Solution when you want to use cv_bridge with python3 (virtualenv)


It was when I was messing around with ROS. Since RGB data is sent from the input node of the camera, I wanted to receive it at another node and was trying to use cv_bridge. It looks like below.

from cv_bridge import CvBridge

def prediction(msg):
    bridge = CvBridge()
    img = bridge.imgmsg_to_cv2(msg, "bgr8")

However, when I built it and ran it, I got the following error.

File "/opt/ros/melodic/lib/python2.7/dist-packages/cv_bridge/", line 91, in encoding_to_cvtype2
    from cv_bridge.boost.cv_bridge_boost import getCvType
ImportError: dynamic module does not define module export function (PyInit_cv_bridge_boost)

When I looked it up, it seemed that the cause was that cv_bridge was built with python2. Since my execution environment was python3 (virtualenv), I had to build cv_bridge from the source code.


First way

At first, I tried it by referring to the method of here. If my local python3 is the runtime environment, it might work, but it didn't work in my environment using virtualenv. I will share it for the time being.

$ cd catkin_ws
$ catkin config -DPYTHON_EXECUTABLE=/usr/bin/python3 -DPYTHON_INCLUDE_DIR=/usr/include/python3.5m -DPYTHON_LIBRARY=/usr/lib/aarch64-linux-gnu/
$ catkin config --install
$ git clone src/vision_opencv
$ apt-cache show ros-melodic-cv-bridge | grep Version
$ cd src/vision_opencv/
#I think it was probably the lower version. apt-It came out in the cache show.
$ git checkout 1.13.0
$ cd ../../
$ catkin build cv_bridge
$ source install/setup.bash --extend

In my environment using virtualenv, cv_bridge is built successfully, but when I build other nodes of my own with catkin_ws together after building, the following error occurs.

RLException: [sample.launch] is neither a launch file in package [sample_proc] nor is [sample_proc] a launch file name

I'm quite addicted to the swamp from here. Share how you have solved it to reduce casualties.

Final solution

The repository environment of here used cv_bridge with python3 on virtualenv like myself, so I got a hint from that. There are two important things. One is to build cv_bridge in a different workspace than your own. The other is to remove the python2.7 path before the line importing cv_bridge in my script. The specific procedure is as follows.

$ sudo apt install python-catkin-tools python3-dev python3-catkin-pkg-modules python3-numpy python3-yaml ros-melodic-cv-bridge
$ mkdir -p cv_bridge_ws/src
$ git clone src/vision_opencv
$ apt-cache show ros-melodic-cv-bridge | grep Version
$ cd src/vision_opencv/
# apt-The version value changes depending on the result of cache show
$ git checkout 1.13.0
$ cd ../../
#Depending on the environment, DPYTHON_LIBRARY aarch64-linux-The gnu part changes
$ catkin config -DCMAKE_BUILD_TYPE=Release -DPYTHON_EXECUTABLE=/usr/bin/python3 -DPYTHON_INCLUDE_DIR=/usr/include/python3.6m -DPYTHON_LIBRARY=/usr/lib/aarch64-linux-gnu/
$ catkin build
$ source devel/setup.bash --extend
$ cd ../catkin_ws
#Build your own node
$ catkin build
$ source devel/setup.bash

Also change your own script.

import sys
from cv_bridge import CvBridge

def prediction(msg):
    bridge = CvBridge()
    img = bridge.imgmsg_to_cv2(msg, "bgr8")

This will cause python3's cv_bridge to be called at runtime instead of python2's cv_bridge.


I don't like melodic anymore (laughs). I want to use noetic or ROS2.

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