Building ROS node with OpenCV



  • Hi,

    I'm trying to build a USB camera ROS node on the VOXL, that requires OpenCV and cv_bridge. I've installed the OpenCV 4.5.1 ipk using opkg, and see the libraries in /usr/lib64. I found the OpenCV cmake files so I added this path to the CMakeLists.txt:

    set(OpenCV_DIR /usr/lib64/cmake/opencv4)

    However, when running catkin_make, I get this error:

    CMake Error at /usr/lib64/cmake/opencv4/OpenCVModules.cmake:404 (message):
    The imported target "opencv_core" references the file

     "/usr/lib/libopencv_core.so.4.5.1"
    

    but this file does not exist.

    It seems like for some reason, the cmake file is looking for the modules in /usr/lib instead of /usr/lib64. What's the best way to fix this?





  • @Chad-Sweet is the voxl-mpa-exposure private? I don't seem to be able to access it.



  • Whoops, please check again



  • @Chad-Sweet Thanks, I will try it out!



  • I had some additional issues with build dependencies, so now I'm trying to build a docker that's a combination of the opencv-opencl one and the roskinetic one. However, I'm now running out of space on the overlays while building the opencv portion:

    symlink ../../Asia/Katmandu /data/overlay/0539659d5706b44f5832c3d53925cfe2647594de8f18b5708435028ddfdd630e/tmproot772012770/usr/share/zoneinfo/posix/Asia/Kathmandu: no space left on device

    However, running df -H, it doesn't appear to be out of space.

    Filesystem Size Used Avail Use% Mounted on
    /dev/root 3.0G 1.7G 1.3G 58% /
    devtmpfs 2.0G 0 2.0G 0% /dev
    tmpfs 2.0G 0 2.0G 0% /dev/shm
    tmpfs 2.0G 18M 2.0G 1% /run
    tmpfs 2.0G 0 2.0G 0% /sys/fs/cgroup
    tmpfs 2.0G 4.1k 2.0G 1% /var/volatile
    /dev/sda2 2.3M 82k 2.1M 4% /persist
    /dev/sde11 100M 35M 66M 35% /firmware
    /dev/sde12 13M 4.3M 7.7M 36% /dsp
    /dev/sda3 61M 41k 59M 1% /cache
    /dev/mmcblk0p1 32G 33k 32G 1% /mnt/sdcard
    /dev/sda9 16G 4.9G 11G 32% /data

    Do you know what might be causing this and how I can get around it? I have a 32 GB micro-sd card, is it possible to use that instead of /data?



  • You can actually build an ARM docker on your Linux desktop. The build process uses tons of space, so tough to say exactly what you're running in to. But, you can definitely try offline and then load it on target. (it will build MUCH faster too!)



  • @Chad-Sweet oh thanks! I'll try that you! I actually want to try to use Ubuntu 18.04 / ROS melodic to be more aligned with our other systems. Do you know if the modalai opencl deb would still be compatible?

    Also, it seems aarch64 is deprecated: https://hub.docker.com/r/aarch64/ubuntu
    for this: https://hub.docker.com/r/arm64v8/ubuntu

    For the base docker image, would it be correct to just change:
    FROM aarch64/ubuntu:xenial
    to:
    FROM arm64v8/ubuntu:bionic

    (Apologies for all the questions, it's actually my first time building a docker image from scratch.)



  • All of those seem likely to work, though we haven't tried them so there could be some debugging required.



  • is there something specific I need to enable in docker (on the amd64 machine side) to be able to build the arm64 docker images? I'm getting this error:

    scripts/build-docker.sh 4.2.0
    Building OpenCV 4.2.0
    Sending build context to Docker daemon 11.16MB
    Step 1/48 : FROM aarch64/ubuntu:xenial
    ---> a7d1ddc47ced
    Step 2/48 : RUN apt-get update
    ---> Running in 23be03a5aa45
    standard_init_linux.go:211: exec user process caused "no such file or directory"
    The command '/bin/sh -c apt-get update' returned a non-zero code: 1



  • @Chad-Sweet any advice on the above message? it doesn't seem to like trying to build an arm64 docker image on an amd64 ubuntu machine.


