OpenCV binaries are certainly available in official software repositories. Depending on your distribution and your package manager, you can install it with commands such as the following:
apt-get install libopencv yum install opencv
When OpenCV is installed on your Linux, you can add this snippet to the filter-plugin-designer.pro file:
linux {
target.path = $$(QTDIR)/../../Tools/QtCreator/lib/Qt/plugins/designer/
CONFIG += link_pkgconfig
PKGCONFIG += opencv
}
This time we do not use the LIBS variable but PKGCONFIG, which relies on pkg-config. It is a helper tool that will insert the correct options into the compile command line. In our case, we will request pkg-config to link our project with OpenCV.