CMake is having problems to compile a project which uses OpenCV - c++

Question

I'm on Linux Mint 19.1 Tessa.
I followed the instructions described here to install OpenCV. Now I have it installed at the directory "/home/dell/opencv".
I tryed to run the example project located at "/home/dell/opencv/samples/cpp/example_cmake/" by running the command "cmake ." on terminal and I got the following error:
CMake Error at CMakeLists.txt:14 (find_package):
By not providing "FindOpenCV.cmake" in CMAKE_MODULE_PATH this project has
asked CMake to find a package configuration file provided by "OpenCV", but
CMake did not find one.
Could not find a package configuration file provided by "OpenCV" with any
of the following names:
OpenCVConfig.cmake
opencv-config.cmake
Add the installation prefix of "OpenCV" to CMAKE_PREFIX_PATH or set
"OpenCV_DIR" to a directory containing one of the above files. If "OpenCV"
provides a separate development package or SDK, be sure it has been
installed.
-- Configuring incomplete, errors occurred!
See also "/home/dell/opencv/samples/cpp/example_cmake/CMakeFiles/CMakeOutput.log".
The "CMakeLists.txt" file contains the following:
# cmake needs this line
cmake_minimum_required(VERSION 3.1)
# Enable C++11
set(CMAKE_CXX_STANDARD 11)
set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
# Define project name
project(opencv_example_project)
# Find OpenCV, you may need to set OpenCV_DIR variable
# to the absolute path to the directory containing OpenCVConfig.cmake file
# via the command line or GUI
find_package(OpenCV REQUIRED)
# If the package has been found, several variables will
# be set, you can find the full list with descriptions
# in the OpenCVConfig.cmake file.
# Print some message showing some of them
message(STATUS "OpenCV library status:")
message(STATUS " config: ${OpenCV_DIR}")
message(STATUS " version: ${OpenCV_VERSION}")
message(STATUS " libraries: ${OpenCV_LIBS}")
message(STATUS " include path: ${OpenCV_INCLUDE_DIRS}")
# Declare the executable target built from your sources
add_executable(opencv_example example.cpp)
# Link your application with OpenCV libraries
target_link_libraries(opencv_example LINK_PRIVATE ${OpenCV_LIBS})
I've searched a lot through the internet and it seems to be a common problem. However, I still didn't manage to solve it by following the intructions that I've found.
One thing that I notice when going through the OpenCV folder is that the version that I have (which I think it's the most recent one) indeed doesn't contain any "OpenCVConfig.cmake" file, as you can see here. However, an oldest version of OpenCV that I found on github have this file, as you can see here.
So, maybe some configuration is set to this oldest version and it's causing conflict? How to change that and get it working for the newest version? I think it must be something simple to solve but I'm quite newbie.
Thanks in advance.

Answer 1

CMake writes you what to do:
Add the installation prefix of "OpenCV" to CMAKE_PREFIX_PATH or set
"OpenCV_DIR" to a directory containing one of the above files.
Another option - please, read comments in the file CMakeLists.txt that you've pasted, once more.
They say:
you may need to set OpenCV_DIR variable
to the absolute path to the directory containing OpenCVConfig.cmake
Another issue - in-source build, performed with passing dot to cmake. It is better to create separate directory for build files.
Building this way allows you easily adding your build directory to .gitignore and not polluting your source code repository with compiled binaries. Also, if something goes wrong, you can just remove that build directory and start again from scratch. That is much easier than messing with files one by one.
So, you need to call CMake like following
mkdir build
cd build
OpenCV_DIR=/home/dell/opencv cmake ..
or
mkdir build
cd build
cmake .. -DCMAKE_PREFIX_PATH=/home/dell/opencv
or the following also should do the trick
mkdir build
cd build
cmake .. -DOpenCV_DIR=/home/dell/opencv

Answer 2

Update your OpenCV search directory path as below,
find_package(OpenCV 3.4 REQUIRED PATHS "/usr/local/opencv/" NO_DEFAULT_PATH)