  • Dev Team

    @sarahl ,

    I've been messing with another project in a similar area, this link helped:

    https://github.com/multiarch/qemu-user-static

    Namely, needing to run this 'magic command' before running my arm64 docker on x86 host (to be honest I'm not fully sure what it's doing)

    docker run --rm --privileged multiarch/qemu-user-static --reset -p yes
    

    Another thing that's helped is adding this to my docker file:

    # required for ARM emulation
    COPY ./bin/qemu-arm-static /usr/bin/qemu-arm-static
    

    Where in my project, I've copied qemu-arm-static into a bin dir in my repo.

    Hope this helps!



  • Hi @modaltb ,

    As I had also storage problems like posted here, I wanted to go with the way suggested by @Chad-Sweet. I am doing the same kind of project. Merging two Docker files(Ros Kinetic and Open CV).

    I am building the images in Raspi aarch64 Ubuntu 20.04.2.
    I was able to build ROS Kinetic image without Open CV and send to VOXL m500 to load and run.

    However, when I merged two files and built I got the following error but am not sure if it is the same error with @sarahl and qemu-arm-static will help me.

    What might be the problem or solution?
    Thank you in advance,

    I pasted the output message and two relevant docker files I merged:
    I used all the docker files for ros kinetic and adapted Open CV files into that.

