Writing Attributes

Writing Attributes

When to Use

Use this skill when the user asks to write an attribute, set a property, change a material, set a color, or modify mesh data.

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.
• Prim paths, attribute name, USD value type, scalar versus array shape, and desired values.
• Data source location, dtype, semantic conversion, ordinal, and write-floor publication.
• Whether the write is one-shot, repeated with a stable target, or needs direct mapped-buffer access.
• 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.
• Use attribute-bindings for repeated writes to the same prims/attribute, and mapping-attributes for zero-copy direct writes.
• Use binding-materials when the request is specifically to set material:binding.

Instructions

1. Identify the target language, prim paths, attribute name, value kind, data shape, memory target, and sync/async requirement.
2. Read the matching source snippet and copy its lifecycle pattern rather than inventing equivalent calls.
3. Validate dtype, shape, semantic, and ownership rules before proposing or editing code.
4. Wait for the ovstage write, then advance the write floor before rendering its ordinal; preserve C compatibility lifetimes when maintaining C code.
5. When changing code, run the narrow docs test or example 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

• Deprecated ovrtx compatibility writes retain their documented unsupported-type limits for string arrays, asset arrays, and timecode attributes.
• For ovstage token and path values, intern IDs through ovstage.PathDictionary and use the matching AttributeSemantic.
• The referenced snippets remain the source of truth; update or add tested snippets before documenting new API usage.

Overview

Beyond transforms, ovstage can write arbitrary attributes to prims: colors, visibility, mesh geometry, string tokens, and more. The DLTensor dtype must exactly match the USD attribute schema.

There are two categories:

• Scalar attributes -- one value per prim (e.g., a color, a transform)
• Array attributes -- variable-length per prim (e.g., mesh points, face vertex counts)

Tensor layout

Ovstage Python and the C compatibility API use lane-based DLTensor attribute layouts.

Python ovstage — lane-based attributes

Scalar values can use NumPy arrays directly. For vectors and matrices, wrap NumPy storage with ovstage.make_dltensor, set the logical element count in shape, and set the component count in dtype.lanes:

Source: tests/docs/python/test_attribute_shapes.py snippets doc-shape-scalar-int32, doc-shape-float3-array, doc-shape-mat4-array.

C — lane-based attributes

The C API uses DLDataType::lanes for multi-component attribute reads and writes. The shape counts logical attribute elements; the lane count holds the vector or matrix component count:

Source: tests/docs/c/test_attribute_shapes.cpp snippets doc-shape-scalar-int32-c, doc-shape-float3-array-c, doc-shape-mat4-array-c.

For a C point3f[] attribute with 10 points, write or read one tensor with shape=[10] and dtype={kDLFloat, 32, 3}. Rendered output/AOV tensors are not attribute tensors; in C they use channel-last shapes such as [height, width, channels] with dtype.lanes=1.

USD Type Lookup

Use this table to answer "how do I write USD type X?" For ovstage, pass is_array=True for array-valued attributes. The C compatibility API sets binding.binding_desc.attribute_type.is_array = true.

Python

Array attribute write (mesh points)

Source: tests/docs/python/test_attribute_shapes.py snippet doc-shape-float3-array

Array attribute write (same pattern for other array schemas)

Source: tests/docs/python/test_attribute_shapes.py snippet doc-shape-float3-array

Prim modes

Source: tests/docs/python/test_attribute_bindings.py snippet doc-bind-attribute-write

Token array attribute

Source: tests/docs/python/test_attribute_bindings.py snippet doc-write-token-array

Path/relationship array attribute

Source: tests/docs/python/test_base.py snippet doc-bind-material

Authored attribute matrix

Python raw snippets:

The snippets listed in this table live in tests/docs/python/test_all_attributes.py.

They write through a reusable ovstage query, wait for each write, and advance the write floor before reading the updated value. Use ovstage.PathDictionary to intern token and relationship strings, then write the resulting IDs with AttributeSemantic.TOKEN_ID, AttributeSemantic.RELATIONSHIP_PATH_ID or AttributeSemantic.ASSET_PATH_ID. String payloads use byte rows with AttributeSemantic.STRING.

