the original object. If the original object is not part of the export, the first duplicate is exported as real object and used as reference.

Limitaciones del exportador

Single-sided and Double-sided Meshes

USD seems to support neither per-material nor per-face-group double-sidedness, so Blender uses the flag from the first material to mark the entire mesh as single/double-sided. If there is no material it defaults to double-sided.

Normales de mallas

The mesh subdivision scheme in USD is “Catmull-Clark” by default, but Blender uses “None” instead, indicating that a polygonal mesh is exported. This is necessary for USD to understand the custom normals; otherwise the mesh is always rendered smooth.

Velocidades de vértices

Currently only fluid simulations (not meshes in general) have explicit vertex velocities. This is the most important case for exporting velocities, though, as the baked mesh changes topology all the time, and thus computing the velocities at import time in a post-processing step is hard.

Orientación del sistema de coordenadas

Blender uses the Z axis as up axis. Since USD supports both Y up and Z up, the USD files written by Blender always use Z up.

Materiales

Very simple versions of the materials are exported, using only the Presentación en vistas color, metallic, and roughness.

When there are multiple materials, the mesh faces are stored as geometry subset and each material is assigned to the appropriate subset. If there is only one material this is skipped. Note that the geometry subsets are not time-sampled, so it may break when an animated mesh changes topology.

Pelo

Only the parent strands are exported, and only with a constant color. No UV coordinates, and no information about the normals.

Cámara

Only perspective cameras are exported.

Luces

USD does not directly support spot lights, so those are not exported.

Partículas

Particles are only written when they are alive, which means that they are always visible. There is currently no code that deals with marking them as invisible outside their lifespan.

Objects instanced by particle system are exported by suffixing the object name with the particle’s persistent ID, giving each particle transform a unique name.

Instanciado y referenciamiento

This is still an experimental feature that can be enabled when exporting to USD. When enabled, instanced object meshes are written to USD as references to the original mesh. The first copy of the mesh is written for real, and the following copies are referencing the first. Which mesh is considered “the first” is chosen more or less arbitrarily.

Importación de archivos USD

USD files typically represent the scene as a hierarchy of primitives, or prims. Individual prims contain data to describe scene entities, such as geometry, lights, cameras and transform hierarchies. Blender’s USD importer converts USD prims to a hierarchy of Blender objects. Like the USD exporter, the importer does not yet handle more advanced USD concepts, such as layers and references.

The following USD data types can be imported as Blender objects:

  • Cámaras

  • Curvas

  • Luces

  • Materiales

  • Mallas

  • Volumen

For more information on how the various data types are handled, see the following descriptions of the Import Options.

Xform and Scope Primitives

USD provides an Xform prim type, containing transform data, which can be used to represent transform hierarchies and to organize the scene. Such Xform prims are imported as Blender empty objects.

USD also supports Scope primitives, which are entities that do not contain transform data, but which serve to group other element of the scene. Blender doesn’t have an exact counterpart to the concept of a scope, so such primitives are imported as Blender empties located at the origin. This is an imperfect representation, because empty objects have a transform and Scopes do not, but this approach nonetheless helps preserve the structure of the scene hierarchy.

Animaciones

The importer supports two types of animation:

  • Animating transforms: If a USD primitive has time-varying transform data, a Transform Cache constraint will be added to the imported Blender object.

  • Animating geometry: Animating mesh and curve geometry is supported by adding a Mesh Sequence Cache modifier to the imported data. Geometry attribute (USD Primvar) animation is currently supported only for Color Attributes and UVs. Note that USD file sequences (i.e. a unique file per frame) are not yet supported.

Materiales

If a USD mesh or geometry subset has a bound material, the importer will assign to the Blender object a material with the same name as the USD material. If a Blender material with the same name already exists in the scene, the existing material will be assigned. Otherwise, a new material will be created.

If the USD material has a USD Preview Surface shader source, the Presentación en vistas color, metallic, and roughness are set to the corresponding USD Preview Surface input values.

There is also an experimental Import USD Preview option to convert USD Preview Surface shaders to Blender Principled BSDF shader nodes. This option can be lossy, as it does not yet handle converting all shader settings and types, but it can generate approximate visualizations of the materials.

Orientación del sistema de coordenadas

En caso de que el archivo USD importado utilice el eje Y hacia arriba, se aplicará una rotación automática a los objetos de primer nivel para convertirlos a la orientación usada por Blender (con el eje Z hacia arriba).

Opciones de importación

The following options are available when importing from USD:

Cámaras

Importará cámaras (perspectiva y ortogonal).

Curvas

Import curve primitives, including USD basis and NURBS curves. (Note that support for Bézier basis is not yet fully implemented.)

Luces

Import lights. Does not currently include USD dome, cylinder or geometry lights.

Materiales

Import materials. See also the experimental Import USD Preview option.

Mallas

Importará mallas.

Volúmenes

Import USD OpenVDB field assets.

Máscara de ruta

Import only the subset of the USD scene rooted at the given primitive.

Escala

Value by which to scale the imported objects in relation to the world’s origin.

Coordenadas UV

Read mesh UV coordinates.

Atributos de color

Convert the USD mesh displayColor values to Blender’s Color Attributes.

Subdivisión

Create Subdivision Surface modifiers based on the USD SubdivisionScheme attribute.

Import Instance Proxies

Create unique Blender objects for USD instances.

Sólo primitivas visibles

Do not import invisible USD primitives. Only applies to primitives with a non-animated visibility attribute. Primitives with animated visibility will always be imported.

Guía

Include primitives with purpose guide.

Reemplazo

Include primitives with purpose proxy.

Procesar

Include primitives with purpose render.

Set Frame Range

Update the scene’s start and end frame to match those of the USD stage.

Rutas relativas

Select the file relative to the blend-file.

Crear colección

Add all imported objects to a new collection.

Escala de intensidad de luces

Permitirá escalar la intensidad de las luces importadas.

Colisión de nombres de materiales

Behavior when the name of an imported material conflicts with an existing material.

Hacer único

Import each USD material as a unique Blender material.

Referenciar existente

If a material with the same name already exists, reference that instead of importing.

Experimental

Importar previsualización USD

Convert USD Preview Surface shaders to Principled BSDF shader networks.

Definir fundido de material

If the Import USD Preview option is enabled, the material blend method will automatically be set based on the opacity and opacityThreshold shader inputs, allowing for visualization of transparent objects.