编译 OpenCV

打开终端进行编译：（-j 是使用 8 个线程进行编译，请根据你的计算机配置合理设置线程数）

E:
cd E:\\opencv_341\\opencv_mingw64_build
mingw32-make -j 8
mingw32-make install

如果 mingw32-make -j 8遇到错误，请看下面的 编译 OpenCV 常见错误，否则执行 mingw32-make install，完成安装。

编译 OpenCV 常见错误

• MinGW-w64 的 aviriff.h 文件注释错误
  表现:

[ 49%] Building CXX   modules/videoio/CMakeFiles/opencv_videoio.dir/src/cap_dshow.cpp.obj
In file included from E:\\opencv_341\\opencv\\sources\\modules\\videoio\\src\\cap_dshow.cpp:113:0:
e:\\mingw-w64\\x64-4.8.1-release-posix-seh-rev5\\mingw64\\x86_64-w64-mingw32\\include\\aviriff.h:2:8: error: expected constructor, destructor, or type conversion before \'file\'
 - This file is part of the mingw-w64 runtime package.
        ^
e:\\mingw-w64\\x64-4.8.1-release-posix-seh-rev5\\mingw64\\x86_64-w64-mingw32\\include\\aviriff.h:3:25: error: \'refer\' does not name a type
 - No warranty is given; refer to the file DISCLAIMER within this package.
                         ^
In file included from e:\\mingw-w64\\x64-4.8.1-release-posix-seh-rev5\\mingw64\\x86_64-w64-mingw32\\include\\aviriff.h:19:0,
                 from E:\\opencv_341\\opencv\\sources\\modules\\videoio\\src\\cap_dshow.cpp:113:
e:\\mingw-w64\\x64-4.8.1-release-posix-seh-rev5\\mingw64\\x86_64-w64-mingw32\\include\\pshpack2.h:7:21: error: expected declaration before end of line
 #pragma pack(push,2)
                     ^
modules\\videoio\\CMakeFiles\\opencv_videoio.dir\\build.make:146: recipe for target \'modules/videoio/CMakeFiles/opencv_videoio.dir/src/cap_dshow.cpp.obj\' failed
mingw32-make[2]: *** [modules/videoio/CMakeFiles/opencv_videoio.dir/src/cap_dshow.cpp.obj] Error 1
CMakeFiles\\Makefile2:3057: recipe for target \'modules/videoio/CMakeFiles/opencv_videoio.dir/all\' failed
mingw32-make[1]: *** [modules/videoio/CMakeFiles/opencv_videoio.dir/all] Error 2
Makefile:161: recipe for target \'all\' failed
mingw32-make: *** [all] Error 2

解决
打开E:\\MinGW-w64\\x64-4.8.1-release-posix-seh-rev5\\mingw64\\x86_64-w64-mingw32\\include\\aviriff.h

发现第一行的多行注释少了个/符号，加上保存，如下：

/**
* This file is part of the mingw-w64 runtime package.
* No warranty is given; refer to the file DISCLAIMER within this package.
*/

然后重新 Configure-Generate-mingw32-make就好了。

1. cap_msmf.cpp capture code 错误
   解决
   在 cmake-gui 编译配置中：

不勾选 WITH_MSMF
然后重新 Configure-Generate-mingw32-make

1. ‘M_PI’ was not declared in this scope 错误
   表现：

E:\\opencv-4.0.0-alpha\\opencv-4.0.0-alpha\\modules\\calib3d\\src\\chessboard.cpp:2879:29: error: \'M_PI\' was not declared in this scope
             if(angle_temp > M_PI*0.5)
                             ^
modules\\calib3d\\CMakeFiles\\opencv_calib3d.dir\\build.make:137: recipe for target \'modules/calib3d/CMakeFiles/opencv_calib3d.dir/src/chessboard.cpp.obj\' failed
mingw32-make[2]: *** [modules/calib3d/CMakeFiles/opencv_calib3d.dir/src/chessboard.cpp.obj] Error 1
CMakeFiles\\Makefile2:3018: recipe for target \'modules/calib3d/CMakeFiles/opencv_calib3d.dir/all\' failed
mingw32-make[1]: *** [modules/calib3d/CMakeFiles/opencv_calib3d.dir/all] Error 2
Makefile:161: recipe for target \'all\' failed
mingw32-make: *** [all] Error 2

解决
在最新的 master 分支已经解决了这个问题，见我的 PR ：M_PI changed to CV_PI

如果你是在 官网 或者 github.com/opencv/opencv/releases 中下的 OpenCV 4.0.0-alpha，可能还会有这个问题，那么你需要把 chessboard.cpp、chessboard.hpp、test_chesscorners.cpp 文件中的 M_PI 全部改为 CV_PI，如我的 commit 所示：M_PI changed to CV_PI (#12645)

然后重新 Configure-Generate-mingw32-make

1. ‘posix_memalign’ was not declared in this scope 错误
   表现:

[ 28%] Building CXX   modules/CMakeFiles/ade.dir/__/3rdparty/ade/ade-0.1.1c/sources/ade/source/alloc.cpp.obj
E:\\opencv-4.0.0-rc\\opencv-4.0.0-rc-build\\3rdparty\\ade\\ade-0.1.1c\\sources\\ade\\source\\alloc.cpp: In function \'void* ade::aligned_alloc(std::size_t, std::size_t)\':
E:\\opencv-4.0.0-rc\\opencv-4.0.0-rc-build\\3rdparty\\ade\\ade-0.1.1c\\sources\\ade\\source\\alloc.cpp:31:16: error: \'posix_memalign\' was not declared in this scope
     auto res = posix_memalign(&ret, std::max(sizeof(void*), alignment), size);
                ^~~~~~~~~~~~~~
mingw32-make[2]: *** [modules\\CMakeFiles\\ade.dir\\build.make:63: modules/CMakeFiles/ade.dir/__/3rdparty/ade/ade-0.1.1c/sources/ade/source/alloc.cpp.obj] Error 1
mingw32-make[1]: *** [CMakeFiles\\Makefile2:884: modules/CMakeFiles/ade.dir/all] Error 2
mingw32-make: *** [Makefile:162: all] Error 2

解决
把 opencv-4.0.0-rc-build\\3rdparty\\ade\\ade-0.1.1c\\sources\\ade\\source\\alloc.cpp 文件的所有 WIN32 改为 _WIN32，如这个 PR 所做的修改：fix check for win32 #6

然后重新 Configure-Generate-mingw32-make
来自：
https://blog.huihut.com/2018/07/31/CompiledOpenCVWithMinGW64/