Ark Messages

The ark_msgs package provides a set of protobuf-based message definitions and lightweight Python helpers used throughout the Ark framework.

In ark_msgs, we use monkeypatching to add convenience methods directly to the generated protobuf message classes. Protobuf code is auto-generated and should not be edited by hand, but many applications benefit from rich, domain-specific helpers (e.g. quaternion and rotation conversions). Monkeypatching allows us to extend the Rotation message with a SciPy-compatible API while keeping the generated files untouched and fully regenerable. This provides a clean, familiar interface without introducing wrapper types or duplicating data structures.

Install

1. Create and activate a conda environment.
2. Clone this repository and change directory cd ark_msgs.
3. Install: pip install -e .

Documentation

Documentation for each message type supported in ark_msgs can be found below.

Table of Contents

• Core
• Geometry
• Sensor
• Control

Core

Core messages used by Ark.

Envelope

This is the message that is actually sent across the network.

Fields

• endpoint_type: the end point type that is handling the message
• channel: channel name that the mmessage was sent across
• src_node_name: the name of the source node
• dst_node_name: the name of the destination node
• sent_timestamp: the timestamp when the message was sent
• recv_timestamp: the timestamp when the message was recieved
• msg_type: the type of the payload message as a string
• payload: the payload of the message as bytes
• req_msg: the request message, this is only used in RESPONSE types

Helpers

• ArkMessage.pack(clock, msg)
  Packs a message as an ArkMessage with the timestamp from the clock (ark.clock.Clock).
• ArkMessage.from_sample(sample) Retreive the ArkMessage from the given Zenoh sample.

Usage

Envelope is not designed for user code, so should not be used.

String

String represents string data.

Fields

• data: string data

Helpers

• String.__str__()
  Use str() in-built method to convert a String message to a string.

Usage

from ark_msgs import String

s = String(data="hello, world")

s_str = str(s) # s_str is "hello, world"

Bool

Bool represents boolean data.

Fields

• data: boolean data

Helpers

• String.__bool__()
  Use bool() in-built method to convert a Bool message to a boolean.

Usage

from ark_msgs import Bool

b = Bool(data=True)

b_bool = bool(s) # b_bool is True

Geometry

Common message types involving geometry calculations. The helper methods for geometry message types are inspired and try to follow (as closely as possible) the scipy.spatial.transform module.

Translation

Translation represents a 3D translation vector with x, y, and z components.

Fields

• x: X component.
• y: Y component.
• z: Z component.

All numeric fields use float32.

Helpers

• Translation.from_array(array)
  Create a Translation from an array-like of shape (3,).

• t.as_array()
  Convert to a numpy.ndarray of shape (3,) with dtype float32.

• t1 + t2, t1 + array_like, array_like + t1
  Vector addition (returns Translation).

• t1 - t2, t1 - array_like, array_like - t1
  Vector subtraction (returns Translation).

Usage

import numpy as np
from ark_msgs.translation import Translation

t1 = Translation.from_array([1.0, 2.0, 3.0])
arr = t1.as_array()  # -> np.ndarray shape (3,), float32

t2 = Translation(x=0.5, y=0.0, z=-1.0)

t3 = t1 + t2
t4 = t1 - [1.0, 0.0, 0.0]
t5 = np.array([0.0, 1.0, 0.0], dtype=np.float32) + t1

Rotation

Rotation represents a 3D rotation as a unit quaternion with components x, y, z, and w.

Fields

• x: Quaternion x component.
• y: Quaternion y component.
• z: Quaternion z component.
• w: Quaternion w (scalar) component.

All numeric fields use float32.

Helpers

• Rotation.from_quat(quat, scalar_first=False)
  Create from a quaternion. Accepts [x, y, z, w] by default, or [w, x, y, z] if scalar_first=True.

• Rotation.from_matrix(matrix)
  Create from a 3×3 rotation matrix.

• Rotation.from_rotvec(rotvec, degrees=False)
  Create from a rotation vector.

• Rotation.from_mrp(mrp)
  Create from Modified Rodrigues Parameters (MRPs).

• Rotation.from_euler(seq, angles, degrees=False)
  Create from Euler angles.

• Rotation.from_davenport(axes, order, angles, degrees=False)
  Create from Davenport angles.

• r.as_quat(canonical=False, scalar_first=False)
  Convert to quaternion array.

• r.as_matrix()
  Convert to 3×3 rotation matrix.

• r.as_rotvec(degrees=False)
  Convert to rotation vector.

• r.as_mrp()
  Convert to MRPs.

• r.as_euler(seq, degrees=False, suppress_warnings=False)
  Convert to Euler angles.

• r.as_davenport(axes, order, degrees=False, suppress_warnings=False)
  Convert to Davenport angles.

• r.inv()
  Invert the rotation.

• r.magnitude()
  Return rotation magnitude in radians.

• Rotation.identity()
  Identity rotation.

• Rotation.random(rng=None)
  Random rotation (uniformly distributed), optionally using a NumPy Generator or integer seed.

• r1 * r2, r1 * Rot, Rot * r1
  Compose rotations using SciPy semantics: r = r1 * r2 applies r2 first, then r1. The result is always an ark_msgs.Rotation.

