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add Python examples for image_transport_py (#17)
Co-authored-by: Alejandro Hernández Cordero <ahcorde@gmail.com>
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‎README.md‎

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# image_transport_tutorials
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# Table of Contents
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1. [Installation](#installation)
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2. [Writing a Simple Image Publisher (C++)](#cpp_simple_image_pub)
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3. [Writing a Simple Image Subscriber (C++)](#cpp_simple_image_sub)
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4. [Running the Simple Image Publisher and Subscriber with Different Transport](#different_publisher_subscriber)
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5. [Writing a Simple Image Publisher (Python)](#py_simple_image_pub)
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6. [Writing a Simple Image Subscriber (Python)](#py_simple_image_sub)
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## Installation
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Before starting any of the tutorials below, create a workspace and clone this repository so you can inspect and manipulate the code:
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```
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Make sure to include the correct setup file (in the above example it is for Iron on Ubuntu and for bash).
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## Writing a Simple Image Publisher (C++)
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## Writing a Simple Image Publisher (C++) <a name="cpp_simple_image_pub"/>
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Description: This tutorial shows how to create a publisher node that will continually publish an image.
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Tutorial Level: Beginner
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Take a look at [my_publisher.cpp](src/my_publisher.cpp).
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Take a look at [my_publisher.cpp](image_transport_tutorials/src/my_publisher.cpp).
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### The code explained
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Now, let's break down the code piece by piece.
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(For example: multiple webcams mounted on a robot record the scene around it and you have to pass the image data to some other node for further analysis).
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The publisher example can be modified quite easily to make it work with a video device supported by `cv::VideoCapture` (in case it is not, you have to handle it accordingly).
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Take a look at [publisher_from_video.cpp](src/publisher_from_video.cpp) to see how a video device can be passed in as a command line argument and used as the image source.
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Take a look at [publisher_from_video.cpp](image_transport_tutorials/src/publisher_from_video.cpp) to see how a video device can be passed in as a command line argument and used as the image source.
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If you have a single device, you do not need to do the whole routine with passing a command line argument.
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In this case, you can hard-code the index/address of the device and directly pass it to the video capturing structure in OpenCV (example: `cv::VideoCapture(0)` if `/dev/video0` is used).
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Multiple checks are also included here to make sure that the publisher does not break if the camera is shut down.
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If the retrieved frame from the video device is not empty, it will then be converted to a ROS message which will be published by the publisher.
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## Writing a Simple Image Subscriber (C++)
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## Writing a Simple Image Subscriber (C++) <a name="cpp_simple_image_sub"/>
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Description: This tutorial shows how to create a subscriber node that will display an image on the screen.
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By using the `image_transport` subscriber to subscribe to images, any image transport can be used at runtime.
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To learn how to actually use a specific image transport, see the next tutorial.
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Tutorial Level: Beginner
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Take a look at [my_subscriber.cpp](src/my_subscriber.cpp).
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Take a look at [my_subscriber.cpp](image_transport_tutorials/src/my_subscriber.cpp).
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### The code explained
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Now, let's break down the code piece by piece.
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In just a few lines of code, we have written a ROS image viewer that can handle images in both raw and a variety of compressed forms.
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## Running the Simple Image Publisher and Subscriber with Different Transports
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## Running the Simple Image Publisher and Subscriber with Different Transports <a name="different_publisher_subscriber"/>
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Description: This tutorial discusses running the simple image publisher and subscriber using multiple transports.
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Tutorial Level: Beginner
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```
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This should display 15.
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## Writing a Simple Image Publisher (Python) <a name="py_simple_image_pub"/>
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Description: This tutorial shows how to create a publisher node that will continually publish an image with random contents from Python.
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Tutorial Level: Beginner
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Take a look at [my_publisher.py](image_transport_tutorials_py/image_transport_tutorials_py/my_publisher.py).
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To publish images using `image_transport_py`, you create an `ImageTransport` object and use it to advertise an image topic. The first parameter for `ImageTransport` is the image transport
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node's name which needs to be unique in the namespace.
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Steps:
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1. Import Necessary Modules:
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```python
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import rclpy
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from rclpy.node import Node
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from sensor_msgs.msg import Image
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from image_transport_py import ImageTransport
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```
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2. Initialize the Node and ImageTransport:
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```python
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def __init__(self):
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super().__init__('my_publisher')
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self.image_transport = ImageTransport(
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'imagetransport_pub', image_transport='compressed'
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)
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self.img_pub = self.image_transport.advertise('camera/image', 10)
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self.bridge = CvBridge()
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timer_period = 0.5
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self.timer = self.create_timer(timer_period, self.timer_callback)
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```
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3. Publish Images in the Callback:
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```python
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def publish_image(self):
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image_msg = .. # Read image from your devices
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image_msg.header.stamp = self.get_clock().now().to_msg()
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self.publisher.publish(image_msg)
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```
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`advertise_camera` can publish `CameraInfo` along with `Image` message.
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## Writing a Simple Image Subscriber (Python) <a name="py_simple_image_sub"/>
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Description: This tutorial shows how to create a subscriber node that will receive the contents of the published
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image. By using the `image_transport` subscriber to subscribe to images, any image transport can be used at runtime.
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Tutorial Level: Beginner
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Take a look at [my_subscriber.py](image_transport_tutorials_py/image_transport_tutorials_py/my_subscriber.py).
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To subscribe to images, use `ImageTransport` to create a subscription to the image topic.
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Steps:
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1. Import Necessary Modules:
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```python
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import rclpy
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from rclpy.node import Node
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from image_transport_py import ImageTransport
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```
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2. Initialize the Node and ImageTransport:
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```python
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class MySubscriber(Node):
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def __init__(self):
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super().__init__('my_subscriber')
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image_transport = ImageTransport(
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'imagetransport_sub', image_transport='compressed'
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)
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image_transport.subscribe('camera/image', 10, self.image_callback)
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```
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3. Handle Incoming Images:
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```python
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def image_callback(self, msg):
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self.get_logger().info('got a new image from frame_id:=%s' % msg.header.frame_id)
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```
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`subscribe_camera` will add `CameraInfo` along with `Image` message for the callback.
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### Transport Selection
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By default, `image_transport` uses the `raw` transport. You can specify a different transport by passing `image_transport` parameter to `ImageTransport`. Alternatively,
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you can use your own ROS2 parameter file for the imagetransport node via `launch_params_filepath` parameter.
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CMakeLists.txt renamed to image_transport_tutorials/CMakeLists.txt

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‎image_transport_tutorials/LICENSE‎

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include/image_transport_tutorials/resized_publisher.hpp renamed to image_transport_tutorials/include/image_transport_tutorials/resized_publisher.hpp

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include/image_transport_tutorials/resized_subscriber.hpp renamed to image_transport_tutorials/include/image_transport_tutorials/resized_subscriber.hpp

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