Converter

In this section we will discuss the concept of a Converter. A converter can be used to convert them convert messages from one type to another. In this section we will go through the process of creating the Ndarray_Float32MultiArray Converter, which will allow to convert a Float32MultiArray to a numpy.ndarray.

The Converter base class has two class variables:

MSG_TYPE_A
MSG_TYPE_B

and has 4 abstract methods:

spec()
initialize()
A_to_B()
B_to_A()

MSG_TYPE_A and MSG_TYPE_B

The class variables MSG_TYPE_A and MSG_TYPE_B specify the two message types that will be converted from one into the other. In this example, MSG_TYPE_A will be of type numpy.ndarray. The second message type will be a Float32MultiArray, since this ROS message can be used for multidimensional data communication over ROS topics.

# ROS IMPORTS
from std_msgs.msg import Float32MultiArray

# RX IMPORTS
import eagerx.core.register as register
from eagerx import Converter
import numpy as np


class Ndarray_Float32MultiArray(Converter):
  MSG_TYPE_A = np.ndarray
  MSG_TYPE_B = Float32MultiArray

spec

The spec() method can be used to specify with which arguments the Converter will be initialized. In our case, we add dtype to the config.

@staticmethod
@register.spec("Ndarray_Float32MultiArray", Converter)
def spec(spec: ProcessorSpec, dtype="float32"):
    spec.config.dtype = dtype

Note

Mind the use of the spec() decorator.

initialize

Next, we implement the initialize() method. Here, the arguments is the ones we have just defined in the spec() method: dtype.

def initialize(self, dtype="float32"):
    self.dtype = dtype

A_to_B

The A_to_B() method takes as an argument a message of type MSG_TYPE_A and converts it into MSG_TYPE_B.

def A_to_B(self, msg):
  return Float32MultiArray(data=msg)

B_to_A

The B_to_A() method takes as an argument a message of type MSG_TYPE_B and converts it into MSG_TYPE_A. In our case, we convert the Float32MultiArray into a

def B_to_A(self, msg):
    return np.array(msg.data, dtype=self.dtype)

make

In order to use this Converter, the user should call the make() method with the arguments of the spec() method.