Rerun Mcap

Rerun MCAP ingestion

McapReader turns an MCAP file into a lazy chunk stream: one entity per topic at the topic's path, message payloads decoded by pluggable decoders. This skill covers the reader's options, what each topic becomes, and the failure modes that yield an empty stream with no error. Stream mechanics (filter, drop, lenses, merge, write) are in rerun-chunk-processing.

The API

from rerun.chunk import McapReader

reader = McapReader(mcap_path)  # see help(McapReader) for the full option set
stream = reader.stream()

A URDF embedded in the MCAP can be ingested as well (then see rerun-urdf).

What a topic becomes

With the default decoders (decoders=None), the message schema name decides what a topic becomes — pass archetypes through, lens only the raw :message topics:

So a camera topic already arrives as Pinhole, its video as VideoStream, and a frame_transforms topic as Transform3D — only custom messages (e.g. a custom joint states schema, a custom gripper status enum) come through reflection or raw only and need lenses.

The foxglove decoder does the schema→archetype mapping; because foxglove messages are protobuf-encoded it rides on the protobuf decoder, so keep decoders=None (verified: decoders=["protobuf"] alone leaves foxglove.CameraCalibration a raw :message; adding foxglove makes it a Pinhole). Confirm on your file: McapReader(path).stream(), then read McapSchema:name and a few Chunk.format() before deciding anything is missing or needs rebuilding.

• Entity path = topic name (/sensors/joint_states stays /sensors/joint_states). Filter early: McapReader(path).stream().filter(content="/sensors/**").
• A reflection-decoded message lands as one struct component named <fully.qualified.MessageName>:message. Navigate it with Selector (Selector(".joint_positions")) inside lenses; this is how custom messages get Rerun semantics attached (see the DeriveLens patterns in rerun-chunk-processing).
• Topic regexes use RE2 syntax and are not anchored: cam matches /external/cam_low and /camera_info. Anchor explicitly (^/external/cam) when it matters. Prefer reader-level topic filtering over .filter(...) when you can, so excluded topics are never decoded at all.

When to use the low-level mcap package instead

McapReader keeps payloads in columnar chunk streams; that is almost always what you want. Drop to mcap.reader.make_reader only when you need raw record metadata without payloads, or when you need to rewrite the container itself (re-registering schemas, channels, and messages).

Gotchas

1. Empty stream, no error: a topic regex that matched nothing, or a channel whose decoder produced no rows. Check Chunk.format() on a few chunks of reader.stream().to_chunks() against a tiny test file, or compare topic names with the mcap CLI / package first.
2. Topic regexes are unanchored RE2; excludes run after includes.
3. timeline_type="timestamp" interprets MCAP log times as wall-clock ns since epoch. If the recording's clock is wrong, fix it at the reader with timestamp_offset_ns rather than mutating timestamps downstream.
4. Decoder subsets silently skip topics no decoder claims; when a topic is missing, retry with decoders=None to rule out decoder selection.
5. Example fix-lenses are dataset-specific. Before copying a MutateLens like the Pinhole:resolution swap from the robot_data_preprocessing example, read the raw component from McapReader(path).stream() and confirm the defect exists in your data — applied blindly it corrupts correct calibration (a correct 648×480 flipped to 480×648).
6. foxglove derives both the camera's Pinhole:child_frame and the video's CoordinateFrame:frame from each message's .frame_id (plus an image-plane suffix), so they match when the calibration and video topics share a frame_id. Only when those topics carry different frame_ids does the video frame diverge and orphan the video from its image plane — re-home it then with a per-camera MutateLens on CoordinateFrame:frame.

References

• End-to-end MCAP pipeline: https://github.com/rerun-io/rerun/tree/main/examples/python/robot_data_preprocessing
• rerun-chunk-processing (stream/lens mechanics), rerun-urdf (FK from joint-state topics), rerun-data-model (modeling decisions)