Why does this package need opencv3-opt?

[jwhendy]

$ pacman -Qi opencv
Name            : opencv
Version         : 4.2.0-1
Required By     : ros-melodic-image-geometry

$ pacman -Qi opencv3-opt
Name            : opencv3-opt
Version         : 3.4.7-2
Required By     : ros-melodic-cv-bridge

I'm trying to build a colleague's code and get errors like this:

Errors     << foo:make /home/jwhendy/catkin_ws/logs/foo/build.make.012.log                                                                                                
/usr/bin/ld: CMakeFiles/foo.cpp.o: in function `foo::foo::foo()':
foo.cpp:(.text+0x2592): undefined reference to `cv::namedWindow(cv::String const&, int)'
/usr/bin/ld: foo.cpp:(.text+0x25d7): undefined reference to `cv::setWindowProperty(cv::String const&, int, double)'
/usr/bin/ld: foo.cpp:(.text+0x2711): undefined reference to `cv::imshow(cv::String const&, cv::_InputArray const&)'
/usr/bin/ld: foo.cpp:(.text+0x2bde): undefined reference to `cv::imshow(cv::String const&, cv::_InputArray const&)'
/usr/bin/ld: foo.cpp:(.text+0x2f76): undefined reference to `cv::destroyWindow(cv::String const&)'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/foo.dir/build.make:242: /home/jwhendy/catkin_ws/devel/.private/foo/lib/foo] Error 1
make[1]: *** [CMakeFiles/Makefile2:1425: CMakeFiles/foo.dir/all] Error 2
make: *** [Makefile:141: all] Error 2

The closest I got was this ROS Answers post. One suggestion was to rearrange some stuff in CMakeLists.txt which didn't help. Another was to compile with -DWITH_OPENCL=OFF which I tried and didn't work. Nothing has changed the error.

The only other thing that stuck out to me was I wondered if my colleague used a different opencv version. I found this post about ld errors and in running this line, I found the results interesting:

$ pkg-config --cflags --libs opencv
Package opencv was not found in the pkg-config search path.
Perhaps you should add the directory containing `opencv.pc'
to the PKG_CONFIG_PATH environment variable
Package 'opencv', required by 'virtual:world', not found

$ PKG_CONFIG_PATH=/opt/opencv3/lib/pkgconfig pkg-config --cflags --libs opencv
-I/opt/opencv3/include/opencv -I/opt/opencv3/include -L/opt/opencv3/lib -lopencv_stitching -lopencv_superres -lopencv_videostab -lopencv_aruco -lopencv_bgsegm -lopencv_bioinspired -lopencv_ccalib -lopencv_dnn_objdetect -lopencv_dpm -lopencv_face -lopencv_freetype -lopencv_fuzzy -lopencv_hdf -lopencv_hfs -lopencv_img_hash -lopencv_line_descriptor -lopencv_optflow -lopencv_reg -lopencv_rgbd -lopencv_saliency -lopencv_stereo -lopencv_structured_light -lopencv_viz -lopencv_phase_unwrapping -lopencv_surface_matching -lopencv_tracking -lopencv_datasets -lopencv_text -lopencv_highgui -lopencv_videoio -lopencv_dnn -lopencv_plot -lopencv_xfeatures2d -lopencv_shape -lopencv_video -lopencv_ml -lopencv_ximgproc -lopencv_xobjdetect -lopencv_objdetect -lopencv_calib3d -lopencv_imgcodecs -lopencv_features2d -lopencv_flann -lopencv_xphoto -lopencv_photo -lopencv_imgproc -lopencv_core

Maybe this is due to running things on the cli from scratch vs. expecting that ROS packages will populate said PKG_CONFIG_PATH, but I still found it worth mentioning. Maybe the issue is in my catkin workspace?

In any case, this brought up the broader question: why are we mixing opencv versions within ros-melodic. This seems like a bad idea in my opinion.

