syncfield 0.1.1

Lightweight timestamp capture SDK for SyncField multi-stream synchronization

syncfield-python

Lightweight Python SDK for SyncField multi-stream synchronization. Captures precise timestamps during multi-camera and sensor recording and produces JSONL files that the SyncField Docker service consumes for frame-level temporal alignment.

Install

pip install syncfield

Zero dependencies -- uses only the Python standard library.

Quick Start

Video Streams

Use stamp() to capture timestamps and link() to associate the saved video file with the stream.

import syncfield as sf

session = sf.SyncSession(host_id="rig_01", output_dir="./sync_data")
session.start()

for i in range(num_frames):
    frame = camera.read()
    session.stamp("cam_left", frame_number=i)
    save_frame_to_video(frame, "cam_left.mp4")

session.link("cam_left", "/data/cam_left.mp4")
session.stop()

Output:

./sync_data/
  sync_point.json
  cam_left.timestamps.jsonl
  manifest.json

Sensor Streams

Use record() to capture timestamps and sensor data in one call. This writes both a .timestamps.jsonl file (for alignment) and a .jsonl file (sensor channel values).

import syncfield as sf

session = sf.SyncSession(host_id="rig_01", output_dir="./sync_data")
session.start()

for i in range(num_samples):
    data = imu.read()
    session.record("imu", frame_number=i, channels={
        "accel_x": data.ax,
        "accel_y": data.ay,
        "accel_z": data.az,
    })

session.stop()

Output:

./sync_data/
  sync_point.json
  imu.timestamps.jsonl
  imu.jsonl
  manifest.json

Complex Sensor Data

Sensors like hand trackers, tactile arrays, and robot joints produce nested data. The SDK handles these natively — leaf values must be numeric (float or int).

# Hand tracker — nested joint positions and gestures
session.record("hand_tracker", frame_number=i, channels={
    "joints": {
        "wrist": [0.1, 0.2, 0.3],
        "thumb_tip": [0.4, 0.5, 0.6],
        "index_tip": [0.7, 0.8, 0.9],
    },
    "gestures": {"pinch": 0.95, "fist": 0.02},
    "finger_angles": [12.5, 45.0, 30.0, 15.0, 5.0],
})

# Tactile grid — 2D pressure array
session.record("tactile", frame_number=i, channels={
    "pressure_grid": [[0.1, 0.2, 0.3, 0.4],
                       [0.5, 0.6, 0.7, 0.8]],
    "total_force": 12.5,
})

# Robot arm — joint states
session.record("robot_arm", frame_number=i, channels={
    "joint_positions": [0.0, -1.57, 0.0, -1.57, 0.0, 0.0],
    "joint_velocities": [0.01, -0.02, 0.0, 0.01, 0.0, 0.0],
    "gripper": {"width": 0.04, "force": 5.2},
})

SyncField automatically flattens nested channels for aggregation using dot-notation keys (e.g., joints.wrist.0, gripper.width).

Multi-Stream Example

A complete example with 2 cameras and 1 IMU, each in its own thread.

import threading
import syncfield as sf

session = sf.SyncSession(host_id="rig_01", output_dir="./sync_data")
session.start()

recording = True

def camera_loop(cam, stream_id, video_path):
    i = 0
    while recording:
        frame = cam.read()
        session.stamp(stream_id, frame_number=i)
        save_frame(frame, video_path)
        i += 1
    session.link(stream_id, video_path)

def imu_loop(imu, stream_id):
    i = 0
    while recording:
        data = imu.read()
        session.record(stream_id, frame_number=i, channels={
            "accel_x": data.ax, "accel_y": data.ay, "accel_z": data.az,
            "gyro_x": data.gx, "gyro_y": data.gy, "gyro_z": data.gz,
        })
        i += 1

threads = [
    threading.Thread(target=camera_loop, args=(cam_left, "cam_left", "/data/cam_left.mp4")),
    threading.Thread(target=camera_loop, args=(cam_right, "cam_right", "/data/cam_right.mp4")),
    threading.Thread(target=imu_loop, args=(imu_device, "imu")),
]

for t in threads:
    t.start()

