Mcap Format

MCAP Format Skill

Format Overview

MCAP is an open-source container format for heterogeneous timestamped data, designed for high-performance reading of robotics/autonomous driving recordings.

Specification: https://mcap.dev/spec

Documentation:

• Full specification (record types, serialization, diagrams): https://mcap.dev/spec
• API reference & library docs: https://mcap.dev/docs/swift/documentation/mcap/
• Feature explanations for implementers: https://mcap.dev/spec/notes#feature-explanations
• Well-known profiles registry: https://mcap.dev/spec/registry
• Kaitai Struct description: https://github.com/foxglove/mcap/blob/main/website/docs/spec/mcap.ksy

Binary Structure

┌─────────────────────────────────────────┐
│ Magic bytes (8 bytes)                   │
├─────────────────────────────────────────┤
│ Header record                           │
│   - profile: "ros1" | "ros2" | ""       │
│   - library: writer identifier          │
├─────────────────────────────────────────┤
│ DATA SECTION                            │
│   Schema records                        │
│   Channel records                       │
│   Message records (bulk of file)        │
│   Attachment records (optional)         │
│   Metadata records (optional)           │
│   ─── grouped into Chunks ───           │
├─────────────────────────────────────────┤
│ DataEnd record                          │
├─────────────────────────────────────────┤
│ SUMMARY SECTION (optional but critical) │
│   Schema records (copy)                 │
│   Channel records (copy)                │
│   ChunkIndex records                    │
│   AttachmentIndex records               │
│   MetadataIndex records                 │
│   Statistics record                     │
├─────────────────────────────────────────┤
│ Footer record                           │
│   - summarySectionOffset                │
│   - summarySectionCrc                   │
├─────────────────────────────────────────┤
│ Magic bytes (8 bytes)                   │
└─────────────────────────────────────────┘

Record Types

Header

• profile: Identifies message conventions (e.g., "ros1", "ros2")
• library: Identifies the writing library (for debugging)

Schema

• id: Unique schema identifier within the file
• name: Schema name (e.g., "sensor_msgs/PointCloud2")
• encoding: How to interpret schema data ("ros1msg", "ros2msg", "protobuf", "flatbuffer", "jsonschema", "ros2idl")
• data: Raw schema definition bytes

Channel

• id: Unique channel identifier
• topic: Topic name (e.g., "/camera/image")
• schemaId: References a Schema record
• messageEncoding: How messages are serialized ("ros1", "cdr", "json", "protobuf", "flatbuffer")
• metadata: Key-value pairs (e.g., callerid for ROS 1 publisher identification)

Message

• channelId: Which channel this message belongs to
• logTime: When the message was recorded (nanoseconds)
• publishTime: When the message was originally published (nanoseconds)
• data: Raw serialized message bytes
• sequence: Optional sequence number

Chunk

• compression: "zstd" | "lz4" | "" (none)
• data: Compressed block containing Schema + Channel + Message records
• Messages within a chunk are ordered by logTime
• Chunk boundaries define the granularity of random access

ChunkIndex (Summary)

• messageStartTime / messageEndTime: Time range of messages in this chunk
• chunkOffset: Byte offset of the chunk in the file
• messageIndexOffsets: Per-channel index within the chunk
• Critical for random access — without these, no seeking is possible

Statistics (Summary)

• messageCount: Total messages in file
• channelMessageCounts: Per-channel message counts
• schemaCount: Number of schemas
• channelCount: Number of channels
• chunkCount: Number of chunks
• messageStartTime / messageEndTime: Global time bounds

Attachment

• Named binary blobs (e.g., calibration files, maps)
• Indexed separately from messages
• Not time-ordered with messages

Metadata

• Named key-value string maps
• Used for recording metadata (robot name, location, session info)

Indexed vs Unindexed Reading

Indexed (preferred for Lichtblick)

• File has Summary section with ChunkIndex records
• Reader reads Footer → jumps to Summary → builds index in memory
• Random access: find chunks overlapping time range → read only those chunks
• Reverse iteration: read chunks in reverse order for backfill
• Lichtblick uses: McapIndexedReader → McapIndexedIterableSource

Unindexed (fallback)

• No Summary section — file must be read sequentially from start
• All messages loaded into memory (Lichtblick limits to 1GB)
• No seeking, no backfill, no progress reporting during load
• Lichtblick uses: McapStreamReader → McapUnindexedIterableSource

Compression Trade-offs

Impact on Lichtblick

• Decompression uses WASM modules (wasm-zstd, wasm-lz4, wasm-bz2)
• Modules are loaded lazily on first MCAP open (adds ~100ms startup)
• loadDecompressHandlers() is a singleton — loads once, shared across all readers
• Larger chunks with zstd = fewer decompressions but more memory per chunk

Best Practices for Optimal Lichtblick Performance

1. Always Include Chunk Indexes

• Without indexes, Lichtblick falls back to streaming (1GB limit, no seek)
• Write summary section at end of file with all ChunkIndex records
• Include Statistics record for immediate UI display of file metadata

2. Put Schemas and Channels in Summary

• Allows Lichtblick to display topic list without reading data section
• Speeds up initialization (only 2 Range requests for remote files)

3. Target ~4MB Chunk Size

• Too small (<100KB): excessive index overhead, many seeks
• Too large (>50MB): wastes memory when seeking (must decompress full chunk)
• ~4MB: good balance for random access granularity vs overhead

4. Use Consistent Chunk Boundaries

• Align chunk boundaries with logical time units when possible
• Avoid chunks spanning very long time ranges (hurts seek precision)

5. Order Messages by logTime Within Chunks

• MCAP spec allows out-of-order, but ordered messages enable streaming reads
• Lichtblick's reader assumes logTime ordering within chunks

6. Include callerid in Channel Metadata (ROS)

• Used by Lichtblick to identify publishers per topic
• Helps with diagnostics and multi-publisher scenarios

7. Choose Compression Based on Use Case

• Remote viewing: zstd (minimize download size, acceptable decompress cost)
• Local playback: lz4 (fastest decompress, acceptable file size)
• Real-time recording: lz4 or none (lowest CPU overhead during recording)

Lichtblick Code Mapping