【C++】CMakeでビルド時にOpenCVのライブラリが見つからない

Question

CMakeでビルド時にOpenCVのライブラリが見つからない（undefined reference to）と怒られます。
CMakeLists.txtではinclude_directories(/usr/local/include/opencv4)としているはずですが、
他に追加せねばならないことがあれば教えてください。

環境はUbuntu18.04です。

CMakeLists.txt

cmake_minimum_required(VERSION 2.8)

# Set the project name
project (sample CXX)

include_directories(/usr/local/include/opencv4)

# Add an executable
add_executable(sample sample.cc)

sample.cc

#include <opencv2/opencv.hpp>
#include <iostream>

int main(int argc, const char* argv[])
{
  cv::Mat img = cv::imread( argv[1], -1);
  if( img.empty() ) return -1;
  cv::namedWindow( "Example", cv::WINDOW_AUTOSIZE);
  cv::imshow( "Example !", img );
  cv::waitKey(0);
  cv::destroyWindow("Example !!!");

  return 0;
}

ビルド時コマンド

mkdir build && cd build
cmake ../
make

エラー結果

$ make
[ 50%] Linking CXX executable sample
CMakeFiles/sample.dir/sample.cc.o: In function `main':
sample.cc:(.text+0x75): undefined reference to `cv::imread(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int)'
sample.cc:(.text+0x9f): undefined reference to `cv::Mat::empty() const'
sample.cc:(.text+0xed): undefined reference to `cv::namedWindow(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, int)'
sample.cc:(.text+0x166): undefined reference to `cv::imshow(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&, cv::_InputArray const&)'
sample.cc:(.text+0x19d): undefined reference to `cv::waitKey(int)'
sample.cc:(.text+0x1d8): undefined reference to `cv::destroyWindow(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
sample.cc:(.text+0x207): undefined reference to `cv::Mat::~Mat()'
sample.cc:(.text+0x2e8): undefined reference to `cv::Mat::~Mat()'
collect2: error: ld returned 1 exit status
CMakeFiles/sample.dir/build.make:94: recipe for target 'sample' failed
make[2]: *** [sample] Error 1
CMakeFiles/Makefile2:67: recipe for target 'CMakeFiles/sample.dir/all' failed
make[1]: *** [CMakeFiles/sample.dir/all] Error 2
Makefile:83: recipe for target 'all' failed
make: *** [all] Error 2

Answer 1

https://docs.opencv.org/4.1.2/db/df5/tutorial_linux_gcc_cmake.html

CMake公式のチュートリアル記載のCMakeでビルドできました。
find_packageとtarget_link_librariesが抜けているようでした。

cmake_minimum_required(VERSION 2.8)
project( sample  )
find_package( OpenCV REQUIRED )
include_directories( ${OpenCV_INCLUDE_DIRS} )
add_executable( sample DisplayImage.cpp )
target_link_libraries( sample  ${OpenCV_LIBS} )