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:

MCAP schema name	decodes to	what to do
foxglove.FrameTransforms	Transform3D	pass through; do not hand-build
foxglove.CameraCalibration	Pinhole	pass through
foxglove.CompressedVideo	VideoStream (real sample bytes) + CoordinateFrame	pass through
other foxglove.* well-known types	the matching archetype	pass through
ros2 well-known types (ros2msg / ros2_reflection)	archetype	pass through
your own schemas.proto.* / custom protobuf	one <schema.name>:message struct	attach semantics with a DeriveLens + Selector

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.