    General configuration for OpenCV 3.4.6 =====================================
    --   Version control:               unknown
    --
    --   Platform:
    --     Timestamp:                   2021-04-22T21:24:55Z
    --     Host:                        Linux 5.4.0-1034-raspi aarch64
    --     CMake:                       3.5.1
    --     CMake generator:             Unix Makefiles
    --     CMake build tool:            /usr/bin/make
    --     Configuration:               Release
    --
    --   CPU/HW features:
    --     Baseline:                    NEON FP16
    --       required:                  NEON
    --       disabled:                  VFPV3
    --
    --   C/C++:
    --     Built as dynamic libs?:      YES
    --     C++11:                       YES
    --     C++ Compiler:                /usr/bin/c++  (ver 5.4.0)
    --     C++ flags (Release):         -std=c++11 -march=armv8-a+simd   -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wno-delete-non-virtual-dtor -Wno-comment -fdiagnostics-show-option -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections    -fvisibility=hidden -fvisibility-inlines-hidden -fopenmp -O3 -DNDEBUG  -DNDEBUG
    --     C++ flags (Debug):           -std=c++11 -march=armv8-a+simd   -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wno-delete-non-virtual-dtor -Wno-comment -fdiagnostics-show-option -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections    -fvisibility=hidden -fvisibility-inlines-hidden -fopenmp -g  -O0 -DDEBUG -D_DEBUG
    --     C Compiler:                  /usr/bin/cc
    --     C flags (Release):           -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wuninitialized -Winit-self -Wno-comment -fdiagnostics-show-option -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections    -fvisibility=hidden -fopenmp -O3 -DNDEBUG  -DNDEBUG
    --     C flags (Debug):             -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wuninitialized -Winit-self -Wno-comment -fdiagnostics-show-option -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections    -fvisibility=hidden -fopenmp -g  -O0 -DDEBUG -D_DEBUG
    --     Linker flags (Release):      -Wl,--gc-sections
    --     Linker flags (Debug):        -Wl,--gc-sections
    --     ccache:                      NO
    --     Precompiled headers:         YES
    --     Extra dependencies:          dl m pthread rt
    --     3rdparty dependencies:
    --
    --   OpenCV modules:
    --     To be built:                 calib3d core dnn features2d flann highgui imgcodecs imgproc ml objdetect photo python2 stitching ts video videoio
    --     Disabled:                    shape superres videostab world
    --     Disabled by dependency:      -
    --     Unavailable:                 cudaarithm cudabgsegm cudacodec cudafeatures2d cudafilters cudaimgproc cudalegacy cudaobjdetect cudaoptflow cudastereo cudawarping cudev java js python3 viz
    --     Applications:                tests perf_tests apps
    --     Documentation:               NO
    --     Non-free algorithms:         NO
    --
    --   GUI:
    --     GTK+:                        YES (ver 2.24.30)
    --       GThread :                  YES (ver 2.48.2)
    --       GtkGlExt:                  NO
    --     VTK support:                 NO
    --
    --   Media I/O:
    --     ZLib:                        /usr/lib/aarch64-linux-gnu/libz.so (ver 1.2.8)
    --     JPEG:                        /usr/lib/aarch64-linux-gnu/libjpeg.so (ver 80)
    --     WEBP:                        build (ver encoder: 0x020e)
    --     PNG:                         /usr/lib/aarch64-linux-gnu/libpng.so (ver 1.2.54)
    --     TIFF:                        /usr/lib/aarch64-linux-gnu/libtiff.so (ver 42 / 4.0.6)
    --     JPEG 2000:                   build (ver 1.900.1)
    --     OpenEXR:                     build (ver 1.7.1)
    --     HDR:                         YES
    --     SUNRASTER:                   YES
    --     PXM:                         YES
    --
    --   Video I/O:
    --     DC1394:                      YES (ver 2.2.4)
    --     FFMPEG:                      YES
    --       avcodec:                   YES (ver 56.60.100)
    --       avformat:                  YES (ver 56.40.101)
    --       avutil:                    YES (ver 54.31.100)
    --       swscale:                   YES (ver 3.1.101)
    --       avresample:                NO
    --     GStreamer:                   NO
    --     libv4l/libv4l2:              NO
    --     v4l/v4l2:                    linux/videodev2.h
    --
    --   Parallel framework:            OpenMP
    --
    --   Trace:                         YES (built-in)
    --
    --   Other third-party libraries:
    --     Lapack:                      NO
    --     Eigen:                       NO
    --     Custom HAL:                  YES (carotene (ver 0.0.1))
    --     Protobuf:                    build (3.5.1)
    --
    --   OpenCL:                        YES (no extra features)
    --     Include path:                /home/root/opencv-opencl/opencv-3.4.6/3rdparty/include/opencl/1.2
    --     Link libraries:              Dynamic load
    --
    --   Python 2:
    --     Interpreter:                 /usr/bin/python2.7 (ver 2.7.12)
    --     Libraries:                   /usr/lib/aarch64-linux-gnu/libpython2.7.so (ver 2.7.12)
    --     numpy:                       /usr/lib/python2.7/dist-packages/numpy/core/include (ver 1.11.0)
    --     install path:                lib/python2.7/dist-packages/cv2/python-2.7
    --
    --   Python (for build):            /usr/bin/python2.7
    --
    --   Java:
    --     ant:                         NO
    --     JNI:                         NO
    --     Java wrappers:               NO
    --     Java tests:                  NO
    --
    --   Install to:                    /usr
    -- -----------------------------------------------------------------
    --
    -- Configuring incomplete, errors occurred!
    
