Loading Usd

Loading USD

When to Use

Use this skill when the user asks to load a USD scene, compose USD content, add cameras or RenderProducts to an existing USD layer, add referenced content, or create runtime geometry.

Inputs

Resolve inputs in this order: existing repository files and referenced snippets, explicit user request, then broader agent context.

• Target API surface: Python, C/C++, or both.
• Application lifecycle stage: renderer creation, scene loading, stepping, warmup, output readback, or cleanup.
• Repository source snippets referenced below. Treat these snippets as the API source of truth.

Prerequisites

• Use an ovrtx checkout that contains the referenced examples and docs tests.
• Read the relevant > **Source:** snippet before writing or explaining API usage.
• For code changes, preserve renderer lifecycle ordering and cleanup semantics for the selected language.

Instructions

1. Identify the requested language and lifecycle stage before choosing an example.
2. Read the referenced snippet that matches the requested stage and language.
3. Preserve the normal ovrtx order: create or initialize the renderer, load or compose USD, step or wait for work, read outputs when needed, then release C resources explicitly.
4. Apply the async, status-query, error-handling, and warmup skills when the workflow crosses those concerns.
5. When changing code, run the narrow example or docs test that owns the snippet whenever practical.

Output Format

• For explanations, cite the relevant API names, source snippets, and caveats.
• For code changes, summarize the files changed, snippets affected, and validation run.

Scripts

This skill has no scripts.

Limitations

• The referenced snippets remain the source of truth; update or add tested snippets before documenting new API usage.

Overview

Before rendering, you must load USD content into the renderer. ovrtx supports three input modes:

1. File path or URL -- open an existing .usd/.usda/.usdc file as the root layer.
2. Inline USDA string -- open runtime-generated USD content as the root layer. The inline root layer may use USD subLayers to compose a base scene and additional authored prims.
3. USD references -- compose additional file or string content under a specific path prefix.

There are two common composition patterns:

• Inline root with subLayers -- use when an existing scene layer does not contain required prims such as Cameras, RenderProducts, or RenderVars. Build one inline root USDA layer that sublayers the original scene and authors the extra prims, then load it with open_usd_from_string() / ovrtx_open_usd_from_string().
• Additive references -- use when a root stage is already open and you need to add removable referenced content at a new prim path. Use add_usd_reference() / add_usd_reference_from_string().

Python

Open from file or URL

Source: examples/python/minimal/main.py snippet add-usd

Open from inline USDA string

Useful for creating RenderProducts, cameras, or runtime geometry without editing the original scene:

Source: tests/docs/python/test_sensor_configuration.py snippet doc-add-render-config-layer

Compose a base scene plus extra prims

When a USD layer has the scene content but lacks render configuration or sensors, compose a new inline root layer: add subLayers = [@existing_scene.usda@], author the missing Camera / RenderProduct / RenderVar prims in that same inline layer, and load it with open_usd_from_string().

USDA source: tests/docs/usd/data/inline_sublayers_camera_renderproduct.usda snippet doc-usda-inline-sublayers-camera-renderproduct

Source: tests/docs/python/test_sensor_configuration.py snippet doc-add-render-config-layer

Query check: tests/docs/python/test_stage_query.py snippet doc-query-inline-sublayer-composition

Add a USD reference with a path prefix

Use add_usd_reference() or add_usd_reference_from_string() after a root stage is open when you need additive content that can be removed later by handle.

Compose multiple inputs

Use inline subLayers for one composed root stage, or reference APIs for incremental additions. See the planet-system example for the reference-addition pattern.

Remove USD

Use remove_usd(handle) with handles returned by add_usd_reference() / add_usd_reference_from_string(). Root layers opened with open_usd* are replaced by the next open_usd* call or cleared with reset_stage().

C

Open from file or URL

Source: examples/c/minimal/main.cpp snippet load-usd-and-wait

Poll for completion

Loading is asynchronous in C. Poll until done:

Source: examples/c/minimal/main.cpp snippet load-usd-and-wait

Or block indefinitely:

Source: examples/c/minimal/main.cpp snippet step-renderer

The step snippet demonstrates blocking with ovrtx_timeout_infinite.

Add a USD reference with a path prefix

Use ovrtx_add_usd_reference_from_file or ovrtx_add_usd_reference_from_string with a path prefix.

Open from inline USDA content

Source: tests/docs/c/test_sensor_configuration.cpp snippet doc-add-render-config-layer-c

Use ovrtx_open_usd_from_file for files/URLs and ovrtx_open_usd_from_string for inline root USDA, including inline roots with subLayers.

Compose a base scene plus extra prims

When a USD layer has the scene content but lacks render configuration or sensors, pass one inline root USDA string to ovrtx_open_usd_from_string(). That inline root can sublayer the existing scene and author missing Camera / RenderProduct / RenderVar prims.

USDA source: tests/docs/usd/data/inline_sublayers_camera_renderproduct.usda snippet doc-usda-inline-sublayers-camera-renderproduct

Source: tests/docs/c/test_sensor_configuration.cpp snippet doc-add-render-config-layer-c

Remove USD

C: ovrtx_enqueue_result_t result = ovrtx_remove_usd(renderer, usd_handle);

Update time-sampled attributes

For animated USD scenes, re-evaluate every time-sampled attribute at a specific stage time. Time is in seconds; the runtime converts to USD timecodes via the stage's timeCodesPerSecond metadata.

Source: tests/docs/python/test_base.py snippet doc-update-from-usd-time-async

Reset stage to empty

Clear all USD content from the runtime stage:

Python: renderer.reset_stage() / renderer.reset_stage_async()

C: ovrtx_reset_stage(renderer)

Key Types / Functions

Troubleshooting

• Only one root layer is allowed. Calling open_usd() followed by open_usd_from_string() (or vice versa) replaces the root stage. To combine a scene file with extra prims such as Cameras and RenderProducts, use a single open_usd_from_string() root layer with subLayers:

  Source: tests/docs/python/test_sensor_configuration.py snippet doc-add-render-config-layer

• Reference additions are a separate pattern. Use add_usd_reference* when a root stage is already open and you want to place additional referenced content at a new absolute path. The prefix_path must not collide with existing prim paths, and inline reference layers must set defaultPrim.

• Authored attributes that are not part of the normal population schema are ignored unless the root layer sets customLayerData.populateAllAuthoredAttributes = true. Use this only for workflows that need generic authored attributes, because populating every authored attribute can dramatically increase memory usage when assets contain many properties the runtime will never read or write.

• In Python, open_usd() blocks until loaded. Use the _async variants if you need non-blocking behavior.

• In C, ovrtx_open_usd_from_file() is always asynchronous -- you must poll or wait.

• Load errors (e.g., file not found) are reported through ovrtx_op_wait_result_t::error_op_ids, not the immediate return value.

References

• Use the > **Source:** directives in this skill to locate tested snippets before reusing API patterns.
• Keep related skills, docs, and snippets synchronized when changing the workflow.