One 、OpenCV download

1. opencv Download address
2. opencv_contrib Download address
   Be careful opencv as well as opencv_contrib The version of the best corresponding

Two 、OpenCV Compilation and installation

2.1 cmake Compilation and installation

After downloading , Enter the source directory （ namely CMakeLists.txt The same level directory ）

1. newly build build Catalog

mkdir build

2. function cmake command , Pay attention to OpenCV Compilation of , The command has many options , Here are only the ones I often use

sudo cmake -D CMAKE_INSTALL_PREFIX=/media/jaychen/Config/Deepin/local/libs/opencv/4.5_vino \
-D CMAKE_BUILD_TYPE=DEBUG \
-D OPENCV_GENERATE_PKGCONFIG=ON \
-D OPENCV_EXTRA_MODULES_PATH=/media/jaychen/Config/Deepin/local/libs/opencv_contrib/modules \
-D WITH_FFMPEG=ON ..

Here is an example of installation options

*   Select installation location 
-D CMAKE_INSTALL_PREFIX=/usr/local/opencv
*  Select the compilation method 
-D CMAKE_BUILD_TYPE=RELEASE/DEBUG
*  Whether to generate PKGCONFIG Needed opencv.pc file 
-D OPENCV_GENERATE_PKGCONFIG=ON/OFF
*  Whether to compile Contrib modular ( there PATH To write contrib Under the modules)
-D OPENCV_EXTRA_MODULES_PATH=/home/..../opencv_contrib/modules
*  Is it just a opencv_world Dynamic link library 
-D BUILD_OPENCV_WORLD=ON/OFF
*  Whether to use Qt As highgui Display frame of module ( You need to install Qt)
-D WITH_Qt=ON/OFF
*  Whether to use FFMPEG Make a video IO
-D WITH_FFMPEG=ON/OFF

3. function make command
4. function sudo make install command ( If installed to /usr/llib Wait for the location to need root jurisdiction )

2.2 Linux Direct installation

This way there is no FindOpenCV.cmake as well as OpenCVConfig.cmake. Only opencv.pc, It's through pkg-config How to organize the library .

sudo apt-get install libopencv-dev

3、 ... and 、 stay Cmake Introduction in OpenCV

3.1 Use OpenCVConfig.cmake

First set up OpenCV_DIR This variable is OpenCVConfig.cmake Directory of files , Then you can use find_package Command found opencv library , You can use it later include_directories as well as target_link_libraries The command introduces opencv The library of .cmake The script code is as follows ：

set(OpenCV_DIR xxxx) # xxxx Directory contains OpenCVConfig.cmake
find_package(opencv required)	#  find opencv library 
include_directories(${OpenCV_INCLUDE_DIRS})
add_executable(${PROJECT_NAME} *.cpp)	# *.cpp Refers to the source files to be compiled 
target_link_libraries(${PROJECT_NAME} ${OpenCV_LIBRARIES})

3.2 Use FindOpenCV.cmake

And 3.1 similar

3.3 Use opencv.pc

cmake_minimum_required(VERSION 3.17)
project(C__test)

set(CMAKE_CXX_STANDARD 14)

set(ENV{
    PKG_CONFIG_PATH} /usr/lib/x86_x64-linux-gnu/pkgconfig)
find_package(PkgConfig REQUIRED)
pkg_check_modules(OPENCV REQUIRED IMPORTED_TARGET opencv)
#MESSAGE(STATUS ${OPENCV_LIBRARIES})
if(${OPENCV_FOUND})
    MESSAGE(${PKG_CONFIG_EXECUTABLE})
    MESSAGE(${OPENCV_LIBRARIES})
    MESSAGE(${OPENCV_LINK_LIBRARIES})
endif()
add_executable(C__test main.cpp) #  Be careful add_executable Put it on target_link_libraries In front of , Otherwise, it will report a mistake 
#  Note that you can directly use PkgConfig::OPENCV This way, CMake file , FindPkgConfig There's something about , Baidu cmake pkgconfig doc There is .
target_link_libraries(${PROJECT_NAME} PkgConfig::OPENCV)

Four 、 stay Linux Middle switch OpenCV Version of

Sometimes different programs need different versions of OpenCV, Compiled and installed multiple OpenCV To different places , You need to switch different versions of the system link . It's recorded below Linux Several ways for the system to find the dynamic link library
Linux Several ways to find dynamic link libraries
By default , The compiler will only use /lib and /usr/lib The library files in these two directories

1. LD_LIBRARY_PATH
   This is a global variable
2. rpath
   gcc Compile options rpath=xxx, Such as gcc -L. -Wl,-rpath=/home/bow/all/program/test/lib_version_test -o test main.o -ltest
3. ld.so.conf
   Generally located /etc/ld.so.conf, Sometimes the content is include /etc/ld.so.conf.d/*.conf, Direct contains /etc/ld.so.conf.d All the conf file
   conf The absolute path of the directory of the record library in the file , And by the /usr/lib/ld The program uses this configuration file
4. pkg-config Program
   pkg-config The program is to configure A program that provides system information , For example, software version , Library version , Library path, etc . install pkg-config There will be a PKG_CONFIG_PATH environment variable , This environment variable stores *.pc Directory path of file ,.pc Files provide all kinds of information about the library ,PKG_CONFIG_PATH Search takes precedence over the default .
   https://blog.csdn.net/weixin_38907560/article/details/81449272
   ./configure Commonly used pkg-config To check the installation environment
   pkg-config --cflags opencv You can see opencv The header directory of
   pkg-config --libs opencv You can see opencv Library file directory
   Pay attention to opencv Is in accordance with the opencv.pc To find the name of , If in PKG_CONFIG_PATH Cannot find... In environment variable , Will go to its default path, such as /usr/local/lib/pkgconfig Next