GoPro Metadata Format Parser

TLDR:

1. ffmpeg -y -i GOPR0001.MP4 -codec copy -map 0:m:handler_name:" GoPro MET" -f rawvideo GOPR0001.bin
2. gopro2json -i GOPR0001.bin -o GOPR0001.json
3. There is no step 3

I spent some time trying to reverse-engineer the GoPro Metadata Format (GPMD or GPMDF) that is stored in GoPro Hero 5 cameras if GPS is enabled. This is what I found.

Extracting the Metadata File

The metadata stream is stored in the .mp4 video file itself alongside the video and audio streams. We can use ffprobe to find it:

[computar][100GOPRO] ➔ ffprobe GOPR0008.MP4
ffprobe version 3.2.4 Copyright (c) 2007-2017 the FFmpeg developers
[SNIP]
    Stream #0:3(eng): Data: none (gpmd / 0x646D7067), 33 kb/s (default)
    Metadata:
      creation_time   : 2016-11-22T23:42:41.000000Z
      handler_name    : 	GoPro MET
[SNIP]

We can identify it by the gpmd in the tag string - in this case it's id 3. We can then use ffmpeg to extract the metadata stream into a binary file for processing:

ffmpeg -y -i GOPR0001.MP4 -codec copy -map 0:3 -f rawvideo out-0001.bin

This leaves us with a binary file with the data.

Data We Get

• ~400 Hz 3-axis gyro readings
• ~200 Hz 3-axis accelerometer readings
• ~18 Hz GPS position (lat/lon/alt/spd)
• 1 Hz GPS timestamps
• 1 Hz GPS accuracy (cm) and fix (2d/3d)
• 1 Hz temperature of camera

The Protocol

Data starts with a label that describes the data following it. Values are all big endian, and floats are IEEE 754. Everything is packed to 4 bytes where applicable, padded with zeroes so it's 32-bit aligned.

• Labels - human readable types of proceeding data
• Type - single ascii character describing data
• Size - how big is the data type
• Count - how many values are we going to get
• Length = size * count

Labels include:

• ACCL - accelerometer reading x/y/z
• DEVC - device
• DVID - device ID, possibly hard-coded to 0x1
• DVNM - devicde name, string "Camera"
• EMPT - empty packet
• GPS5 - GPS data (lat, lon, alt, speed, 3d speed)
• GPSF - GPS fix (none, 2d, 3d)
• GPSP - GPS positional accuracy in cm
• GPSU - GPS acquired timestamp; potentially different than "camera time"
• GYRO - gryroscope reading x/y/z
• SCAL - scale factor, a multiplier for subsequent data
• SIUN - SI units; strings (m/s², rad/s)
• STRM - ¯\_(ツ)_/¯
• TMPC - temperature
• TSMP - total number of samples
• UNIT - alternative units; strings (deg, m, m/s)

Types include:

• c - single char
• L - unsigned long
• s - signed short
• S - unsigned short
• f - 32 float

For implementation details, see reader.go and other corresponding files in telemetry/.