Universal Scene Description

Exporting to USD Files

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.

The following objects can be exported to USD:

• Meshes (of different kinds, see below).

• Cameras (perspective cameras only at the moment, not orthogonal ones).

• Light (all types except area lights).

• Hair (exported as curves, and limited to parent strands).

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.

Shot from Spring exported to USD and opened in USDView.

Export Options

The following options are available when exporting to USD:

Selection Only

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.

Visible Only

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

애니메이션

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

Hair

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

UV Maps

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.

Normals

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

Materials

When checked, exports the viewport materials of meshes. 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.

Use Settings for

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.

Experimental

Instancing

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.

Exporter Limitations

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.

Mesh Normals

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.

Vertex Velocities

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.

Coordinate System Orientation

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.

Materials

Very simple versions of the materials are exported, using only the Viewport Display 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.

Hair

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

카메라

Only perspective cameras are exported.

Lights

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

Particles

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.

Instancing/Referencing

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.

Importing USD Files

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:

• Cameras

• 곡선

• Lights

• Materials

• 메쉬(Meshes)

• Volume

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.

Animations

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 mesh vertex colors and UVs. Note that USD file sequences (i.e. a unique file per frame) are not yet supported.

Materials

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 Viewport Display 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.

Coordinate System Orientation

If the imported USD is Y up, a rotation will be automatically applied to root objects to convert to Blender’s Z up orientation.

Import Options

The following options are available when importing from USD:

Cameras

Import cameras (perspective and orthographic).

곡선

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

Lights

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

Materials

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

메쉬(Meshes)

Import meshes.

Volumes

Import USD OpenVDB field assets.

Path Mask

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

축척(Scale)

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

UV Coordinates

Read mesh UV coordinates.

Vertex Colors

Convert the USD mesh displayColor values to Blender mesh vertex colors.

Subdivision

Create Subdivision Surface modifiers based on the USD SubdivisionScheme attribute.

Import Instance Proxies

Create unique Blender objects for USD instances.

Visible Primitives Only

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

Guide

Include primitives with purpose guide.

Proxy

Include primitives with purpose proxy.

Render

Include primitives with purpose render.

Set Frame Range

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

Relative Path

Select the file relative to the blend-file.

Create Collection

Add all imported objects to a new collection.

Light Intensity Scale

Scale for the intensity of imported lights.

Experimental

Import USD Preview

Convert USD Preview Surface shaders to Principled BSDF shader networks.

Set Material Blend

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.