# ... record for desired duration ...
recording = False

for t in threads:
    t.join()

counts = session.stop()
# counts == {"cam_left": 900, "cam_right": 900, "imu": 9000}

Output directory:

./sync_data/
  sync_point.json
  cam_left.timestamps.jsonl
  cam_right.timestamps.jsonl
  imu.timestamps.jsonl
  imu.jsonl
  manifest.json

Best Practices

Call stamp()/record() immediately after I/O read

The timestamp should reflect when data arrived on the host, not when processing finished.

# GOOD -- timestamp reflects when data arrived on the host
data = device.read()
session.stamp("sensor", frame_number=i)  # immediately after read

# BAD -- processing delay adds jitter to timestamp
data = device.read()
processed = expensive_transform(data)
session.stamp("sensor", frame_number=i)  # too late!

Use one thread per device

Each device should have its own thread with a tight read loop. Both stamp() and record() are thread-safe.

import threading

def camera_thread(cam, stream_id, session):
    i = 0
    while recording:
        frame = cam.read()
        session.stamp(stream_id, frame_number=i)
        i += 1

def sensor_thread(imu, stream_id, session):
    i = 0
    while recording:
        data = imu.read()
        session.record(stream_id, frame_number=i, channels={
            "accel_x": data.ax, "accel_y": data.ay, "accel_z": data.az,
        })
        i += 1

t1 = threading.Thread(target=camera_thread, args=(camera, "cam_left", session))
t2 = threading.Thread(target=sensor_thread, args=(imu_device, "imu", session))
t1.start()
t2.start()

Integration with SyncField Docker

Using manifest.json (recommended)

After stop(), the SDK writes a manifest.json that maps all streams to their files. Use it to construct the API request body programmatically.

import json
import requests

# Read the manifest produced by the SDK
with open("./sync_data/manifest.json") as f:
    manifest = json.load(f)

host_id = manifest["host_id"]

# Build the streams list from manifest entries
streams = []
for stream_id, info in manifest["streams"].items():
    stream_entry = {"stream_id": stream_id}

    if "path" in info:
        stream_entry["path"] = info["path"]

    if info.get("type") == "sensor":
        stream_entry["stream_type"] = "sensor"

    streams.append(stream_entry)

# Mark the first video stream as primary
for s in streams:
    entry = manifest["streams"][s["stream_id"]]
    if entry.get("type") == "video":
        s["is_primary"] = True
        break

# Submit to SyncField Docker
resp = requests.post("http://localhost:8080/api/v1/sync", json={
    "hosts": [
        {
            "host_id": host_id,
            "streams": streams,
        }
    ],
    "timestamps_dir": "/timestamps",
})
print(resp.json())  # {"job_id": "a1b2c3d4"}

Volume-mounted mode

Mount your data and timestamp directories into the container and call the API directly.

docker run -v ./data:/data -v ./sync_data:/timestamps \
  syncfield-app:latest

curl -X POST http://localhost:8080/api/v1/sync \
  -H "Content-Type: application/json" \
  -d '{
    "hosts": [
      {
        "host_id": "rig_01",
        "streams": [
          {"path": "/data/cam_left.mp4", "stream_id": "cam_left", "is_primary": true},
          {"path": "/data/cam_right.mp4", "stream_id": "cam_right"},
          {"stream_id": "imu", "stream_type": "sensor"}
        ]
      }
    ],
    "timestamps_dir": "/timestamps"
  }'

The service automatically matches {stream_id}.timestamps.jsonl and {stream_id}.jsonl files to streams using the timestamps_dir path.

