python 3.x - Unable to use cv_bridge with ROS Kinetic and Python3

Question

I had a computer vision project on Ubuntu 14.04 with ROS indigo and python3, then I had to move on Ubuntu 16.04 with ROS kinetic. Here I ran into multiple issues:

1) I installed opencv, but couldn't import it in python3, the error message was:

Traceback (most recent call last):
  File "test.py", line 2, in <module>
    import cv2
ImportError: /opt/ros/kinetic/lib/python2.7/dist-packages/cv2.so: 
undefined symbol: PyCObject_Type

What I figured out is just rename cv2.so

cd /opt/ros/kinetic/lib/python2.7/dist-packages/
sudo mv cv2.so cv2_ros.so

Then I was able to import cv2 and use it

2) Then I couldn't import rospy, but installation of python3-catkin-pkg-modules and python3-rospkg-modules fixed that problem

3) Finally I ran into an issue with cv_bridge it says:

[ERROR] [1520780674.845066]: bad callback: <bound method ViewsBuffer.update of <__main__.ViewsBuffer object at 0x7f5f45a07f28>>
Traceback (most recent call last):
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rospy/topics.py", line 750, in _invoke_callback
    cb(msg)
  File "test.py", line 48, in update
    im = self.bridge.imgmsg_to_cv2(im, "bgr8")
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/cv_bridge/core.py", line 163, in imgmsg_to_cv2
    dtype, n_channels = self.encoding_to_dtype_with_channels(img_msg.encoding)
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/cv_bridge/core.py", line 99, in encoding_to_dtype_with_channels
    return self.cvtype2_to_dtype_with_channels(self.encoding_to_cvtype2(encoding))
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/cv_bridge/core.py", 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)

I believe the problem is in the cv_bridge_boost.so file. I also tried to build cv_bridge from https://github.com/ros-perception/vision_opencv, but it automatically builds for python2.7, I was trying to modify CMakeLists.txt a little bit to specify python3 there, but I don't have much experience with CMakeLists, so it didn't work. I also tried to copy cv_bridge module to my project folder, but it didn't change much and it still points on that cv_bridge_boost.so file. Another thing to mention is that cv_bridge works fine with python2.7, but my project requires python3.5.

Answer 1

You are right, you should build cv_bridge with python3.

You can do it with passing

-DPYTHON_EXECUTABLE=/usr/bin/python3 -DPYTHON_INCLUDE_DIR=/usr/include/python3.5m -DPYTHON_LIBRARY=/usr/lib/x86_64-linux-gnu/libpython3.5m.so

args to cmake. Or, if you are using catkin to build packages, you can do next steps:

# `python-catkin-tools` is needed for catkin tool
# `python3-dev` and `python3-catkin-pkg-modules` is needed to build cv_bridge
# `python3-numpy` and `python3-yaml` is cv_bridge dependencies
# `ros-kinetic-cv-bridge` is needed to install a lot of cv_bridge deps. Probaply you already have it installed.
sudo apt-get install python-catkin-tools python3-dev python3-catkin-pkg-modules python3-numpy python3-yaml ros-kinetic-cv-bridge
# Create catkin workspace
mkdir catkin_workspace
cd catkin_workspace
catkin init
# Instruct catkin to set cmake variables
catkin config -DPYTHON_EXECUTABLE=/usr/bin/python3 -DPYTHON_INCLUDE_DIR=/usr/include/python3.5m -DPYTHON_LIBRARY=/usr/lib/x86_64-linux-gnu/libpython3.5m.so
# Instruct catkin to install built packages into install place. It is $CATKIN_WORKSPACE/install folder
catkin config --install
# Clone cv_bridge src
git clone https://github.com/ros-perception/vision_opencv.git src/vision_opencv
# Find version of cv_bridge in your repository
apt-cache show ros-kinetic-cv-bridge | grep Version
    Version: 1.12.8-0xenial-20180416-143935-0800
# Checkout right version in git repo. In our case it is 1.12.8
cd src/vision_opencv/
git checkout 1.12.8
cd ../../
# Build
catkin build cv_bridge
# Extend environment with new package
source install/setup.bash --extend

And

$ python3

Python 3.5.2 (default, Nov 23 2017, 16:37:01) 
[GCC 5.4.0 20160609] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from cv_bridge.boost.cv_bridge_boost import getCvType
>>> 

If you encountered an next error

CMake Error at /usr/share/cmake-3.5/Modules/FindBoost.cmake:1677 (message):
  Unable to find the requested Boost libraries.

  Boost version: 1.58.0

  Boost include path: /usr/include

  Could not find the following Boost libraries:

          boost_python3

  No Boost libraries were found.  You may need to set BOOST_LIBRARYDIR to the
  directory containing Boost libraries or BOOST_ROOT to the location of
  Boost.
Call Stack (most recent call first):
  CMakeLists.txt:11 (find_package)

It is because CMake tries to find libboost_python3.so library, but in ubuntu it is libboost_python-py35.so(/usr/lib/x86_64-linux-gnu/libboost_python-py35.so), so you should change line

find_package(Boost REQUIRED python3)

to

find_package(Boost REQUIRED python-py35)

in file src/vision_opencv/cv_bridge/CMakeLists.txt and rebuild package.