Universal Scene Description

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

• Curves

• Lights

• Materials

• Meshes

• Primitive Shapes

• Volume

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

ملاحظة

When importing a USDZ archive, it is important to carefully consider the Import Textures option to determine whether and how to copy texture files from the zip archive.

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.

PointInstancer Primitives

USD provides a UsdGeomPointInstancer prim type, containing instances that are scattered on a primitive's points.

These are imported into Blender as Point Clouds using a Geometry Nodes Modifier and the Instance on Points Node.

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 Color Attributes 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 may be used, depending on the Material Name Collision option. 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 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:

General

Path Mask

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

Include

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.

Defined Primitives Only

When disabled this allows importing USD primitives which are not defined, such as those with an override specifier.

Set Frame Range

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

Create Collection

Add all imported objects to a new collection.

Relative Path

Select the file relative to the blend-file.

Scale

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

Light Intensity Scale

Scale for the intensity of imported lights.

Custom Properties

Behavior when importing USD attributes as Custom Properties.

None:

Does not import USD custom attributes.

User:

Imports USD attributes in the userProperties namespace as custom properties. The namespace will be stripped from the property names.

All Custom:

Imports all USD custom attributes as custom properties. Namespaces will be retained in the property names.

Object Types

Cameras

Import cameras (perspective and orthographic).

Curves

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.

Meshes

Import meshes.

Volumes

Import USD OpenVDB field assets.

Point Clouds

Imports USD UsdGeomPoints as a Point Cloud object.

USD Shapes

Imports USD primitive shapes (cubes, spheres, cones, ect) as Blender meshes.

Display Purpose

Render

Include primitives with purpose render.

Proxy

Include primitives with purpose proxy.

Guide

Include primitives with purpose guide.

Material Purpose

Attempt to import materials with the given purpose. If no material with this purpose is bound to the primitive, fall back on loading any other bound material.

All Purpose:

Attempt to import allPurpose materials.

Preview:

Attempt to import preview materials. Load allPurpose materials as a fallback.

Full:

Attempt to import full materials. Load allPurpose or preview materials, in that order, as a fallback".

Geometry

UV Coordinates

Read mesh UV coordinates.

Color Attributes

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

Mesh Attributes

Read USD Primvars as mesh attributes.

Subdivision

Create Subdivision Surface modifiers based on the USD SubdivisionScheme attribute.

Validate Meshes

Check the imported mesh for corrupt data and fix it if necessary. When disabled, erroneous data may cause crashes displaying or editing the meshes. This option will make the importing slower but is recommended, as data errors are not always obvious.

Rigging

Shape Keys

Imports USD blend shapes as Blender's Shape Keys.

Armatures

Imports USD skeletons as Blender's Armatures.

Materials

Import All Materials

Also import materials that are not used by any geometry. Note, when this option is false, materials referenced by geometry will still be imported.

Import USD Preview

Convert USD Preview Surface shaders to Principled BSDF shader networks.

Create World Material

Converts the first discovered USD dome light to a world background shader.

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.

Material Name Collision

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

Make Unique:

Import each USD material as a unique Blender material.

Reference Existing:

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

Textures

When importing a USDZ package, the following options specify whether and how texture asset dependencies of the USD should be copied from the zip archive so they can be loaded into Blender.

Import Textures

Behavior when importing textures from a USDZ archive.

None:

Don't import textures. Note that, with this option, material textures may fail to be resolved in Blender.

Packed:

Import textures as packed data in the Blender file.

Copy:

Copy files to the directory specified in the Textures Directory option.

Textures Directory

Path to the directory where imported textures will be copied, when the Import Textures mode is Copy.

Note that the default textures directory is the relative path //textures, which requires the Blender file to have been saved before importing, so the relative path can be resolved.

File Name Collision

Behavior when the name of an imported texture file conflicts with an existing file.

Use Existing:

If a file with the same name already exists, use that instead of copying.

Overwrite:

Overwrite existing files.

Particles and Instancing

Scene Instancing

Import USD scene graph instances as collection instances, otherwise they are imported as copies.

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, 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).

• Volume (both static and animated volumes).

• Armatures

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.

ملاحظة

To export the Blender scene as a USDZ archive, set the file extension of the output file to .usdz. The exported USDZ package will be a zip archive containing the USD and its texture file dependencies.

Shot from Spring exported to USD and opened in USDView.