C

Generic scalar write in C

Source: tests/docs/c/test_attribute_bindings.cpp snippet doc-write-bound-attribute-c

Supported authored attribute write snippets in C

The snippets below are ovstage-native: they issue ovstage_write_attribute against a DocsQueryAndToken (single-prim query handle + interned attribute token) with an ovstage_write_data_t describing the DLTensor payload, is_array, and OVSTAGE_SEMANTIC_*. Each block advances the write floor and bumps the caller's tracked ordinal so a follow-up latest(ordinal) read observes the write. The ovrtx ovrtx_write_attribute + ovrtx_binding_desc_or_handle_t compatibility shim is still present for standalone-renderer callers but is not used by these snippets — the recommended path is attached ovstage.

C raw snippets:

The snippets listed in this table live in tests/docs/c/test_all_attributes.cpp. As of ovstage 0.2, writing ASSET_PATH_ID id pairs to a USD-populated scalar asset attribute is supported and updates the existing column in place. On 0.1.x a write of this shape created a second same-named column of a conflicting type instead. The scalar-asset snippets are runtime-validated by AllAttributesTest.AssetReadWriteSnippets. Intern both paths through the path dictionary before the write, and use token id 0 for the resolved half when the path resolves to nothing.

Key Types / Functions

Ovstage C write payload (ovstage_write_data_t):

• .tensors / .tensor_count — DLTensor array. tensor_count==1 for the packed-uniform layout used by every snippet.
• .is_array — sole authority for fixed (false) vs. array (true) storage. Does NOT auto-derive from the DLTensor's shape.
• .semantic — ovstage_attribute_semantic_t, tag the write with the same geometric role the column was created under, or OVSTAGE_SEMANTIC_NONE to preserve an existing column's authored role.

Semantics (Python: ovstage.AttributeSemantic; C ovstage: ovstage_attribute_semantic_t; C ovrtx compat: ovrtx_attribute_semantic_t):

• AttributeSemantic.NONE / OVSTAGE_SEMANTIC_NONE / OVRTX_SEMANTIC_NONE -- generic data.
• AttributeSemantic.MATRIX / OVSTAGE_SEMANTIC_MATRIX / OVRTX_SEMANTIC_XFORM_MAT4x4 -- matrix data.
• OVRTX_SEMANTIC_XFORM_POS3d_ROT4f_SCALE3f -- decomposed transform (C ovrtx compat only).
• OVRTX_SEMANTIC_XFORM_POS3d_ROT3x3f -- decomposed transform (C ovrtx compat only).
• AttributeSemantic.RELATIONSHIP_PATH_ID / OVSTAGE_SEMANTIC_RELATIONSHIP_PATH_ID -- interned relationship path IDs (write is_array=true).
• AttributeSemantic.TOKEN_ID / OVSTAGE_SEMANTIC_TOKEN_ID / OVRTX_SEMANTIC_TOKEN_ID -- interned token IDs.
• AttributeSemantic.STRING / OVSTAGE_SEMANTIC_STRING -- UTF-8 USD string bytes (write is_array=true, dtype {kDLUInt, 8, 1}).
• AttributeSemantic.ASSET_PATH_ID / OVSTAGE_SEMANTIC_ASSET_PATH_ID -- interned (authored, resolved) asset token ids (write is_array=false, dtype {kDLUInt, 64, 2}).

In C, ovstage_write_attribute returns ovstage_enqueue_result_t which contains both .status (check for OVSTAGE_OK) and .op_index (for async tracking via ovstage_wait_op). Every doc-test snippet uses the shared docs_wait_ovstage_no_errors(stage, op_index) helper from tests/docs/c/helpers.h to wait + assert on op errors.

The deprecated ovrtx_write_attribute still returns ovrtx_enqueue_result_t and tracks via ovrtx_wait_op for standalone-mode callers.