    See also "/home/root/opencv-opencl/opencv-3.4.6/build/CMakeFiles/CMakeOutput.log".
    See also "/home/root/opencv-opencl/opencv-3.4.6/build/CMakeFiles/CMakeError.log".
    The command '/bin/sh -c cmake -DCMAKE_BUILD_TYPE=Release           -DCMAKE_INSTALL_PREFIX=/usr           -DWITH_OPENMP=ON           -DWITH_OPENCL=ON           -DHAVE_OPENCL=1           -DOPENCV_EXTRA_MODULES_PATH=/home/opencv-opencl/opencv_contrib-$opencv_version/modules           -DBUILD_opencv_aruco=ON           -DBUILD_opencv_bgsegm=OFF           -DBUILD_opencv_bioinspired=OFF           -DBUILD_opencv_ccalib=OFF           -DBUILD_opencv_cnn_3dobj=OFF           -DBUILD_opencv_cudaarithm=OFF           -DBUILD_opencv_cudabgsegm=OFF           -DBUILD_opencv_cudacodec=OFF           -DBUILD_opencv_cudafeatures2d=OFF           -DBUILD_opencv_cudafilters=OFF           -DBUILD_opencv_cudaimgproc=OFF           -DBUILD_opencv_cudalegacy=OFF           -DBUILD_opencv_cudaobjdetect=OFF           -DBUILD_opencv_cudaoptflow=OFF           -DBUILD_opencv_cudastereo=OFF           -DBUILD_opencv_cudawarping=OFF           -DBUILD_opencv_cudev=OFF           -DBUILD_opencv_cvv=OFF           -DBUILD_opencv_datasets=OFF           -DBUILD_opencv_dnn_objdetect=ON           -DBUILD_opencv_dnns_easily_fooled=OFF           -DBUILD_opencv_dnn_superres=OFF           -DBUILD_opencv_dpm=OFF           -DBUILD_opencv_face=OFF           -DBUILD_opencv_freetype=OFF           -DBUILD_opencv_fuzzy=OFF           -DBUILD_opencv_hdf=OFF           -DBUILD_opencv_hfs=OFF           -DBUILD_opencv_img_hash=OFF           -DBUILD_opencv_line_descriptor=OFF           -DBUILD_opencv_matlab=OFF           -DBUILD_opencv_optflow=OFF           -DBUILD_opencv_ovis=OFF           -DBUILD_opencv_phase_unwrapping=OFF           -DBUILD_opencv_plot=OFF           -DBUILD_opencv_quality=OFF           -DBUILD_opencv_reg=OFF           -DBUILD_opencv_rgbd=OFF           -DBUILD_opencv_saliency=OFF           -DBUILD_opencv_sfm=OFF           -DBUILD_opencv_shape=OFF           -DBUILD_opencv_stereo=OFF           -DBUILD_opencv_structured_light=OFF           -DBUILD_opencv_superres=OFF           -DBUILD_opencv_surface_matching=OFF           -DBUILD_opencv_text=OFF           -DBUILD_opencv_tracking=OFF           -DBUILD_opencv_videostab=OFF           -DBUILD_opencv_viz=OFF           -DBUILD_opencv_xfeatures2d=OFF           -DBUILD_opencv_ximgproc=OFF           -DBUILD_opencv_xobjdetect=OFF           -DBUILD_opencv_xphoto=OFF           -DCMAKE_CXX_FLAGS="${CMAKE_CXX_FLAGS} -std=c++11 -march=armv8-a+simd" ..' returned a non-zero code: 1
    

    Dockerfile.xenial-base

    FROM aarch64/ubuntu:xenial
    
    RUN apt-get update
    RUN apt-get -y upgrade
    RUN sh -c 'echo "deb http://packages.ros.org/ros/ubuntu xenial main" > /etc/apt/sources.list.d/ros-latest.list'
    RUN apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654
    RUN apt-get update && apt-get install -y python-rosdep \
      python-pip \
      python-rosinstall-generator \
      python-wstool \
      python-rosinstall \
      python-pandas \
      python-numpy \
      python-dev \
      python-defusedxml \
      build-essential \
      libboost-all-dev \
      libconsole-bridge-dev \
      libtinyxml-dev \
      sbcl-source \
      sbcl-doc \
      vim \
      sudo \
      nano \
      strace \
      libglib2.0-0 \
      libgtk2.0-dev \
      pkg-config \
      libavcodec-dev \
      libavformat-dev \
      libswscale-dev \
      libtbb2 \
      libtbb-dev \
      libjpeg-dev \
      libpng-dev \
      libtiff-dev \
      libdc1394-22-dev \
      wget \
      unzip \
      cmake \
      git
    
    RUN mkdir -p /home/root/opencv-opencl
    
    WORKDIR /home/root/opencv-opencl
    
    COPY modalai-opencl-dev_1.0-1.deb .
    
