Ubuntu之OpenCV

★ 查看linux下的opencv安装版本：

pkg-config opencv --modversion

★ 查看linux下的opencv安装路径：

sudo find / -iname "*opencv*"

★ Ubuntu下多版本OpenCV共存和切换 安装OpenCV参考https://blog.youkuaiyun.com/cocoaqin/article/details/78163171
1、装ROS会自带个OpenCV，默认路径为/usr/local下。如果需要装另一个版本的，就不能再装到/usr/local下了，而是选择其他路径，否则会覆盖掉之前的版本。
2、首先去https://opencv.org/releases.html下载所需版本的Sources版，也可去https://github.com/opencv/opencv/tree/3.4.1下载。 假设我们安装的第二个OpenCV版本为3.4.1。
3、解压下载下来的zip包

$ unzip opencv-3.4.1.zip

4、安装依赖包

[compiler]$ sudo apt-get install build-essential
[required]$ sudo apt-get install cmake git libgtk2.0-dev pkg-config libavcodec-dev libavformat-dev libswscale-dev
[optional]$ sudo apt-get install python-dev python-numpy libtbb2 libtbb-dev libjpeg-dev libpng-dev libtiff-dev libjasper-dev libdc1394-22-dev

5、编译安装

$ cd opencv-3.4.1
$ mkdir build
$ cd build
$ mkdir installed
$ cmake \
-DCMAKE_BUILD_TYPE=RELEASE \
-DCMAKE_INSTALL_PREFIX=~/opencv-3.4.1/build/installed \
\
-DWITH_CUDA=OFF \
\
-DBUILD_DOCS=OFF \
-DBUILD_EXAMPLES=OFF \
-DBUILD_TESTS=OFF \
-DBUILD_PERF_TESTS=OFF \ 
..
$ make -j4
$ sudo make install

注意： ⅰ、别漏了.. ⅱ、其中~/opencv-3.4.1/build/installed为安装OpenCV3.4.1的路径，这个十分关键 ⅲ、设置OFF的理由如下，可大大加快编译速度，当然还要根据需求进行设置
6、多版本OpenCv切换

$ gedit ~/.bashrc

注释掉

export PKG_CONFIG_PATH=/usr/lib/pkgconfig  
export LD_LIBRARY_PATH=/usr/local/lib 

在文件末尾增加以下内容

export PKG_CONFIG_PATH=～/opencv-3.4.1/build/installed/lib/pkgconfig
export LD_LIBRARY_PATH=～/opencv-3.4.1/build/installed/lib

$ source ~/.bashrc 

查询OpenCV版本

$ pkg-config --modversion opencv

如果输出3.4.1，就表明配置成功。 如果想使用之前的版本，在~/.bashrc中注释掉增加的内容，去掉之前的注释，然后source ~/.bashrc即可。 如果只有一个版本的OpenCV，在CMakeList.txt中使用以下语句即可

FIND_PACKAGE(OpenCV REQUIRED)

上面的这些过程并不能保证在写makefile中调用的是3.41版本的库，可能还是调用之前的2.4.10的库，具体还是要在CMakeList.txt中指定要找的opencv的目录。 在OpenCV编译好后，所在目录中会生成OpenCVConfig.cmake文件，这个文件中指定了CMake要去哪里找OpenCV，其.h文件在哪里等。 存在多版本OpenCV时，需要找到所需版本对应的OpenCVConfig.cmake文件，并将其路径添加到工程的CMakeLists.txt中。

# cmake needs this line
cmake_minimum_required(VERSION 2.8)
set (OpenCV_DIR "~/opencv-3.4.1/build")# 改回默认就注释掉它，换回默认的
# Define project name
project(Test)
# 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 "    version: ${OpenCV_VERSION}")
message(STATUS "    libraries: ${OpenCV_LIBS}")
message(STATUS "    include path: ${OpenCV_INCLUDE_DIRS}")
if(CMAKE_VERSION VERSION_LESS "2.8.11")
  # Add OpenCV headers location to your include paths
  include_directories(${OpenCV_INCLUDE_DIRS})
endif()
# Declare the executable target built from your sources
add_executable(demo demo.cpp)
# Link your application with OpenCV libraries
target_link_libraries(demo ${OpenCV_LIBS})