Deprecated renderer data access modes (Python: from ovrtx import DataAccess):

• DataAccess.SYNC -- copies data during the call, safe to free after return
• DataAccess.ASYNC -- data accessed later during stream execution, must keep alive; pass cuda_stream= or cuda_event= for GPU synchronization. Not allowed with string data.

Troubleshooting

• Array attribute dtype must exactly match the USD schema. Using numpy's default float64 for a float3[] attribute (which expects float32) will cause errors.
• In the current runtime, authored scalar USD float3 values may be created but populated as zero by populateAllAuthoredAttributes. If a value needs to come from USD, author it as a role-bearing type such as vector3f, point3f, normal3f, or color3f. Direct runtime writes to scalar float3 still work.
• Quaternion tensors use ovrtx runtime lane order (i, j, k, real). USDA quat* values are authored as (real, i, j, k), so reading quatd, quatf, or quath attributes reorders the components into (i, j, k, real), and writes should use that runtime tensor order.
• Deprecated renderer string writes using Semantic.PATH_STRING or Semantic.TOKEN_STRING require DataAccess.SYNC.
• Deprecated renderer compatibility writes do not support string arrays, asset arrays, or timecode attributes.
• Custom relationships are not populated by the generic authored-attribute path. Specific relationships used by supported schemas, such as material:binding and shader connections, are handled by their schema/population code paths; arbitrary custom relationships are ignored today.
• Unsupported authored types are covered by negative tests in tests/docs/python/test_all_attributes.py and tests/docs/c/test_all_attributes.cpp; if one starts populating, keep this documentation and those tests in sync.
• For array attributes in Python, pass a list of tensors (one per prim), not a single tensor. NumPy arrays, Warp arrays, and any __dlpack__-compatible objects are accepted directly.
• PrimMode.UPSERT creates absent prims and updates existing prims. PrimMode.INSERT is create-only. When a write creates a column whose authored interpretation is not generic, pass the matching AttributeSemantic.
• For ovstage token and relationship writes, intern strings with PathDictionary and write the resulting IDs with TOKEN_ID or RELATIONSHIP_PATH_ID semantics.
• The ovstage-native "binding" is the pair (ovstage_query_handle_t, ovx_token_t attr_token) reserved with ovstage_query_from_path_list + path_dictionary_create_tokens_from_strings. Both are stable across writes/reads until released via ovstage_release_query and path_dictionary_release_path_list_reference. tests/docs/c/helpers.h factors the setup into DocsQueryAndToken + docs_make_query_and_token / docs_release_query_and_token — every C attribute doc-test uses those helpers to keep the boilerplate out of [snippet:] blocks. The deprecated ovrtx_make_binding_desc still exists for standalone-mode callers and borrows its input ovx_string_t prim path array; keep it alive until the write completes.
• dirty_bits is a bitvector with 1 bit per prim -- the byte array size must be (prim_count + 7) / 8.
• In C, dirty_bits support three combination modes via ovrtx_write_bits_t in the ovrtx_input_buffer_t.dirty_bits_mode field: OVRTX_DIRTY_MASK_REPLACE (default -- replace existing mask), OVRTX_DIRTY_MASK_OR (merge with existing), OVRTX_DIRTY_MASK_AND (intersect with existing).

C convenience helpers for string attributes (#include <ovrtx/ovrtx_attributes.h>):

• ovrtx_set_path_attributes(renderer, paths, count, attr_name, path_values) -- write path/relationship attributes. Each prim gets a single-element array (relationships are always arrays in USD).
• ovrtx_set_token_attributes(renderer, paths, count, attr_name, token_values) -- write token string attributes (one per prim).

Source: tests/docs/c/test_attribute_helpers.cpp snippet doc-set-token-attributes-c

Deprecated Standalone APIs

The renderer attribute-write APIs in this skill are deprecated in 0.4 and retained for compatibility. New code should write through ovstage with an application-owned ordinal, wait for completion, and advance the write floor before rendering.

See docs/core/ovstage_integration.rst, skills/update-0_3-0_4-c/SKILL.md and skills/update-0_3-0_4-python/SKILL.md.

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.