win10下cmake编译opencv库问题

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1.未配置Android NDK工具链目录

CMake Error at platforms/android/android.toolchain.cmake:451 (message):
Could not find neither Android NDK nor Android standalone toolchain.

You should either set an environment variable:
export ANDROID_NDK=~/my-android-ndk

根据提示配置ANDROID_NDK到环境变量PATH中，即可
例如：ANDROID_NDK = D:\android-ndk-r14b

2.ndk版本不匹配。

CMake Error at cmake/OpenCVUtils.cmake:235 (TRY_COMPILE):
Failed to configure test project build system.
Call Stack (most recent call first):
cmake/OpenCVUtils.cmake:296 (ocv_check_compiler_flag)
cmake/OpenCVCompilerOptions.cmake:69 (ocv_check_flag_support)
cmake/OpenCVCompilerOptions.cmake:93 (add_extra_compiler_option)
CMakeLists.txt:495 (include)

发现这个问题时，先是使用Android Studio中链接下载的最新ndk目录（版本是17）。会报错
之后，从https://developer.android.google.cn/ndk/downloads/ 中下载最新的16b，还是报错。

最后 查看源码文件夹下opencv-3.2.0\platforms\android\android.toolchain.cmake文件里有两行注释

Android CMake toolchain file, for use with the Android NDK r5-r10d
Requires cmake 2.6.3 or newer (2.8.9 or newer is recommended).

提示使用r5到r10版本的NDK（我去网上下只剩r10e的，下载来编译后就可以通过了。）

3.下载依赖文件超时，未下载成功。比对hash码不匹配

CMake Error at D:/opencv/opencv-3.2.0/cmake/OpenCVUtils.cmake:1043 (file):
file DOWNLOAD HASH mismatch

for file: [D:/opencv/opencv_contrib-3.2.0/modules/dnn/.download/bd5e3eed635a8d32e2b99658633815ef/v3.1.0/protobuf-cpp-3.1.0.tar.gz]
  expected hash: [bd5e3eed635a8d32e2b99658633815ef]
    actual hash: [d41d8cd98f00b204e9800998ecf8427e]
         status: [28;"Timeout was reached"]

Call Stack (most recent call first):
D:/opencv/opencv_contrib-3.2.0/modules/dnn/cmake/download_protobuf.cmake:23 (ocv_download)
D:/opencv/opencv_contrib-3.2.0/modules/dnn/cmake/download_protobuf.cmake:51 (ocv_protobuf_download)
D:/opencv/opencv_contrib-3.2.0/modules/dnn/cmake/OpenCVFindLibProtobuf.cmake:30 (include)
D:/opencv/opencv_contrib-3.2.0/modules/dnn/CMakeLists.txt:35 (include)

CMake Error at D:/opencv/opencv-3.2.0/cmake/OpenCVUtils.cmake:1047 (message):
Failed to download v3.1.0/protobuf-cpp-3.1.0.tar.gz. Status=28;"Timeout
was reached"
Call Stack (most recent call first):
D:/opencv/opencv_contrib-3.2.0/modules/dnn/cmake/download_protobuf.cmake:23 (ocv_download)
D:/opencv/opencv_contrib-3.2.0/modules/dnn/cmake/download_protobuf.cmake:51 (ocv_protobuf_download)
D:/opencv/opencv_contrib-3.2.0/modules/dnn/cmake/OpenCVFindLibProtobuf.cmake:30 (include)
D:/opencv/opencv_contrib-3.2.0/modules/dnn/CMakeLists.txt:35 (include)

根据提示D:/opencv/opencv_contrib-3.2.0/modules/dnn/cmake/download_protobuf.cmake:23
找到该行信息
可以看出，他需要libprotobuf这个库，并且需要下载到 ${CMAKE_CURRENT_BINARY_DIR}/3rdparty/protobuf/sources 目录下，而下载地址就是 https://github.com/google/protobuf/releases ，于是我们只要手动下载3.1.0，然后把文件放到这个目录下即可。