[jwhendy]

This ultimately fixed my issue:

target_link_libraries(
  ${OpenCV_LIBS}
  /opt/opencv3/lib/libopencv_highgui.so
  ${catkin_LIBRARIES}
 )

This does support my theory that at least something is awry with this install. It might be an opencv3 issue, but I don't know how to track down what, exactly, is wrong. I just know that OpenCV_LIBS is not appearing to work. I'm wondering if it's linking to opencv vs. opencv3-opt stuff and getting confused.

---

edit: I figured out which library to link directly by grep-ing for the erroring functions, which all happened to come from libopencv_highgui.so.

[jwhendy]

Further detail. This is the include line in my colleague's file:

#include <opencv2/highgui.hpp>

I know almost zilch about cpp, includs, ld, etc. so I'm having a hard time figuring out how to see why this path doesn't work.

$ locate highgui
/opt/opencv3/include/opencv/highgui.h
/opt/opencv3/include/opencv2/highgui
/opt/opencv3/include/opencv2/highgui.hpp
/opt/opencv3/include/opencv2/highgui/highgui.hpp
/opt/opencv3/include/opencv2/highgui/highgui_c.h
/opt/opencv3/lib/libopencv_highgui.so
/opt/opencv3/lib/libopencv_highgui.so.3.4
/opt/opencv3/lib/libopencv_highgui.so.3.4.7
/usr/include/boost/compute/interop/opencv/highgui.hpp
/usr/include/opencv4/opencv2/highgui
/usr/include/opencv4/opencv2/highgui.hpp
/usr/include/opencv4/opencv2/highgui/highgui.hpp
/usr/include/opencv4/opencv2/highgui/highgui_c.h
/usr/lib/libopencv_highgui.so
/usr/lib/libopencv_highgui.so.4.2
/usr/lib/libopencv_highgui.so.4.2.0

So maybe back to the original theory: the .hpp is found for one version of opencv, but the linker is looking for the corresponding libopencv_highgui.so for a different version of opencv?

[acxz]

Just gonna drop the upstream issue here: ros-perception/vision_opencv#272

[jwhendy]

Nice. That may help, though we also may have an issue with a fully functioning ros-melodic ecosystem. My current questions:

• any input into how to understand exactly what's going on above? Mainly why is including OpenCV_LIBS insufficient and I have to point directly to the .so itself?
• if this is related to using both opencv3 and opencv4, what is the right solution?
• edited to add: is it troublesome to anyone else that the pkg-config command doesn't work as-is, and one needs to do PKG_CONFIG_PATH=/opt/opencv... for this to work?

[acxz]

I'm afraid I do not have the time to pry into the issues you have been facing.

The right solution I think is to use the latest opencv.

[jwhendy]

Completely understood. I'll leave this open as I think it's one thing to compile the package successfully, but another if there's something odd in our setup that doesn't let users actually do something they expect.

I may try building -cv-bridge with opencv4 using the breadcrumbs at that upstream thread, ditch opencv3-opt completely, and see if that helps.

[jhbruhn]

I did also run into troubles trying to compile packages which depend on both cv_bridge and image_geometry, resulting in a linking error with opencv functions. I fixed it by building image_geometry with opencv3 (specified in CMakeLists.txt). It did automatically pick up my OpenCV-4 installation which I had to prevent, although OpenCV3 ist listed as an explicit dependency!

[acxz]

Thanks for pointing that out @jhbruhn I believe this issue should be resolved with the noetic release since ROS will not target opencv3 anymore

[jhbruhn]

I agree, but shouldn't we be consistent with the OpenCV3 dependencies for melodic still? Or at least decide whether we

• depend on opencv3-opt and actually use it to build this package
• or depend on opencv and let CMake choose OpenCV4?

[acxz]

yeah we should be consistent esp considering that we get errors when compiling packages depending on it.

Feel free to create a PR regarding this!
I think the first option sounds more appealing.

[acxz]

With the merge I'll close this issue.