• 问题描述：使用cmake-gui配置opencv库时会遇到ffmpeg无法被正常下载的现象
• 问题分析：raw.git*在部分地区无法ping通
• 解决办法：编辑${opencv}/sources/3rdparty/ffmpeg/ffmpeg.cmake，在原下载地址前增加https://ghproxy.com/，修改具体如下

"https://ghproxy.com/https://raw.githubusercontent.com/opencv/opencv_3rdparty/${FFMPEG_BINARIES_COMMIT}/ffmpeg/"

而后就可以正常在cmake-gui中执行configure和Generate，具体log如下：

===
OpenCV Python: during development append to PYTHONPATH: W:/packages/opencv/ming_build/python_loader
FFMPEG: Download: opencv_videoio_ffmpeg.dll
FFMPEG: Download: opencv_videoio_ffmpeg_64.dll
FFMPEG: Download: ffmpeg_version.cmake
Consider adding OPENCV_ALLOCATOR_STATS_COUNTER_TYPE=int/int64_t according to your build configuration
===
  Video I/O:
    DC1394:                      NO
    FFMPEG:                      YES (prebuilt binaries)
      avcodec:                   YES (58.91.100)
      avformat:                  YES (58.45.100)
      avutil:                    YES (56.51.100)
      swscale:                   YES (5.7.100)
      avresample:                YES (4.0.0)
    GStreamer:                   NO
    DirectShow:                  YES

  Parallel framework:            none

  Trace:                         YES (built-in)

  Other third-party libraries:
    Lapack:                      NO
    Eigen:                       NO
    Custom HAL:                  NO
    Protobuf:                    build (3.5.1)

  OpenCL:                        YES (no extra features)
    Include path:                W:/packages/opencv/sources/3rdparty/include/opencl/1.2
    Link libraries:              Dynamic load

  Python (for build):            NO

  Java:                          
    ant:                         NO
    JNI:                         NO
    Java wrappers:               NO
    Java tests:                  NO

  Install to:                    W:/packages/opencv/ming_build/install
-----------------------------------------------------------------

Configuring done
Generating done

而后在cmd中${target}目录下执行mingw32-make -j 32即可启动编译（我用的是mingw，其他编译器使用对应的命令），执行成功log如下：

W:/packages/opencv/sources/modules/gapi/include/opencv2/gapi/own/saturate.hpp:75:0:
                           ignoring #pragma warning  [         ignoring #pragma warning  []
 #pragma warning(disable: 4244)

                 ignoring #pragma warning  [
 #pragma warning(default: 4244)

W:/packages/opencv/sources/modules/gapi/include/opencv2/gapi/own/saturate.hpp:75:0:-Wunknown-pragmas]
 #pragma warning(default: 4244)

 ]
 #pragma warning(default: 4244)

warning: ignoring #pragma warning  [-Wunknown-pragmas]
 #pragma warning(default: 4244)

[100%] Linking CXX executable ..\..\bin\opencv_test_gapi.exe
[100%] Built target opencv_test_gapi

再执行mingw32-make install即可安装到build文件夹下供QT使用