# How do Alembic point caches (OPoints/IPoints) differ from PC2/MDD in identity, topology change and time sampling, and what does a mismatch look like?

- id: art_01M5MDWY4976J9XWXTHMGZB8GT
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T07, T15 · tags: none
- updated: 2026-10-23T13:06:12.737Z
- url: https://veyrul.com/articles/art_01M5MDWY4976J9XWXTHMGZB8GT

Alembic point samples carry positions with optional uint64 ids, velocities and widths; the API comments describe ids on the first sample and re-stated ids/topology to mark a topology change, and time is per-property TimeSampling (Uniform, Cyclic or Acyclic). The writer enforces positions on sample 0 but substitutes an empty ids array, so readers cannot assume ids exist. PC2/MDD carry neither ids nor topology.

Alembic point samples carry positions with optional uint64 ids, velocities and widths; the API comments describe ids on the first sample and re-stated ids/topology to mark a topology change, and time is per-property TimeSampling (Uniform, Cyclic or Acyclic). The writer enforces positions on sample 0 but substitutes an empty ids array, so readers cannot assume ids exist. PC2/MDD carry neither ids nor topology.

Curator qualifications: Code and manual read 2026-10-22; no cache file written or loaded; no Blender run. Importer behaviour (Blender, Unreal, Houdini) for Alembic ids not covered (producer limitation kept).

Limitations: Code and manual read 2026-10-22; no cache file written or loaded; no Blender run.

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://docs.blender.org/manual/en/latest/modeling/modifiers/modify/mesh_cache.html",
      "note": "alembic OPoints.h at 6f59e4d0 (raw), Sample constructors; alembic OPoints.cpp at 6f59e4d0 (raw), OPointsSchema::set; alembic TimeSampling.h at 6f59e4d0 (raw); alembic OPolyMesh.h at 6f59e4d0 (raw), Sample comments"
    },
    {
      "url": "https://github.com/blender/blender/blob/d13f752e3b9c4f8c261cda552b1021f8bcc0382c/source/blender/modifiers/intern/MOD_meshcache_pc2.cc",
      "note": "alembic OPoints.h at 6f59e4d0 (raw), Sample constructors; alembic OPoints.cpp at 6f59e4d0 (raw), OPointsSchema::set; alembic TimeSampling.h at 6f59e4d0 (raw); alembic OPolyMesh.h at 6f59e4d0 (raw), Sample comments"
    },
    {
      "url": "https://github.com/alembic/alembic/blob/6f59e4d0c9012c67e242da510b31d96504db8e01/lib/Alembic/AbcGeom/OPoints.h",
      "note": "alembic OPoints.h at 6f59e4d0 (raw), Sample constructors; alembic OPoints.cpp at 6f59e4d0 (raw), OPointsSchema::set; alembic TimeSampling.h at 6f59e4d0 (raw); alembic OPolyMesh.h at 6f59e4d0 (raw), Sample comments"
    },
    {
      "url": "https://github.com/alembic/alembic/blob/6f59e4d0c9012c67e242da510b31d96504db8e01/lib/Alembic/AbcGeom/OPoints.cpp",
      "note": "alembic OPoints.h at 6f59e4d0 (raw), Sample constructors; alembic OPoints.cpp at 6f59e4d0 (raw), OPointsSchema::set; alembic TimeSampling.h at 6f59e4d0 (raw); alembic OPolyMesh.h at 6f59e4d0 (raw), Sample comments"
    },
    {
      "url": "https://github.com/alembic/alembic/blob/6f59e4d0c9012c67e242da510b31d96504db8e01/lib/Alembic/AbcCoreAbstract/TimeSampling.h",
      "note": "alembic OPoints.h at 6f59e4d0 (raw), Sample constructors; alembic OPoints.cpp at 6f59e4d0 (raw), OPointsSchema::set; alembic TimeSampling.h at 6f59e4d0 (raw); alembic OPolyMesh.h at 6f59e4d0 (raw), Sample comments"
    },
    {
      "url": "https://github.com/alembic/alembic/blob/6f59e4d0c9012c67e242da510b31d96504db8e01/lib/Alembic/AbcGeom/OPolyMesh.h",
      "note": "alembic OPoints.h at 6f59e4d0 (raw), Sample constructors; alembic OPoints.cpp at 6f59e4d0 (raw), OPointsSchema::set; alembic TimeSampling.h at 6f59e4d0 (raw); alembic OPolyMesh.h at 6f59e4d0 (raw), Sample comments"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] Alembic point samples carry positions with optional uint64 ids, velocities and widths; the API comments describe ids on the first sample and re-stated ids/topology to mark a topology change, and time is per-property TimeSampling (Uniform, Cyclic or Acyclic). The writer enforces positions on sample 0 but substitutes an empty ids array, so readers cannot assume ids exist. PC2/MDD carry neither ids nor topology. · locator: alembic OPoints.h at 6f59e4d0 (raw), Sample constructors; alembic OPoints.cpp at 6f59e4d0 (raw), OPointsSchema::set; alembic TimeSampling.h at 6f59e4d0 (raw); alembic OPolyMesh.h at 6f59e4d0 (raw), Sample comments · source: https://docs.blender.org/manual/en/latest/modeling/modifiers/modify/mesh_cache.html

Rights:
- documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources.

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