1.在安装了opencv4的基础上安装opencv3

opencv版本在这[Releases · opencv/opencv · GitHub]，contrib版本在这[GitHub - opencv/opencv_contrib at 3.4.3]

环境依赖项

Ubuntu16.04安装配置opencv3.4.3+opencv_contrib3.4.3_林深时见鹿echo的博客-CSDN博客_ubuntu 安装opencv3

​​​​​​-D WITH_IPP=OFF, OPENCV_NON_FREE=ON

*编译中遇到的问题IPPICV下载失败
参考：源码编译opencv卡在IPPICV: Download: ippicv_2017u3_lnx_intel64_general_20170822.tgz解决办法_eternal_truth的博客-CSDN博客_ippicv
在Github上下载对应需要下载的文件，随便放在哪里，比如直接/home/xxx/opencv下
Github下载： 

opencv_3rdparty/ippicv_2017u3_lnx_intel64_general_20180518.tgz at ippicv/master_20180518 · opencv/opencv_3rdparty · GitHub先自己手动（百度）下载ippicv 包，不过要在cmake 的最开头加上-D WITH_IPP=OFF  （因为已经下载过了，相应的下载关掉即可）  ,这样再cmake下就好了
 

*opencv3所缺少的boostdesc_bgm.i等文件 

采用的是源码编译的方式，所以可以查看 build 文件夹下的日志文件 CMakeDownloadLog.txt，在日志文件CMakeDownloadLog.txt中搜索 boostdesc_bgm.i 关键词 (不是在文件夹中搜索)，
发现这个文件下载失败了。日志文件里就有它的下载地址，直接复制其下载地址到网页可以看该到文件的源码，直接拷贝源码并生存同名文件，放在 opencv_contrib/modules/xfeatures2d/src/ 路径下即可。

*/stitching/include/opencv2/stitching/detail/matchers.hpp

fatal error: opencv2/xfeatures2d/cuda.hpp: 没有那个文件或目录

/home/tsingroo/Documents/3rdparty/opencv-3.4.3/modules/stitching/include/opencv2/stitching/detail/matchers.hpp打开，找到# include "opencv2/xfeatures2d/cuda.hpp"语句
改为绝对路径#include "/home/tsingroo/Documents/3rdparty/opencv-3.4.3/opencv_contrib-3.4.3/modules/xfeatures2d/include/opencv2/xfeatures2d/cuda.hpp"
 

第三步：编译安装
    make -j11
    make install
 
第四步：配置环境
    sudo vi  /etc/ld.so.conf.d/opencv.conf
    输入：/home/tsingroo/Documents/package/opencv-3.4.3/lib
 
第五步：配置bashrc
vi ~/.bashrc,在最后添加：
 
    #opencv-3.4.3
    export PKG_CONFIG_PATH=${PKG_CONFIG_PATH}:/home/tsingroo/Documents/package/opencv-3.4.3/lib/pkgconfig
    export LD_LIBRARY_PATH=${LD_LIBRARY_PATH}:/home/tsingroo/Documents/package/opencv-3.4.3/lib
 
保存后输入source ~/.bashrc

pkg-config --modversion opencv  //显示版本

2.项目中区分使用

ubuntu 安装Opencv4版本后安装Opencv3，并在项目中区分使用__yuan_的博客-CSDN博客_opencv4兼容opencv3嘛

【解决方案】在已有OpenCV4的情况下安装OpenCV3_幻世至上的博客-CSDN博客

默认opencv4情况下，使用opencv3.4.3，编辑CMakeLists.txt
 
set(${OpenCV_INCLUDE_DIRS} 
"/home/tsingroo/Documents/package/opencv-3.4.3/include/opencv2" "/home/tsingroo/Documents/package/opencv-3.4.3/include/opencv" "/home/tsingroo/Documents/package/opencv-3.4.3/include") # set include path
set(${OpenCV_LIBS}  "/home/tsingroo/Documents/package/opencv-3.4.3/lib") # set lib path
find_package(OpenCV 3.4.3 REQUIRED)
include_directories(${OpenCV_INCLUDE_DIRS})
 
如果只依赖了opencv，或：
 
set(CMAKE_PREFIX_PATH "/home/tsingroo/Documents/package/opencv-3.4.3") # opencv3 install path
FIND_PACKAGE(OpenCV 3.4.3 REQUIRED)
include_directories(${OpenCV_INCLUDE_DIRS})
 
或：
 
set(OpenCV_DIR "/home/tsingroo/Documents/package/opencv-3.4.3")
set(${OpenCV_LIBS}  "/home/tsingroo/Documents/package/opencv-3.4.3/lib") # set lib path
find_package(OpenCV 3.4.3 REQUIRED)
include_directories(${OpenCV_INCLUDE_DIRS})

默认opencv3情况下，使用opencv4.5.0，编辑CMakeLists.txt
 
set(${OpenCV_INCLUDE_DIRS} 
"/home/tsingroo/Documents/package/opencv-3.4.3/include/opencv2") # set include path
set(${OpenCV_LIBS}  "/home/tsingroo/Documents/package/opencv-3.4.3/lib") # set lib path
find_package(OpenCV 3.4.3 REQUIRED)
include_directories(${OpenCV_INCLUDE_DIRS})
 
如果只依赖了opencv，或：
 
set(CMAKE_PREFIX_PATH "/home/tsingroo/Documents/package/opencv-3.4.3") # opencv3 install path
FIND_PACKAGE(OpenCV REQUIRED)
include_directories(${OpenCV_INCLUDE_DIRS})
 
 
 

3.安装opencv4

 安装OpenCV4.5+OpenCV-contrib

 ubuntu 20.04安装 opencv-4.5和opencv-contrib-4.5_路漫漫 修远兮的博客-CSDN博客

cmake ..替换为(路径修改）

sudo cmake -D CMAKE_BUILD_TYPE=Release \
           -D CMAKE_INSTALL_PREFIX=/usr/local \
           -D OPENCV_PYTHON3_INSTALL_PATH=/usr/local/lib/python3.5/dist-packages \
           -D OPENCV_EXTRA_MODULES_PATH=/home/tsingroo/Downloads/opencv-4.5.4/opencv_contrib/modules/ \
           -DOPENCV_ENABLE_NONFREE=True ..

cmake-gui

ubuntu16.04+opencv4.1+opencv_contrib编译及安装（SIFT、SURF）_他们叫我一代大侠的博客-CSDN博客