Make sure you have opencv installed:
- sudo apt install build-essential cmake git libgtk2.0-dev pkg-config libavcodec-dev libavformat-dev libswscale-dev
- git clone https://github.com/opencv/opencv.git
- mkdir opencv/build
- cd opencv/build
- cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local ..
- make && sudo make install
Make sure you have 'convert' command i.e. ImageMagick installed.