opencv源码编译之下载超时问题

• 错误：安装opencv时，xfeatures2d模块缺失boostdesc_bgm.i文件，下载超时问题。

• 原因：无法访问国外的网站导致下载超时而失败。

• 分析：打开文件build/CMakeDownloadLog.txt，如下所示：

  指定缓存文件夹
  #use_cache "/home/user/Project/OpenCV/opencv/.cache"
  解压路径
  #do_unpack "ippicv_2019_lnx_intel64_general_20180723.tgz" "c0bd78adb4156bbf552c1dfe90599607" "https://raw.githubusercontent.com/opencv/opencv_3rdparty/32e315a5b106a7b89dbed51c28f8120a48b368b4/ippicv/ippicv_2019_lnx_intel64_general_20180723.tgz" "/home/user/Project/OpenCV/build/3rdparty/ippicv"
  文件校验
  #check_md5 "/home/user/Project/OpenCV/opencv/.cache/ippicv/c0bd78adb4156bbf552c1dfe90599607-ippicv_2019_lnx_intel64_general_20180723.tgz"
  文件校验失败
  #mismatch_md5 "/home/user/Project/OpenCV/opencv/.cache/ippicv/c0bd78adb4156bbf552c1dfe90599607-ippicv_2019_lnx_intel64_general_20180723.tgz" "d41d8cd98f00b204e9800998ecf8427e"
  删除下载文件
  #delete "/home/user/Project/OpenCV/opencv/.cache/ippicv/c0bd78adb4156bbf552c1dfe90599607-ippicv_2019_lnx_intel64_general_20180723.tgz"
  下载文件
  #cmake_download "/home/user/Project/OpenCV/opencv/.cache/ippicv/c0bd78adb4156bbf552c1dfe90599607-ippicv_2019_lnx_intel64_general_20180723.tgz" "https://raw.githubusercontent.com/opencv/opencv_3rdparty/32e315a5b106a7b89dbed51c28f8120a48b368b4/ippicv/ippicv_2019_lnx_intel64_general_20180723.tgz"
  #   Trying 0.0.0.0...
  # TCP_NODELAY set
  # connect to 0.0.0.0 port 443 failed: Connection refused
  #   Trying ::...
  # TCP_NODELAY set
  # connect to :: port 443 failed: Connection refused
  # Failed to connect to raw.githubusercontent.com port 443: Connection refused
  # Closing connection 0
  # 
  

• 办法：只需要下载文件那一行中找到下载地址和存放地址即可，例如：

  #cmake_download 下载后保存地址 下载地址
  
  #cmake_download "/home/user/Project/OpenCV/opencv/.cache/ippicv/c0bd78adb4156bbf552c1dfe90599607-ippicv_2019_lnx_intel64_general_20180723.tgz" "https://raw.githubusercontent.com/opencv/opencv_3rdparty/32e315a5b106a7b89dbed51c28f8120a48b368b4/ippicv/ippicv_2019_lnx_intel64_general_20180723.tgz"
  
  下载地址：https://raw.githubusercontent.com/opencv/opencv_3rdparty/32e315a5b106a7b89dbed51c28f8120a48b368b4/ippicv/ippicv_2019_lnx_intel64_general_20180723.tgz
  保存地址：
  /home/user/Project/OpenCV/opencv/.cache/ippicv/
  保存文件名
  c0bd78adb4156bbf552c1dfe90599607-ippicv_2019_lnx_intel64_general_20180723.tgz
  注：保存文件名格式：md5-文件名.扩展名
  

  最后将build/CMakeDownloadLog.txt的所有所需文件通过梯子下载后保存到指定位置即可。最后再次执行：

  cmake -DOPENCV_EXTRA_MODULES_PATH=../opencv_contrib/modules ../opencv