# MCAP Layout

`cvmmap-streamer` currently deals with three active MCAP layout shapes:

- bundled multi-camera timeline export
- multi-camera copy export
- single-source `zed_svo_to_mcap` output, which now uses the one-camera `copy` shape

This document covers the current bundled and copy contracts.

Legacy `/camera/*` is documented separately in [mcap_legacy_single_camera_layout.md](./mcap_legacy_single_camera_layout.md).
Conceptually, it is compatible with one-camera `copy` if you treat the camera label as the literal `camera` instead of `zedN` or another derived label.

## Bundled Multi-Camera Layout

Bundled export namespaces each camera stream and adds one bundle manifest topic:

| Topic | Schema / Encoding | Notes |
|------|------|------|
| `/bundle` | `cvmmap_streamer.BundleManifest` | One message per emitted bundle |
| `/zedN/video` | `foxglove.CompressedVideo` | Per-camera encoded video |
| `/zedN/depth` | `cvmmap_streamer.DepthMap` | Per-camera depth |
| `/zedN/calibration` | `foxglove.CameraCalibration` | Per-camera video intrinsics |
| `/zedN/depth_calibration` | `foxglove.CameraCalibration` | Written only when exported depth resolution differs from video |
| `/zedN/pose` | `foxglove.PoseInFrame` | Optional per-camera pose |
| `/zedN/body` | raw `cvmmap.body_tracking.v1` | Optional raw body packets; see [mcap_body_tracking.md](./mcap_body_tracking.md) |

`nearest` is the default bundled policy. It emits one bundle on a common timeline, but it keeps the original per-camera timestamps on `/zedN/video`, `/zedN/depth`, and optional `/zedN/pose`.

`strict` is still available. In strict mode, bundle membership is thresholded by timestamp skew and `--sync-tolerance-ms` applies. In `nearest` mode, `--sync-tolerance-ms` is not used.

Because `nearest` emits on the slowest common timeline, faster cameras can legitimately end up with the same message count as slower cameras in bundled output.

## Multi-Camera Copy Layout

`copy` export also namespaces each camera stream under `/zedN/...`, but it does not emit `/bundle`.

| Topic | Schema / Encoding | Notes |
|------|------|------|
| `/zedN/video` | `foxglove.CompressedVideo` | Per-camera encoded video |
| `/zedN/depth` | `cvmmap_streamer.DepthMap` | Per-camera depth |
| `/zedN/calibration` | `foxglove.CameraCalibration` | Per-camera video intrinsics |
| `/zedN/depth_calibration` | `foxglove.CameraCalibration` | Written only when exported depth resolution differs from video |
| `/zedN/pose` | `foxglove.PoseInFrame` | Optional per-camera pose |
| `/zedN/body` | raw `cvmmap.body_tracking.v1` | Optional raw body packets; see [mcap_body_tracking.md](./mcap_body_tracking.md) |

`copy` preserves each camera topic exactly as an independent stream inside one MCAP:

- no `/bundle` topic
- no shared bundle index
- no resampling to a common timeline
- original per-camera timestamps and cadence are preserved

`--copy-range common` trims each camera to the common overlap window without resampling. `--copy-range full` preserves each camera’s full readable timestamp range from the grouped segment.

Single-source `zed_svo_to_mcap` now writes this same wire shape with one derived label, for example:

- `/zed4/video`, `/zed4/depth`, `/zed4/calibration` for `*_zed4.svo2`
- `/cam1/video`, `/cam1/depth`, `/cam1/calibration` when the filename has no `zedN` suffix

## Bundle Manifest Contract

`/bundle` is the authoritative grouping contract for bundled MCAP files. Consumers should not infer grouping from identical MCAP `logTime` values or from matching per-camera timestamps.

`cvmmap_streamer.BundleManifest` contains:

- `timestamp`: the nominal bundle timestamp on the common timeline
- `bundle_index`: zero-based emitted bundle index
- `policy`: `BUNDLE_POLICY_NEAREST` or `BUNDLE_POLICY_STRICT`
- `members`: one entry per camera label in the bundle

Each bundle member contains:

- `camera_label`
- `timestamp`: the original camera sample timestamp when a payload is present
- `delta_ns`: `member.timestamp - bundle.timestamp` when a payload is present
- `status`
- `corrupted_frames_skipped`

Current member statuses:

- `BUNDLE_MEMBER_STATUS_PRESENT`
- `BUNDLE_MEMBER_STATUS_CORRUPTED_GAP`

`CORRUPTED_GAP` means the exporter skipped one or more unreadable ZED frames and intentionally emitted no video, depth, or pose payload for that camera for this bundle. `corrupted_frames_skipped` reports the size of the skipped run that led to the next recovered readable frame.

## Timestamp Semantics

The bundled MCAP contract intentionally separates bundle time from per-camera sample time:

- `/bundle.logTime` and `/bundle.publishTime` use the nominal bundle timestamp
- `/zedN/video`, `/zedN/depth`, and `/zedN/pose` use the original camera sample timestamp
- calibration and depth-calibration messages use the timestamp of the first emitted sample on that camera

This means a single bundle can legitimately contain different per-camera timestamps, especially in `nearest` mode.

`copy` has no separate bundle time. Its `/zedN/video`, `/zedN/depth`, and `/zedN/pose` messages all use the original per-camera sample timestamp directly.

## Corruption And Partial Bundles

Bundled `nearest` export is resilient to ZED `CORRUPTED FRAME` runs:

- unreadable tail frames are treated as end-of-stream
- mid-stream corruption is skipped until a readable frame is found
- bundles inside the unreadable gap still exist
- the affected camera is marked `CORRUPTED_GAP` in `/bundle`
- no `/zedN/video`, `/zedN/depth`, or `/zedN/pose` message is written for that camera for those bundles

Bundled `strict` export stays strict:

- corruption is skipped internally until recovery
- only fully present, threshold-qualified groups are emitted

`copy` is also resilient to ZED `CORRUPTED FRAME` runs:

- unreadable tail frames are treated as end-of-stream
- mid-stream corruption is skipped until a readable frame is found
- there is no placeholder or manifest entry because `copy` has no grouping contract
- the affected camera topic simply resumes at the recovered readable frame

## Validation Expectations

For bundled MCAP files, the current validation contract is:

- `/bundle` must exist
- every bundle must contain one member per camera label
- for each camera, the count of `BUNDLE_MEMBER_STATUS_PRESENT` members must match the number of `/zedN/video` messages
- for each camera, the count of `BUNDLE_MEMBER_STATUS_PRESENT` members must match the number of `/zedN/depth` messages

That is why a partially written MCAP with topic presence but mismatched counts is treated as invalid.

For multi-camera `copy` MCAP files, the current validation contract is:

- `/bundle` must not exist
- each camera must have `/zedN/video`, `/zedN/depth`, and `/zedN/calibration`
- for each camera, `/zedN/video` and `/zedN/depth` message counts must match

Legacy `/camera/*` validation expectations are documented in [mcap_legacy_single_camera_layout.md](./mcap_legacy_single_camera_layout.md).

The repository-level Python helper [scripts/mcap_bundle_validator.py](../scripts/mcap_bundle_validator.py) understands bundled, copy, and legacy `/camera/*` layouts and reports which one it found before applying the corresponding validation rules.