    RUN dpkg -i modalai-opencl-dev_1.0-1.deb
    
    
    CMD /bin/bash
    
    

    Dockerfile.opencv

    FROM roskinetic-xenial:base
    
    ARG opencv_version=4.2.0
    
    WORKDIR /home/root/opencv-opencl
    
    RUN wget https://github.com/opencv/opencv/archive/$opencv_version.zip
    
    RUN unzip $opencv_version.zip
    RUN rm $opencv_version.zip
    
    RUN wget https://github.com/opencv/opencv_contrib/archive/$opencv_version.zip
    
    RUN unzip $opencv_version.zip
    RUN rm $opencv_version.zip
    
    WORKDIR opencv-$opencv_version/build
    
    RUN cmake -DCMAKE_BUILD_TYPE=Release \
              -DCMAKE_INSTALL_PREFIX=/usr \
              -DWITH_OPENMP=ON \
              -DWITH_OPENCL=ON \
              -DHAVE_OPENCL=1 \
              -DOPENCV_EXTRA_MODULES_PATH=/home/opencv-opencl/opencv_contrib-$opencv_version/modules \
              -DBUILD_opencv_aruco=ON \
              -DBUILD_opencv_bgsegm=OFF \a
              -DBUILD_opencv_bioinspired=OFF \
              -DBUILD_opencv_ccalib=OFF \
              -DBUILD_opencv_cnn_3dobj=OFF \
              -DBUILD_opencv_cudaarithm=OFF \
              -DBUILD_opencv_cudabgsegm=OFF \
              -DBUILD_opencv_cudacodec=OFF \
              -DBUILD_opencv_cudafeatures2d=OFF \
              -DBUILD_opencv_cudafilters=OFF \
              -DBUILD_opencv_cudaimgproc=OFF \
              -DBUILD_opencv_cudalegacy=OFF \
              -DBUILD_opencv_cudaobjdetect=OFF \
              -DBUILD_opencv_cudaoptflow=OFF \
              -DBUILD_opencv_cudastereo=OFF \
              -DBUILD_opencv_cudawarping=OFF \
              -DBUILD_opencv_cudev=OFF \
              -DBUILD_opencv_cvv=OFF \
              -DBUILD_opencv_datasets=OFF \
              -DBUILD_opencv_dnn_objdetect=ON \
              -DBUILD_opencv_dnns_easily_fooled=OFF \
              -DBUILD_opencv_dnn_superres=OFF \
              -DBUILD_opencv_dpm=OFF \
              -DBUILD_opencv_face=OFF \
              -DBUILD_opencv_freetype=OFF \
              -DBUILD_opencv_fuzzy=OFF \
              -DBUILD_opencv_hdf=OFF \
              -DBUILD_opencv_hfs=OFF \
              -DBUILD_opencv_img_hash=OFF \
              -DBUILD_opencv_line_descriptor=OFF \
              -DBUILD_opencv_matlab=OFF \
              -DBUILD_opencv_optflow=OFF \
              -DBUILD_opencv_ovis=OFF \
              -DBUILD_opencv_phase_unwrapping=OFF \
              -DBUILD_opencv_plot=OFF \
              -DBUILD_opencv_quality=OFF \
              -DBUILD_opencv_reg=OFF \
              -DBUILD_opencv_rgbd=OFF \
              -DBUILD_opencv_saliency=OFF \
              -DBUILD_opencv_sfm=OFF \
              -DBUILD_opencv_shape=OFF \
              -DBUILD_opencv_stereo=OFF \
              -DBUILD_opencv_structured_light=OFF \
              -DBUILD_opencv_superres=OFF \
              -DBUILD_opencv_surface_matching=OFF \
              -DBUILD_opencv_text=OFF \
              -DBUILD_opencv_tracking=OFF \
              -DBUILD_opencv_videostab=OFF \
              -DBUILD_opencv_viz=OFF \
              -DBUILD_opencv_xfeatures2d=OFF \
              -DBUILD_opencv_ximgproc=OFF \
              -DBUILD_opencv_xobjdetect=OFF \
              -DBUILD_opencv_xphoto=OFF \
              -DCMAKE_CXX_FLAGS="${CMAKE_CXX_FLAGS} -std=c++11 -march=armv8-a+simd" ..
    
    RUN make -j4
    RUN make install
    
    WORKDIR /home/root/opencv-opencl
    
    CMD ["/bin/bash"]
    
    


  • I overcame that problem by adjusting the WORKDIR and PATH. Moreover adding sudo apt-update.