Usage

import numpy as np
from scipy.spatial.transform import Rotation as Rot
from ark_msgs.rotation import Rotation

# Construction
r1 = Rotation.from_euler("z", 90, degrees=True)
r2 = Rotation.from_quat([0.0, 0.0, 0.0, 1.0])

# Conversions
q = r1.as_quat()          # (x, y, z, w)
R = r1.as_matrix()        # 3x3
v = r1.as_rotvec()        # radians

# Composition (apply r2, then r1)
r3 = r1 * r2

# Works with SciPy rotations too (result is protobuf Rotation)
rs = Rot.from_euler("x", 30, degrees=True)
r4 = r1 * rs
r5 = rs * r1

# Inversion / magnitude
r_inv = r1.inv()
angle = r1.magnitude()

# Identity / random
r_id = Rotation.identity()
r_rand = Rotation.random(rng=np.random.default_rng(0))

RigidTransform

RigidTransform represents a rigid transform (translation + rotation) between two coordinate frames.

Fields

• translation: Translation component (ark_msgs.Translation).
• rotation: Rotation component (ark_msgs.Rotation).
• child_id: Child frame ID.
• parent_id: Parent frame ID.

Helpers

• RigidTransform.from_matrix(matrix, child_id="child", parent_id="parent")
  Create from a 4×4 homogeneous transform matrix.

• RigidTransform.from_rotation(rotation, child_id="child", parent_id="parent")
  Create from a rotation (protobuf Rotation or SciPy Rotation).

• RigidTransform.from_translation(translation, child_id="child", parent_id="parent")
  Create from a translation (protobuf Translation or array-like (3,)).

• RigidTransform.from_components(translation, rotation, child_id="child", parent_id="parent")
  Create from translation and rotation components.

• RigidTransform.from_exp_coords(exp_coords, child_id="child", parent_id="parent")
  Create from exponential coordinates (6,).

• RigidTransform.from_dual_quat(dual_quat, scalar_first=True, child_id="child", parent_id="parent")
  Create from a dual quaternion (8,).

• t.as_matrix()
  Convert to a 4×4 homogeneous transform matrix.

• t.as_components(proto=False)
  Return (translation, rotation) components. If proto=True, returns ark_msgs.Translation and ark_msgs.Rotation; otherwise returns SciPy-compatible arrays.

• t.as_exp_coords()
  Convert to exponential coordinates (6,).

• t.as_dual_quat(scalar_first=True)
  Convert to dual quaternion (8,).

• t1 * t2, t1 * matrix, matrix * t1
  Compose transforms (SciPy semantics). Frame IDs are not propagated; users should assign child_id and parent_id manually if needed.

• t.inv()
  Invert the transform (swaps parent_id and child_id).

Usage

import numpy as np
from ark_msgs.translation import Translation
from ark_msgs.rotation import Rotation
from ark_msgs.rigid_transform import RigidTransform

# Construction
t = Translation.from_array([1.0, 2.0, 3.0])
r = Rotation.from_euler("z", 90, degrees=True)

T1 = RigidTransform.from_components(
    translation=t,
    rotation=r,
    parent_id="world",
    child_id="camera",
)

# Conversions
M = T1.as_matrix()
tr_proto, r_proto = T1.as_components(proto=True)
exp = T1.as_exp_coords()
dq = T1.as_dual_quat()

# Composition (apply T2, then T1)
T2 = RigidTransform.from_translation([0.1, 0.0, 0.0])
T3 = T1 * T2

# Inversion
T_inv = T1.inv()

Twist

Twist roughly represents velocity.

Fields

• linear: Linear velocity (ark_msgs.Translation).
• angular: Angular velocity (ark_msgs.Translation).

Usage

from ark_msgs.twist import Twist

# Create from array [vx, vy, vz, wx, wy, wz]
t = Twist.from_array([1.0, 0.0, 0.0, 0.0, 0.0, 0.1])
arr = t.as_array()

Wrench

Wrench represents force and torque.

Fields

• force: Force (ark_msgs.Translation).
• torque: Torque (ark_msgs.Translation).

Usage

from ark_msgs.wrench import Wrench

# Create from array [fx, fy, fz, tx, ty, tz]
w = Wrench.from_array([10.0, 0.0, 0.0, 0.0, 0.0, 1.0])
arr = w.as_array()

Sensor

Common sensor message types.

JointState

JointState represents the joint states for a robot at an instance in time.

Fields

• name: Joint names.
• position: Joint positions (e.g. radians or meters).
• velocity: Joint velocities.
• effort: Joint efforts or currents (e.g. torque or force or amps)
• ext_torque: External torques acting on the robot joints.

All numeric fields use float32. Elements at the same index across fields refer to the same joint.

Usage

from ark_msgs.joint_state import JointState

js = JointState(
    name=["joint1", "joint2"],
    position=[0.1, 1.57],
    velocity=[0.0, 0.1],
    effort=[0.001, 0.002],
    effort=[-0.2, -0.1],
)

Image

Generic image message (supports RGB, BGR, Mono, etc.).

Fields

• height, width: Image dimensions.
• encoding: String encoding (e.g., "rgb8", "bgr8").
• step: Row length in bytes.
• data: Raw bytes.

Usage

from ark_msgs.image import Image
import numpy as np

# Create from numpy array
data = np.zeros((100, 100, 3), dtype=np.uint8)
img = Image.from_array(data, encoding="rgb8")

# Convert back to numpy
arr = img.as_array()

Joystick

Joystick input state.

Fields

• axes: Float values for axes.
• buttons: Integer values for buttons.

IMU

Inertial Measurement Unit data.

Fields

• orientation (Rotation).
• angular_velocity (Translation).
• linear_acceleration (Translation).

Control

ParallelGripperCommand

Command for parallel grippers.

Fields

• width: Target width (meters).
• max_force: Maximum force (Newtons).

JointArrayCommand

Command for a set of joints.

Fields

• name: List of joint names.
• value: List of command values.
• mode: POSITION, VELOCITY, or TORQUE.