centos6.10 下编译opencv3.4.6 , opencv_contrib、ffmpeg

第一步，先安装ffmpeg

                按该网页https://trac.ffmpeg.org/wiki/CompilationGuide/Centos一步步安装

                 仅其中git clone --depth 1 https://chromium.googlesource.com/webm/libvpx.git 地址无效

                 换https://github.com/webmproject/libvpx/releases 地址下载了libvpx-1.7.0.tar.gz

                 make时报错信息如下

                 third_party/libyuv/source/row_gcc.cc:831: Error: suffix or operands invalid for vpmaddubsw'
                 third_party/libyuv/source/row_gcc.cc:832: Error: suffix or operands invalid forvpmaddubsw’
                 third_party/libyuv/source/row_gcc.cc:833: Error: suffix or operands invalid for vpmaddubsw'
                 third_party/libyuv/source/row_gcc.cc:834: Error: suffix or operands invalid forvpmaddubsw’
                 third_party/libyuv/source/row_gcc.cc:836: Error: suffix or operands invalid for vphaddw'
                 third_party/libyuv/source/row_gcc.cc:837: Error: suffix or operands invalid forvphaddw’
                 …..
                 third_party/libyuv/source/row_gcc.cc:5321: Error: no such instruction: `vpermq $0xd8,%ymm0,%ymm0’
                 make[1]: * [third_party/libyuv/source/row_gcc.cc.o] Error 1
                 make: * [.DEFAULT] Error 2
                 按该博客方法https://blog.csdn.net/happybird100/article/details/50707026禁用avx2可解

摘录——“https://blog.csdn.net/happybird100/article/details/50707026
经过一番谷歌,找到一段话:
As a workaround if your hardware does not support avx2 (grep avx2
/proc/cpuinfo) you can disable it with –disable-avx2

也就是说，我的系统可能不支持avx2这个东东,grep avx2
/proc/cpuinfo果然没有, avx2是一个指令集，支持更多位数的浮点数运算，因此视频解码上更好 效率更高。但是系统不支持，所以得编译一个不包含avx2的VP8/VP9编解码库则可,最后用以下命令编译通过：
libvpx-v1.3.0.tar.bz2
./configure –enable-shared –disable-avx2
make && make install”

               我的命令./configure --prefix="/usr/local/ffmpeg_build" --disable-avx2 --disable-examples --disable-unit-tests --enable-vp9-highbitdepth --as=yasm

    安装完ffmpeg之后，配置环境变量：

                  echo $PKG_CONFIG_PATH   居然为空
                  export PKG_CONFIG_PATH=/usr/local/ffmpeg_build/lib/pkgconfig:/usr/local/lib/pkgconfig:/usr/lib/pkgconfig

                  export PATH=$PATH:/usr/local/ffmpeg_build/bin

或者：vi /etc/profile

                  export PKG_CONFIG_PATH=/usr/local/ffmpeg_build/lib/pkgconfig:/usr/local/lib/pkgconfig:/usr/lib/pkgconfig

                  export PATH=$PATH:/usr/local/ffmpeg_build/bin

                   export LD_LIBRARY_PATH="/usr/local/ffmpeg_build/lib"

           source /etc/profile

                  进一步，把ffmpeg的库加入/etc/ld.so.conf中
                  cd /etc/ld.so.conf.d/
                  vi ffmpeg.conf
                  /usr/local/ffmpeg_build/lib
                  ldconfig

第二步： 编译opencv3.4.6   opencv_contrib3.4.6  

                 https://github.com/opencv/opencv 下载opencv与opencv_contrib，版本必须相同

                 https://github.com/opencv/opencv/releases/tag/3.4.6
                 https://github.com/opencv/opencv_contrib/releases/tag/3.4.6

                  tar -zxvf /home/jello/Downloads/opencv-3.4.6.tar.gz -C /usr/local/
                  tar -zxvf /home/jello/Downloads/opencv_contrib-3.4.6.tar.gz -C /usr/local/

                 安装依赖项yum install cmake gcc gcc-c++ gtk+-devel gimp-devel gimp-devel-tools gimp-help-browser zlib-devel libjpeg-devel libpng-devel gstreamer-devel libavc1394-devel libraw1394-devel libdc1394-devel jasper-devel jasper-utils swig python libtool nasm

                 cd /usr/local/opencv-3.4.6

                 mkdir build && cd build
                 cmake -D CMAKE_BUILD_TYPE=Release -D CMAKE_INSTALL_PREFIX=/usr/local -D WITH_FFMPEG=ON -D OPENCV_EXTRA_MODULES_PATH=/usr/local/opencv_contrib-3.4.6/modules ..

                 make

                 make install

                 cmake若有相关错误可参考https://blog.csdn.net/lj402159806/article/details/84975294

                  我之前安装依赖项时装了libtiff-devel,cmake报错如下

                 grfmt_tiff.cpp:132:12: error: 'tmsize_t' does not name a type
                 grfmt_tiff.cpp:213:86: error: 'read' is not a member of 'cv::TiffDecoderBufHelper'
                 按上面博客方法yum remove libtiff-devel后重新编译成功

                 cmake成功后能看到ffmpeg为YES

                 Video I/O:
--     DC1394:                      YES (ver 2.1.2)
--     FFMPEG:                      YES
--       avcodec:                   YES (ver 58.53.101)
--       avformat:                  YES (ver 58.28.102)
--       avutil:                    YES (ver 56.30.100)
--       swscale:                   YES (ver 5.4.101)
--       avresample:                NO
--     GStreamer:                   NO

                   补记：之前安装opencv3.2.0时，遇到了安装了各种版本和各种方式的ffmpeg, 并且配置好了PKG_CONFIG_PATH等环境变量，opencv3.2.0的cmake后ffmpeg始终为NO的情况，这是opencv3.2.0的源码bug，按下面博客方式解决

                http://www.opencv.org.cn/forum.php?mod=viewthread&tid=264912     

                要在 /usr/local/include/libavutil/common.h 前边加上一行

                #ifndef UINT64_C
                #define UINT64_C(value) __CONCAT(value, ULL)
                #endif

                原因不理解，但是确实有效！

第三步： 最后按照自己2018/07/17博客安装配置qt即可。

                 在ldconfig配置动态链接库时遇到如下问题：

                  ldconfig: /usr/local/lib/libstdc++.so.6.0.18-gdb.py is not an ELF file - it has the wrong magic bytes at the start.

                  网搜直接删除该文件，是可解决，为预防，做了备份

                  mv /usr/local/lib/libstdc++.so.6.0.18-gdb.py /home/jello/libstdc++.so.6.0.18.gdb.py.bak