Export Options

The following options are available when exporting to USD:

General

Root Prim

If set, add a transform primitive with the given path to the stage as the parent of all exported data.

Include

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.

Animation

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

Blender Data

Custom Properties

Exports Custom Properties as USD attributes. The Namespace property is used to determine the namespace that the attributes are written to.

Namespace

If set, add the given namespace as a prefix to exported custom property names. This only applies to property names that do not already have a prefix (e.g., it would apply to name bar but not foo:bar) and does not apply to Blender object and data names which are always exported in the userProperties:blender namespace.

By default, userProperties namespace is used.

Blender Names

Author USD custom attributes containing the original Blender object and object data names.

Allow Unicode

Preserves UTF-8 encoded characters when writing USD prim and property names (requires software utilizing USD 24.03 or greater when opening the resulting files).

File References

Relative Paths

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

Convert Orientation

Convert orientation axis to a different convention to match other applications. Blender uses Y Forward, Z Up (since the front view looks along the +Y direction). For example, its common for applications to use Y as the up axis, in that case -Z Forward, Y Up is needed.

Forward / Up Axis

By mapping these to different axes you can convert rotations between applications default up and forward axes.

Xform Ops

The type of transform operators to use to transform prims.

Translate, Rotate, Scale:

Export with translate, rotate, and scale Xform operators.

Translate, Orient, Scale:

Export with translate, orient quaternion, and scale Xform operators.

Matrix:

Export matrix operator.

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.

Object Types

Meshes

Exports Mesh Objects

Lights

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

Cameras

Exports Camera Objects Only perspective cameras are exported.

Curves

Exports Curve Objects

Point Clouds

Exports Point Cloud Objects as UsdGeomPoints primitives.

Volumes

Exports Volume Objects

Hair

Exports parent hair strands are exported as a curve system. Hair strand colors are not exported.

Geometry

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.

Rename UV Maps

Exports UV maps using the USD default name (st) as opposed to Blender's default name (UVMap).

Normals

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

Triangulate

Triangulates the mesh before writing. For more detail on the specific option see the Triangulate modifier.

Rigging

Shape Keys

Export shape keys as USD blend shapes.

Absolute shape keys are not supported.

Armatures

Export Armatures and meshes with Armature Modifiers as USD skeletons and skinned meshes.

Limitations:

• Modifiers in addition to Armature modifiers will not be applied.

• Bendy bones are not supported.

Only Deform Bones

Only export deform bones and their parents.

Materials

Exports material information of the object. By default the exporter approximates the Principled BSDF node tree by converting it to USD's Preview Surface format.

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.

ملاحظة

If USD Preview Surface Network and MaterialX Network are disabled, the material is set to the viewport materials of meshes.

USD Preview Surface Network

Approximates a Principled BSDF node tree to by converting it to USD's Preview Surface format.

تحذير

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

MaterialX Network

Generates material shading graphs using the MaterialX standard. This standard is designed to support a high amount of interoperability among DCCs <Digital Content Creation>. In Blender, MaterialX supports most of the shader nodes and their functionality but has a few caveats (see below).

Implementation Caveats

When using the Principled BSDF, the resulting graph is very usable. However, when using some of the other BSDFs, some of the generated shading graphs are difficult for other DCC's to understand.

Convert World Material

Convert the world material to a USD dome light. Currently works for simple materials, consisting of an environment texture connected to a background shader, with an optional vector multiply of the texture color.

Export Textures

Method for exporting textures.

Keep:

Use original location of textures.

Preserve:

Preserve file paths of textures from already imported USD files. Export remaining textures to a 'textures' folder next to the USD file.

New Path:

Export textures to a 'textures' folder next to the USD file.

Overwrite Textures

Allow overwriting existing texture files when exporting textures.

USDZ Texture Downsampling

Choose a maximum size for all exported textures.

Keep:

Keep all current texture sizes.

256:

Resize to a maximum of 256 pixels.

512:

Resize to a maximum of 512 pixels.

1024:

Resize to a maximum of 1024 pixels.

2048:

Resize to a maximum of 2048 pixels.

4096:

Resize to a maximum of 4096 pixels.

Custom:

Specify a custom size.

USDZ Custom Downscale Size

The size in pixels of the Custom downsampling.

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.

Materials

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.

Camera

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.

USDZ

Due to a current limitation in the USD library, UDIM textures cannot be include in the USDZ archive. This limitation will likely be addressed in a future version of USD. (See USD pull request #2133.)