Replay

The replay_trossen_mcap_jointstate tool reads joint state data from a recorded TrossenMCAP episode and plays it back onto connected arms (and optionally a SLATE mobile base). It is the fastest way to verify a recorded trajectory on hardware.

What You Need

• A completed build. The replay tool is always built as part of the standard SDK build (see Installation and Setup).

• At least one .mcap episode recorded with the SDK. If you have not recorded anything yet, follow Record.

• Hardware connected and reachable. The same arms (and optional SLATE base) used when the episode was recorded, powered on and on the same subnet.

Building the Tool

The tool is produced by the standard build:

cd build
cmake ..
make replay_trossen_mcap_jointstate

The resulting binary lives at build/scripts/replay_trossen_mcap_jointstate.

Usage

./build/scripts/replay_trossen_mcap_jointstate <path/to/episode.mcap> [config.json]

If no config file is specified, the tool loads scripts/replay_trossen_mcap_jointstate/config.json from the repository root.

Example:

SoloStationaryMobile

./build/scripts/replay_trossen_mcap_jointstate \
    ~/.trossen_sdk/solo_dataset/episode_000000.mcap \
    scripts/replay_trossen_mcap_jointstate/config.json

Configuring the Replay

The replay config maps MCAP stream IDs back to physical hardware. Each entry in arms must have a stream_id that matches a joint-state channel in the MCAP file. For example, follower/joints/state maps to stream_id: "follower". Arms not listed in the config are skipped.

SoloStationaryMobile

{
  "replay": {
    "playback_speed": 1.0,
    "arms": [
      {
        "stream_id":    "follower",
        "ip_address":   "192.168.1.4",
        "model":        "wxai_v0",
        "end_effector": "wxai_v0_follower",
        "goal_time":    0.066
      }
    ]
  }
}

Key fields:

Field	Meaning
playback_speed	Multiplier on the inter-frame delay. 1.0 plays in real time, 0.5 plays at half speed.
arms[].stream_id	Must match a joint-state channel in the MCAP file.
arms[].ip_address	IP of the physical controller that should receive the trajectory.
arms[].goal_time	Time (seconds) for the arm to reach each commanded position. A value of 2.0 / fps (for example, 0.066 at 30 Hz) produces smooth motion.
slates[]	Include one entry per mobile base stream to replay recorded velocities. Omit for solo or stationary episodes.

Playback Behavior

• Playback is paced using log_time (monotonic) timestamps in the MCAP file. playback_speed scales the inter-frame delay.

• Before playback begins, the tool moves each listed arm to the recorded starting position so the trajectory starts from a known state.

• The tool requires libtrossen_arm to be installed for arm control. This is the same library the SDK uses for recording.

• For episodes containing mobile-base streams, the tool uses trossen_slate to drive the base.

What’s Next

• Convert the dataset to LeRobot V2 for training. See Convert.