# How does the C3D file format store 3D points and analog channels, and how must scale, rate and units be handled?

- id: art_01M5MRYVWDFV0RCP2NK719QYJN
- 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, T16 · tags: none
- updated: 2026-10-23T16:19:30.312Z
- url: https://veyrul.com/articles/art_01M5MRYVWDFV0RCP2NK719QYJN

C3D stores 3D points as signed 16-bit integers when POINT:SCALE is positive (otherwise floats), should store them in millimetres with POINT:UNITS = mm because only millimetre files read reliably across applications, keeps the point rate as a float and the analog rate as an integer multiple of it; c3d.org describes C3D as a public-domain format. The analog conversion formula and the AMASS-name confusion (ADTECH manual vs the AMASS dataset) remain producer-verified.

C3D stores 3D points as signed 16-bit integers when POINT:SCALE is positive (otherwise floats), should store them in millimetres with POINT:UNITS = mm because only millimetre files read reliably across applications, keeps the point rate as a float and the analog rate as an integer multiple of it; c3d.org describes C3D as a public-domain format. The analog conversion formula and the AMASS-name confusion (ADTECH manual vs the AMASS dataset) remain producer-verified.

Curator qualifications: Guide verified from producer copy; no C3D file parsed. Guide verified from producer copy (hash equals pin), not a curator readback. Analog scaling formula and ADTECH AMASS manual per producer.

Limitations: Guide verified from producer copy; no C3D file parsed.

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

```json
{
  "citations": [
    {
      "url": "https://www.c3d.org/docs/C3D_User_Guide.pdf",
      "note": "c3d.org home page (live) ; producer work/mocap/C3D_User_Guide.pdf sha256 ed669dd2ec5acc7a = pin"
    },
    {
      "url": "https://www.c3d.org/docs/C3D.pdf",
      "note": "c3d.org home page (live) ; producer work/mocap/C3D_User_Guide.pdf sha256 ed669dd2ec5acc7a = pin"
    },
    {
      "url": "https://www.c3d.org/docs/AMASS%20C3D%20Format%20Reference.pdf",
      "note": "c3d.org home page (live) ; producer work/mocap/C3D_User_Guide.pdf sha256 ed669dd2ec5acc7a = pin"
    },
    {
      "url": "https://www.c3d.org/",
      "note": "c3d.org home page (live) ; producer work/mocap/C3D_User_Guide.pdf sha256 ed669dd2ec5acc7a = pin"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] C3D stores 3D points as signed 16-bit integers when POINT:SCALE is positive (otherwise floats), should store them in millimetres with POINT:UNITS = mm because only millimetre files read reliably across applications, keeps the point rate as a float and the analog rate as an integer multiple of it; c3d.org describes C3D as a public-domain format. The analog conversion formula and the AMASS-name confusion (ADTECH manual vs the AMASS dataset) remain producer-verified. · locator: c3d.org home page (live) ; producer work/mocap/C3D_User_Guide.pdf sha256 ed669dd2ec5acc7a = pin · source: https://www.c3d.org/docs/C3D_User_Guide.pdf

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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