Preprocessing FunctionsΒΆ

Preprocessing Functions are introduced as a mean to optimize on expensive computations on datapacks attached to a single engine. In some cases there might be the need for applying the same operations on a particular datapack in multiple TFs. An example of this might be to apply a filter to a datapack containing an image coming from a physics simulator. In order to allow to execute this operations just once and let other TFs to access the processed datapack data, PreprocessingFunctions (PFs) are introduced.

They are similar to Transceiver Functions both in implementation and behavior. Both are Python functions, their input and output are DataPacks and they are linked to an specific engine. PFs are also executed if and only if its linked Engine is synchronized.

They show two main differences with respect to TFs:

  • Their output datapacks are not sent to the corresponding Engines, they are kept in a local datapack cache and can be used as input in TFs

  • PFs just can take input datapacks from the Engine they are linked to

The latter is necessary to guarantee that new datapacks retrieved from a particular Engine are always processed by its connected PFs. In this way PFs can be thought as simple filters that read and transform datapacks coming from a certain Engine and store the processed data in the local datapack cache.

To declare a function as PreprocessingFunction, the decorator:


must be prepended to its definition.

In order to use the datapacks returned by PFs in other TFs, a dedicated decorator is available and must be used:

@PreprocessedDataPack(keyword, id)

The difference between this decorator and EngineDataPack is that with the latter it is indicated that the datapack should be requested from its linked engine. While the use the PreprocessedDataPack decorator tells that the datapack can be directly taken from the local datapack cache.

Since the output of PFs is stored in the local cache and does not need to process on the Engine Server side, PFs can return any type of DataPack without restrictions.

There is a DataPack type particularly convenient to use as PF output: JsonDataPack. This type of datapack stores a JSON object, and thus any type of data can be attached to it. Below is an example taking a camera image from Gazebo and returning the processed data as a JsonDataPack object.

from nrp_core import *
from import *
import numpy as np
from PIL import Image
import time
import cv2

@EngineDataPack(keyword='camera', id=DataPackIdentifier('husky::eye_vision_camera::camera', 'gazebo'))
def detect_red(camera):
    Performs a very simple image detection as used in the Braitenberg demo.
    Copied and modified from original NRP, see hbp_nrp_cle/hbp_nrp_cle/tf_framework/

    :param camera: The DataPack containing the image to process
    :returns: A JsonDataPack with three properties:
        - *left*: This is the percentage of red pixels in the left half of the image
        - *right*: This is the percentage of red pixels in the right half of the image
        - *go_on*: This is the percentage of non-red pixels of the overall image

    :example: A completely red image (255,0,0) results in (1,1,0)
    :example: A completely yellow image (255,255,0) results in (0,0,1)

    The lightest color that is recognized as red is (255,127,127).

    red_left = red_right = green_blue = 0
    if not camera.isEmpty():

        # Set to True to display camera image data and pause for 10 s
        show_image = False
        if show_image:
            d = np.frombuffer(, np.uint8)
            cv_image = d.reshape((,, 3))
            img = Image.fromarray(cv_image)

        lower_red = np.array([0, 30, 30])
        upper_red = np.array([0, 255, 255])

        # Reshape to proper size
        d = np.frombuffer(, np.uint8)
        cv_image = d.reshape((,, 3))

        # Transform image to HSV (easier to detect colors).
        hsv_image = cv2.cvtColor(cv_image, cv2.COLOR_RGB2HSV)

        # Create a mask where every non red pixel will be a Zero.
        mask = cv2.inRange(hsv_image, lower_red, upper_red)
        image_size = (cv_image.shape[0] * cv_image.shape[1])
        if image_size > 0:
            half = cv_image.shape[1] // 2
            # Get the number of red pixels in the image.
            red_left = cv2.countNonZero(mask[:, :half])
            red_right = cv2.countNonZero(mask[:, half:])
            green_blue = (image_size - (red_left + red_right)) / image_size
            # We have to multiply the rate by two since it is for an half image only.
            red_left = 2 * (red_left / image_size)
            red_right = 2 * (red_right / image_size)

    processed_image = JsonDataPack("processed_image", "gazebo")["left"] = red_left["right"] = red_right["go_on"] = green_blue

    # print("------------------")
    # print("Left Red:  {}".format(["left"]))
    # print("Right Red: {}".format(["right"]))
    # print("Go On:     {}".format(["go_on"]))

    return [processed_image]