Installing OpenCV on an embedded device

There is a very easy way to install OpenCV and all its dependencies on a Debian-based embedded device such as Raspberry Pi:

sudo apt-get install libopencv-dev

However, that might install an old version of OpenCV from one or two years ago.

To install the latest version of OpenCV on an embedded device such as Raspberry Pi, we need to build OpenCV from the source code. First, we install a compiler and build system, then libraries for OpenCV to use, and finally OpenCV itself. Note that the steps for compiling OpenCV from source on Linux are the same whether you are compiling for desktop or for embedded systems. A Linux script, install_opencv_from_source.sh, is provided with this book; it is recommended you copy the file onto your Raspberry Pi (for example, with a USB flash stick) and run the script to download, build, and install OpenCV, including potential multi-core CPU and ARM NEON SIMD optimizations (depending on hardware support):

chmod +x install_opencv_from_source.sh
 ./install_opencv_from_source.sh

It's highly recommended to build and run a few OpenCV samples every time you install OpenCV, so when you have problems building your own code, at least you will know whether the problem is the OpenCV installation or a problem with your code.

Let's try to build the simple edge sample program. If we try the same Linux command to build it from OpenCV 2, we get a build error:

cd ~/opencv-4.*/samples/cpp
 g++ edge.cpp -lopencv_core -lopencv_imgproc -lopencv_highgui
 -o edge
 /usr/bin/ld: /tmp/ccDqLWSz.o: undefined reference to symbol '_ZN2cv6imreadERKNS_6StringEi'
 /usr/local/lib/libopencv_imgcodecs.so.4..: error adding symbols: DSO missing from command line
 collect2: error: ld returned 1 exit status

The second to last line of that error message tells us that a library was missing from the command line, so we simply need to add -lopencv_imgcodecs in our command next to the other OpenCV libraries we linked to. Now, you know how to fix the problem anytime you are compiling an OpenCV 3 program and you see that error message. So, let's do it correctly:

cd ~/opencv-4.*/samples/cpp
 g++ edge.cpp -lopencv_core -lopencv_imgproc -lopencv_highgui
 -lopencv_imgcodecs -o edge

It worked! So, now you can run the program:

./edge

Hit Ctrl + C on your keyboard to quit the program. Note that the edge program might crash if you try running the command in an SSH Terminal and you don't redirect the window to display on the device's LCD screen. So, if you are using SSH to remotely run the program, add DISPLAY=:0 before your command:

DISPLAY=:0 ./edge

You should also plug a USB webcam into the device and test that it works:

g++ starter_video.cpp -lopencv_core -lopencv_imgproc
 -lopencv_highgui -lopencv_imgcodecs -lopencv_videoio \
 -o starter_video
 DISPLAY=:0 ./starter_video 0

Note: if you don't have a USB webcam, you can test using a video file:

DISPLAY=:0 ./starter_video ../data/768x576.avi

Now that OpenCV is successfully installed on your device, you can run the Cartoonifier applications we developed earlier. Copy the Cartoonifier folder onto the device (for example, by using a USB flash stick, or using scp to copy files over the network). Then, build the code just like you did for the desktop:

cd ~/Cartoonifier
 export OpenCV_DIR="~/opencv-3.1.0/build"
 mkdir build
 cd build
 cmake -D OpenCV_DIR=$OpenCV_DIR ..
 make

And run it:

DISPLAY=:0 ./Cartoonifier

And if all is fine, we will see a window with our application running as follows: