Possibility of importing and using an external library in ROS nodes

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Yes, it is definitely possible (and very common) to use external libraries in ROS nodes. Depending on the language and library you want to use, the required steps will change somewhat, but in general ROS tries to follow the typical usage patterns of the language (e.g., CMake for C++, etc.).

However, for libraries that extremely popular to use with ROS, such as OpenCV, pre-baked distributions are already built for several platforms. This is done so that you don't have to build (and continually update) it yourself, and to guarantee a consistent library version for other packages in the ROS build farm that also require OpenCV.

For example, if you're using ROS Indigo on Ubuntu, you can just do this:

sudo apt-get install ros-indigo-vision-opencv

And then you can follow the documentation and tutorials on the ROS vision_opencv page for examples on how to use it.