多版本OpenCV过程记录
环境
ubuntu18.04
python2.7 python3.6 python3.9
opencv 3.2
OpenCV 4.4.0安装
Ubuntu18.04 安装 Opencv4.4.0 及 Contrib (亲测有效)
暂时不清楚Contrib的作用，所以没安装，只安装最基础的
下载opencv4.4.0并解压
opencv下载地址
# 没验证过wget，但是我觉得可以
cd ~/tools/opencv
wget https://github.com/opencv/opencv/archive/4.4.0.zip
unzip opencv-4.4.0.zip
依赖
sudo apt update
sudo apt install cmake
sudo apt install build-essential libgtk2.0-dev libavcodec-dev libavformat-dev libjpeg-dev libswscale-dev libtiff5-dev
sudo apt install pkg-config
自定义安装
多版本OpenCV+OpenCV_contrib安装及使用
Ubuntu下多版本OpenCV共存和切换
# 安装地址需要注意
cd opencv-4.4.0
mkdir build && cd buildcmake -DCMAKE_BUILD_TYPE=RELEASE \-DCMAKE_INSTALL_PREFIX=/home/toniht/tools/opencv/opencv-4.4.0/build/installed \-DWITH_GTK=ON \-DOPENCV_GENERATE_PKGCONFIG=YES \..make
make install
ERROR 1：
/home/toniht/tools/opencv/opencv-4.4.0/modules/videoio/src/ffmpeg_codecs.hpp:81:26: error: ‘CODEC_ID_H264’ was not declared in this scope
#define AV_CODEC_ID_H264 CODEC_ID_H264
满篇飘红，全是错误
FIX：
发现是按照别的教程，曾经安装过FFmpeg，又去把它卸载了。然后删除build重新构建，解决。
版本切换
在bashrc文件中添加
# OpenCV 4.4.0
# export OPENCV_440_PATH=/home/toniht/tools/opencv/opencv-4.4.0/build
export PKG_CONFIG_PATH=$PKG_CONFIG_PATH:/home/toniht/tools/opencv/opencv-4.4.0/build/installed/lib/pkgconfig
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/home/toniht/tools/opencv/opencv-4.4.0/build/installed/lib
企图添加环境变量OPENCV_440_PATH方便后续调用，但是发现设置后在CMakeLists中无论如何都读取不到，即便在终端中echo OPENCV_440_PATH都能正确输出路径，但是CMakeLists中还是不行。很玄学。
在项目（以ORB_SLAM3为例）中的CMakeLists.txt文件中修改
# set(OpenCV_DIR "${OPENCV_440_PATH}")
set(OpenCV_DIR "/home/toniht/tools/opencv/opencv-4.4.0/build")
message(STATUS "OpenCV_DIR:${OPENCV_440_PATH}")find_package(OpenCV 4.4)if(NOT OpenCV_FOUND)message(FATAL_ERROR "OpenCV > 4.4 not found.")endif()
项目正常编译。
ERROR 2：
玄学bug，${OPENCV_440_PATH}在终端中都正常输出了，但是CMakeLists怎么输出都是空，希望有人能解决我的疑问。
)