    However, I always come back to the same problem no matter what i do.
    I decided to build in my host computer and load in target to head off "No Space Problem" in the target but the problem also occurred in the host as well. I cannot proceed any how.

    Any suggestions from your side?

    /home/opencv-opencl/opencv-3.4.6/build/modules/calib3d/test_precomp.hpp:20:1: fatal error: can't write PCH file: No space left on device
     } // namespace
     ^
    compilation terminated.
    In file included from /home/opencv-opencl/opencv-3.4.6/build/modules/features2d/perf_precomp.hpp:5:0:
    /home/opencv-opencl/opencv-3.4.6/modules/features2d/include/opencv2/features2d.hpp:1426:1: fatal error: can't write PCH file: No space left on device
     } /* namespace cv */
     ^
    
    make: *** [all] Error 2
    The command '/bin/sh -c make -j4' returned a non-zero code: 2
    
    REPOSITORY                   TAG       IMAGE ID       CREATED             SIZE
    <none>                       <none>    f1a9b038119e   18 minutes ago      1.81GB
    roskinetic-xenial            v1.0      e76c6e4f806e   About an hour ago   1.98GB
    roskinetic-xenial            minimal   fcdda3d30808   3 hours ago         1.56GB
    roskinetic-xenial            base      d22679ae8e50   3 hours ago         1.37GB
    multiarch/qemu-user-static   latest    a217e72a8293   3 months ago        280MB
    hello-world                  latest    a29f45ccde2a   15 months ago       9.14kB
    aarch64/ubuntu               xenial    a7d1ddc47ced   3 years ago         110MB
    
    

    Could you @sarahl solve your issue?


  • Dev Team

    Hi @Voxlady ,

    I'm scratching my head on this one, I don't have anything helpful yet but will keep an eye out!



  • @modaltb thanks so much! I was actually wondering if I needed something like buildx for the multi-arch building, but I couldn't get it to work with the Ubuntu version of docker. But this seemed to fix my previous issue, so I'll see how this goes. Just trying the first command for now without the COPY line.

    @Voxlady I'll let you know if this fixes my issue. Haven't gotten to the OpenCV portion yet. I don't know what your disk storage constraints are on the raspberry pi, but I'm running on an amd64 Ubuntu 18.04 machine which hopefully will not run into storage issues. I noticed on the Voxl that what was taking up a lot of space was the docker overlays. I wonder if condensing all the dockerfiles into a single one would help (if you don't need all the intermediate images)?



  • Hi @modaltb and @sarahl ,

    Thank you for your messages. It took me some trial-and-error time but I finally solved the problem by reducing the number of Docker Files as well as images. Now, everything seems working again.

    BR,
    Ayberk


  • Dev Team

    Thanks for the update. We really appreciate the feedback and community response!



  • @modaltb okay it finally built, but when I tried to load it onto the Voxl, it ran out of space. So now I'm trying to build a new version with fewer intermediate images and with opencl removed. Are there any other ways to make more space for docker?

    @Voxlady were you able to load your roskinetic+opencv image to the Voxl? Did you use 4.2.0 or 3.4.6 or another opencv version?



  • @sarahl,

    Yes, I was able to load the image and even download additional packages in the container.

    1. I merged two base files from Opencv/cl and kinetic. Moreover, I put everything that's in Dockerfile.opencv into Dockerfile.xenial-packages. So, in the end I didn't have any extra Opencv-cl file. I also disabled Opencl.

    2. Most importantly, after I built roskinetic-xenial:v1.0, I deleted other two images base and minimal. I think that's okay to delete since no error or failure has occurred so far.

    3. Last, I used some best practices suggested by Docker. Especially, for Dockerfile.xenial-base. See sorting multi-line arguments .

    4. I think 3.4.6 version doesn't have a corresponding release in GitHub. So, ARG opencv_version=4.2.0 in Dockerfile.opencv assigns automatically. Therefore, I went with 4.2.0.

    Hopefully it helps.
    All the best


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