ROS Melodic OpenCV xfeatures2d

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Since xfeatured2d module is not part of OpenCV anymore, but now included in opencv_contrib ( https://github.com/opencv/opencv_cont... ) you need to build and install from source and add those modules to the build configuration.

Summarizing what you can find in the following link ( https://linuxize.com/post/how-to-inst... ).

1. Install the necessary packages
2. Clone the OpenCV source code and mostly important for you, clone the opencv_contrib repository
3. Set up the OpenCV build with CMake. Since xfeatured2d is a nonfree module, you will need to set OPENCV_ENABLE_NONFREE=ON. The whole command would look something like this:

cmake -D CMAKE_BUILD_TYPE=RELEASE \  
        -D CMAKE_INSTALL_PREFIX=/usr/local \  
        -D INSTALL_C_EXAMPLES=ON \  
        -D INSTALL_PYTHON_EXAMPLES=ON \  
        -D OPENCV_EXTRA_MODULES_PATH=~/opencv_build/opencv_contrib/modules \  
        -D BUILD_EXAMPLES=ON .. \  
        -D OPENCV_ENABLE_NONFREE=ON ..

1. Compile OpenCV
2. Install OpenCV