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Image Processing with ROS

Overview

This lecture is about how to interface ROS with cameras and vision processing libraries.

Cameras On Linux

  • Cameras on Linux are viewed as files (like any other device).
  • A driver in the Linux Kernel enables users to work with cameras using a common interface
  • The driver framework for USB cameras in Linux is called V4l2
    • Some machine-vision cameras use vendor-specific or proprietary Linux drivers rather than V4L2
    • Some machine-vision cameras don't provide drivers for Linux
  • USB cameras on Linux are usually called /dev/videoX, where X is a number
    • Some cameras create two video devices, the lower numbered one is usually the one to use
  • v4l2-ctl is a command line tool for interfacing with cameras. See man v4l2-ctl
  • v4l2-ctl --list-formats-ext prints useful information about the formats supported by cameras connected to your computer
    • You often need to specify these parameters to the ROS camera node, as parameters
  • v4l2-ctl --all prints all information about your cameras
  • v4l2-ctl --list-devices Lists all the usb cameras. Some cameras have multiple /dev/video* devices so it is useful to see what goes with what
  • When working with cameras you should use udev rules to give your cameras the proper permissions and a persistent name.
    • These rules are triggered when hardware is connected to the system and can (among other things) set permissions of the devices

ROS Camera "Drivers"

  • A ros driver is a node (or set of nodes) that interacts with hardware and provides a standard ROS interface
  • There are at least two packages that contain ROS drivers for interacting with USB cameras that operate via the v4l2 interface
  • usb_cam and v4l2_camera
    • Sometimes you need to experiment with camera drivers, as which one works better may depend on your hardware
  • Nodes in these package access your camera via the video device file (/dev/videoX)
    • Parameters control options for the camera (such as framerate and resolution)
    • Sometimes, you need to experiment with various options to get a usable webcam image, because every camera is different
  • The image display type in rviz can be used to see the video from a camera
  • However, all camera drivers ultimately provide a consistent ROS interface

Using usb_cam

  1. The usb_cam package can be used with many cameras
  2. The camera is one of the /dev/videoX devices, and can be specified with the video_device parameter
  3. Run the usb_cam node. A command with parameters that work well with the webcam on the System76 laptops are
    • ros2 run usb_cam usb_cam_node_exe --ros-args -p framerate:=30.0 -p pixel_format:=yuyv
    • Cameras have many settings that are important for machine vision. Use v4l2-ctl to view and tweak them
    • The camera_info_url parameter can be passed to provide a camera calibration file that for most cameras you will need to generate.
      • Without this file images can still be processed but pixel coordinates in the images cannot be converted to real world coordinates via ROS
  4. After running you can view the image in several ways
    1. Use rviz2 and add an Image view
    2. ros2 run usb_cam show_image.py (Image might be laggy at high resolutions)
    3. ros2 run rqt_image_view rqt_image_view
    4. ros2 run image_view image_view --ros-args -r image:=/topic_image_is_published_on to view the image

Using v4l2_camera

  • The v4l2_camera package is designed to expose many v4l2 camera options to ROS via parameters
    • It automatically enumerates all available v4l2 options (which vary by camera model) and exposes them
  • It can be used with ros2 run v4l2_camera v4l2_camera_node

The Image Pipeline

Image Pipeline

The image_pipeline contains several packages relating to image manipulation in ROS.

  • The idea is to chain various calibration and image processing steps together to complete computer vision tasks.
  • The pipeline also applies to stereo-vision cameras and 3D point clouds as well as monocular images.

Camera Calibration

camera_calibration finds the intrinsic parameters of a camera using a checkerboard pattern and OpenCV.

  • See this tutorial for information on how to calibrate a monocular camera in ROS 2.
  • The calibration can be stored and used to provide a camera_info topic so that the calibration can be used by other nodes.
    • The CameraInfo has useful information about camera calibration
  • Calibrations can be loaded by setting the camera_info_url parameter
    • Each driver may be different, camera_info_url is a ROS convention however
  • It is also possible to perform a stereo calibration for two cameras, which results in receiving depth information
  • The camera_check node can be used to check the calibration
    • ros2 run camera_calibration cameracheck --ros-args -r /monocular/image:=image_rect-r /monocular/camera_info:=camera_info

Example

Here is an example for how to start the camera calibration using usb_cam as the camera source. 

ros2 run usb_cam usb_cam_node_exe --ros-args -p framerate:=30 -p pixel_format:=yuyv -r __ns:=/camera
ros2 run camera_calibration cameracalibrator --size 8x6 --square 0.108 --ros-args -r camera/set_camera_info:=usb_cam/set_camera_info -r image:=/raw_image
ros2 launch image_proc image_proc.launch.py

