OpenCV教學之使用cmake生成MakeFile時下載檔案

在編譯OpenCV以及其附加模組時，有時會需要一些第三方的庫，如果本地沒有，會自動下載，下載地址一般為GitHub，結果當然就是卡死在那裡，根本無法下載，下面教大家如何解決這種問題。

問題重現

比如我在使用cmake設定的過程中出現以下情況：

Call Stack (most recent call first):
  /home/ryan/build/opencv_contrib-master/modules/dnn/cmake/download_protobuf.cmake:23 (ocv_download)
  /home/ryan/build/opencv_contrib-master/modules/dnn/cmake/download_protobuf.cmake:51 (ocv_protobuf_download)
  /home/ryan/build/opencv_contrib-master/modules/dnn/cmake/OpenCVFindLibProtobuf.cmake:30 (include)
  /home/ryan/build/opencv_contrib-master/modules/dnn/CMakeLists.txt:38 (include)


-- Downloading v3.1.0/protobuf-cpp-3.1.0.tar.gz...

解決思路

從上面的紀錄檔中可以得到以下資訊：

1. 需要下載protobuf-cpp-3.1.0.tar.gz這個檔案
2. 最先報錯的是download_protobuf.cmake這個檔案

一般情況下，最先報錯的那個檔案能獲得一些與錯誤相關的資訊，於是，我們開啟這個檔案，如下

set(PROTOBUF_CPP_NAME "libprotobuf")
set(PROTOBUF_CPP_DOWNLOAD_HASH "bd5e3eed635a8d32e2b99658633815ef")
set(PROTOBUF_CPP_PATH "${CMAKE_CURRENT_BINARY_DIR}/3rdparty/protobuf/sources") # /protobuf-3.1.0 subdirectory

set(OPENCV_PROTOBUF_CPP_DOWNLOAD_URL ${OPENCV_PROTOBUF_URL};$ENV{OPENCV_PROTOBUF_URL};https://github.com/google/protobuf/releases/download/)

function(ocv_protobuf_download file ID)
  if(DEFINED ${ID}_DOWNLOADED_HASH
       AND ${ID}_DOWNLOADED_HASH STREQUAL ${ID}_DOWNLOAD_HASH
       AND EXISTS ${${ID}_PATH})
    # Files have been downloaded and checked by the previous CMake run
    return()
  else()
    if(EXISTS ${${ID}_PATH})
      message(STATUS "${${ID}_NAME}: Removing previous unpacked files: ${${ID}_PATH}")
      file(REMOVE_RECURSE ${${ID}_PATH})
    endif()
  endif()
  unset(${ID}_DOWNLOADED_HASH CACHE)

  file(MAKE_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/.download)
  file(WRITE "${CMAKE_CURRENT_SOURCE_DIR}/.download/.gitignore" "*n")
  ocv_download(PACKAGE ${file}
               HASH ${${ID}_DOWNLOAD_HASH}
               URL ${OPENCV_${ID}_DOWNLOAD_URL}
               DOWNLOAD_DIR ${CMAKE_CURRENT_SOURCE_DIR}/.download)
  set(${ID}_ARCHIVE "${DOWNLOAD_PACKAGE_LOCATION}")

  ocv_assert(EXISTS "${${ID}_ARCHIVE}")
  ocv_assert(NOT EXISTS "${${ID}_PATH}")
  file(MAKE_DIRECTORY ${${ID}_PATH})
  ocv_assert(EXISTS "${${ID}_PATH}")
  file(WRITE "${${ID}_PATH}/.gitignore" "*n")

  message(STATUS "${${ID}_NAME}: Unpacking ${file} to ${${ID}_PATH}...")
  execute_process(COMMAND ${CMAKE_COMMAND} -E tar xz "${${ID}_ARCHIVE}"
                  WORKING_DIRECTORY "${${ID}_PATH}"
                  RESULT_VARIABLE __result)

  if(NOT __result EQUAL 0)
    message(FATAL_ERROR "${${ID}_NAME}: Failed to unpack ${ID} archive from ${${ID}_ARCHIVE} to ${${ID}_PATH} with error ${__result}")
  endif()

  ocv_assert(EXISTS "${${ID}_PATH}")

  set(${ID}_DOWNLOADED_HASH "${${ID}_DOWNLOAD_HASH}" CACHE INTERNAL "${ID} hash")

  #message(STATUS "${${ID}_NAME}: Successfully downloaded")
endfunction()

ocv_protobuf_download(v3.1.0/protobuf-cpp-3.1.0.tar.gz PROTOBUF_CPP)

可以看出，他需要libprotobuf這個庫，並且需要下載到 ${CMAKE_CURRENT_BINARY_DIR}/3rdparty/protobuf/sources 目錄下，而下載地址就是 https://github.com/google/protobuf/releases/download/ ，於是我們只要手動下載，然後把檔案放到這個目錄下即可。

OpenCV官方教學中文版（For Python） PDF  http://www.linuxidc.com/Linux/2015-08/121400.htm

Ubuntu 14.04安裝OpenCV2.4.9  http://www.linuxidc.com/Linux/2016-07/132884.htm

Ubuntu 16.04上用CMake圖形介面交叉編譯樹莓派的OpenCV3.0 http://www.linuxidc.com/Linux/2016-10/135914.htm

Ubuntu 16.04中安裝OpenCV 2.4.11  http://www.linuxidc.com/Linux/2016-07/132882.htm

Ubuntu 16.04下Matlab2014a+Anaconda2+OpenCV3.1+Caffe安裝  http://www.linuxidc.com/Linux/2016-07/132860.htm

Ubuntu 16.04下TensorFlow+Caffe+OpenCV3.1+Theano部署  http://www.linuxidc.com/Linux/2017-01/139503.htm

Ubuntu 16.04 編譯安裝OpenCV 3.1及OpenCV多版本切換 http://www.linuxidc.com/Linux/2017-01/139325.htm