File upload mode

Upload files directly without volume mounts. Use host_ids to group streams by host.

import requests

files = [
    ("files", open("cam_left.mp4", "rb")),
    ("files", open("cam_right.mp4", "rb")),
    ("timestamp_files", open("sync_data/cam_left.timestamps.jsonl", "rb")),
    ("timestamp_files", open("sync_data/cam_right.timestamps.jsonl", "rb")),
]
data = {
    "stream_ids": "cam_left,cam_right",
    "host_ids": "rig_01,rig_01",
    "primary_id": "cam_left",
}
resp = requests.post("http://localhost:8080/api/v1/sync/upload", files=files, data=data)
print(resp.json())  # {"job_id": "a1b2c3d4"}

Format Specification

This section defines the output format for implementors in other languages.

sync_point.json

{
  "sdk_version": "0.1.0",
  "monotonic_ns": 1234567890123456789,
  "wall_clock_ns": 1709890101000000000,
  "host_id": "rig_01",
  "timestamp_ms": 1709890101000,
  "iso_datetime": "2024-03-08T12:00:01.000000"
}

{stream_id}.timestamps.jsonl

One JSON object per line (no trailing comma, no array wrapper):

{"frame_number":0,"capture_ns":1234567890123456789,"clock_source":"host_monotonic","clock_domain":"rig_01","uncertainty_ns":5000000}
{"frame_number":1,"capture_ns":1234567890156789012,"clock_source":"host_monotonic","clock_domain":"rig_01","uncertainty_ns":5000000}

Field	Type	Description
frame_number	int	0-based sequential index
capture_ns	int	Monotonic nanoseconds at data arrival
clock_source	string	Always "host_monotonic" for SDK output
clock_domain	string	Must match host_id -- identifies the clock
uncertainty_ns	int	Timing uncertainty (default: 5000000 = 5ms)

Key rules:

• capture_ns must be monotonically non-decreasing within each stream
• clock_domain must be identical across all streams on the same host
• File name must be {stream_id}.timestamps.jsonl for auto-matching

{stream_id}.jsonl (Sensor Data)

One JSON object per line, combining timestamp and channel values:

{"frame_number":0,"capture_ns":1234567890123456789,"clock_source":"host_monotonic","clock_domain":"rig_01","uncertainty_ns":5000000,"channels":{"accel_x":0.12,"accel_y":-9.8,"accel_z":0.05}}
{"frame_number":1,"capture_ns":1234567890133456789,"clock_source":"host_monotonic","clock_domain":"rig_01","uncertainty_ns":5000000,"channels":{"accel_x":0.13,"accel_y":-9.7,"accel_z":0.06}}

Field	Type	Description
frame_number	int	0-based sequential index
capture_ns	int	Monotonic nanoseconds at data arrival (same clock as video timestamps)
clock_source	string	Origin of the timestamp (always "host_monotonic" for SDK)
clock_domain	string	Host identifier -- must match across all streams on the same host
uncertainty_ns	int	Timing uncertainty (default: 5000000 = 5ms)
channels	object	Sensor values as key-value pairs (e.g. {"accel_x": 0.12})

manifest.json

Written by stop(). Maps all streams in the session to their output files.

{
  "sdk_version": "0.1.0",
  "host_id": "rig_01",
  "streams": {
    "cam_left": {
      "type": "video",
      "timestamps_path": "cam_left.timestamps.jsonl",
      "frame_count": 900,
      "path": "/data/cam_left.mp4"
    },
    "cam_right": {
      "type": "video",
      "timestamps_path": "cam_right.timestamps.jsonl",
      "frame_count": 900,
      "path": "/data/cam_right.mp4"
    },
    "imu": {
      "type": "sensor",
      "sensor_path": "imu.jsonl",
      "timestamps_path": "imu.timestamps.jsonl",
      "frame_count": 9000
    }
  }
}

Field	Type	Description
sdk_version	string	SDK version that produced this file
host_id	string	Host identifier for this recording session
streams	object	Map of stream_id to stream metadata
streams.*.type	string	"video" or "sensor"
streams.*.timestamps_path	string	Relative path to the timestamps JSONL file
streams.*.frame_count	int	Number of frames/samples recorded
streams.*.path	string	(video only) Path set via link()
streams.*.sensor_path	string	(sensor only) Relative path to the sensor data JSONL file

License

Apache-2.0