MrCal

  1. The mrcal package provides routines for calibrating (and verifying the calibration) in a way that is much more rigorous and precise than the usual calibration methods

Image Proc

  • The image_proc package handles rectifying and debayering images.
  • Rectification is a method to compensate for camera lens distortion (which can make parallel lines appear curved in an image)
  • Debayering is a compensation that accounts for the individual color pixel layout on color cameras More Info
  • Run image_proc with ros2 launch image_proc image_proc.launch.py
    • This launchfile launches several component nodes (that is nodes that all run in the same process) that subscribe to /image_raw and /camera_info to create the rectified and debayerd images
  • image_proc publishes the following
    1. image_mono: A monochrome unrectified image
    2. image_rect: A monochrome rectified image
    3. image_color: A color unrectified image. These images are de-omsaiced (that is it accounts for the rgb pixel pattern of the camera)
    4. image_rect_color A color rectified image.
    5. The subtopics (e.g., image_rect/compressed) are various compressed versions of the image. These are referred to as transports.
    6. Transmitting uncompressed images takes a lot of bandwidth (even with modern networks), so compressed images are preferred
    7. However, there is no convenient way in python of subscribing to compressed images

Depth Images

depth_image_proc Processes depth image

  • In a depth image, each pixel corresponds to the depth (i.e., how far light has to travel before hitting something) in the scene.
  • Used by RGB-D cameras such as the intel RealSense.
  • Used to make point clouds (basically 3 dimensional pixelized images) from calibrated depth cameras

Other Image Tools

  1. image_transport determines how image data is sent across the system.

Rather than raw camera data, the image data can be compressed to use less bandwidth.

  • For python usage, the republish node is the most important.
    • It can subscribe to a compressed image, decompress it, and publish it as a raw image
    • Thus, you can publish a compressed image on a remote computer on the network, subscribe to it with the republish node, and have it republish the uncompressed image, allowing easier access from python.
  • web_video_server is used to create an http stream of video, useful if interfacing your robot to a website.

OpenCV

OpenCV is a major image processing library. Their website has many tutorials on how to process images.

Some packages that use/relate to OpenCV are:

  1. cv_bridge
    • Converts between OpenCV data and ROS messages.
    • If you want to use opencv with ROS, you should use this package to convert between OpenCV formats and ROS 2 image messages
  2. image_geometry Using a camera calibration,
    • lets you convert between pixel coordinates and the other frames in your system..
    • Python API documentation is not hosted properly but can be build locally with rosdoc2.
    • For those too lazy to build the documentation, the ROS 1 python API is substantially similar and is documented online Python API documentation

Not In ROS 2 (yet)

  1. opencv_apps Each "app" is a node that performs a single image processing function using opencv. Thus you can chain together many computer vision algorithms by connecting some nodes together in a launchfile.

Tag Tracking

Tags are special 2-Dimensional patterns that are designed to be viewed with a camera. Computer vision algorithms can then determine the 6 Degree of freedom pose of the tag and often an identifier (thus Tags are like QR codes but they also contain geometric information.

April Tags in ROS

  1. The apriltag_ros provides april-tag related functionality for ROS
    • This is a meta-package that also installs the ROS-independent apriltag library (ros-jazzy-apriltag) and messages for apriltags (ros-jazzy-apriltag-msgs).
  2. apriltag has documentation for apriltags.
  3. Choose a Tag Family
    • Download images here: https://github.com/AprilRobotics/apriltag-imgs
    • Resize the images and print them (for example, use gimp):
      • Choose Image->Scale Image
      • Set the Size of the image (choose the resolution and then set the size in real-world units (e.g. meters)
      • Choose "None" for interpolation
      • Print it out
      • It is important that the tag's scaling is correct or the tag will not work.
      • April tags must have a boarder around them. There are some tag families that have a white border and others that have a black border
  4. See apriltag documentation for more instructions on how to use them.

Other Image Packages and Libraries

  1. Darknet ROS 2 - realtime object detection (last released for foxy)
  2. Up to date list of visual SLAM solutions
  3. face_detector
  4. facial_recognition You will need to compile this since it was last released for ROS indigo.
  5. 2D tracking of multiple objects
  6. Find object 2D Feature detection to find objects
  7. YoloV8 wrapper for ROS 2

Other Learning Resources

Author: Matthew Elwin.