修改错误命令，完善编译命令

CentOS7CompileOpencvFFmpeg.md

@@ -1,6 +1,6 @@
-# 编译ffmpeg
+# 编译FFmpeg
 
-省略下载步骤，下载内容见[FFmpeg官方编译Wiki](https://trac.ffmpeg.org/wiki/CompilationGuide)
+省略下载或同步源代码步骤，需要的源代码内容见[FFmpeg官方编译Wiki](https://trac.ffmpeg.org/wiki/CompilationGuide) 。为了后面编译动态库，编译时需要加上`-fPIC`参数。
 
 ## 编译需要的容器
 
@@ -9,6 +9,8 @@ docker run -it -d --privileged=true --name centos7 -p 2333:22 centos:7 /usr/sbin
 docker exec -it centos7 bash
 ```
 
+## 编译
+
 ```shell
 # 编译需要的环境
 yum install autoconf automake bzip2 bzip2-devel freetype-devel gcc gcc-c++ git libtool make pkgconfig zlib-devel
@@ -17,22 +19,26 @@ yum install autoconf automake bzip2 bzip2-devel freetype-devel gcc gcc-c++ git l
 tar xzvf ffmpeg_sources.tar.gz
 cd ~/ffmpeg_sources
 
+# 解压
 tar xjvf nasm-2.15.05.tar.bz2
 tar xzvf yasm-1.3.0.tar.gz
 tar xzvf lame-3.100.tar.gz
 tar xzvf opus-1.3.1.tar.gz
-tar zxvf libvpx.tar.gz -m
+tar zxvf libvpx.tar.gz -m # -m 强制时间同步
 tar zxvf ffmpeg-4.4.tar.gz
 tar zxvf x264-stable.tar.gz
 mv x264-stable x264
 tar zxvf x265_git.tar.gz
 tar zxvf fdk-aac.tar.gz
 
-# 手动安装cmake
+# 安装cmake
 ./cmake-3.21.3-linux-x86_64.sh --prefix=/usr/local
 mv /usr/local/cmake-3.21.3-linux-x86_64 /usr/local/cmake
-# 添加到环境变量并生效
+
+# 添加编译需要的环境变量
 echo "export PATH=\$PATH:/usr/local/cmake/bin" >> ~/.bashrc
+echo "export PATH=\$HOME/bin:\$PATH" >> ~/.bashrc
+echo "export PKG_CONFIG_PATH=\$HOME/ffmpeg_build/lib/pkgconfig:\$PKG_CONFIG_PATH" >> ~/.bashrc
 source ~/.bashrc
 
 # 编译nasm
@@ -48,7 +54,7 @@ make -j6 && make install
 
 # 编译x264
 cd ~/ffmpeg_sources/x264
-PATH="$HOME/bin:$PATH" PKG_CONFIG_PATH="$HOME/ffmpeg_build/lib/pkgconfig" ./configure --prefix="$HOME/ffmpeg_build" --bindir="$HOME/bin" --enable-static --extra-cflags=-fPIC
+./configure --prefix="$HOME/ffmpeg_build" --bindir="$HOME/bin" --enable-static --extra-cflags=-fPIC
 make -j6 && make install
 
 # 编译x265
@@ -79,7 +85,7 @@ make -j6 && make install
 
 # 编译ffmpeg
 cd ~/ffmpeg_sources/ffmpeg-4.4
-PATH="$HOME/bin:$PATH" PKG_CONFIG_PATH="$HOME/ffmpeg_build/lib/pkgconfig" ./configure \
+./configure \
   --prefix="$HOME/ffmpeg_build" \
   --pkg-config-flags="--static" \
   --extra-cflags="-I$HOME/ffmpeg_build/include -fPIC" \
@@ -100,13 +106,30 @@ PATH="$HOME/bin:$PATH" PKG_CONFIG_PATH="$HOME/ffmpeg_build/lib/pkgconfig" ./conf
   --enable-shared
 make -j6 && make install
 ```
---enable-shared \
-# 编译opencv[^1]
+
+如果`./configure`时去掉`--enable-shared`参数将会编译静态库。但使用FFmpeg编译的静态库的时候需要注意：**链接动态库**时要加上`-Wl,-Bsymbolic`选项，这个参数告诉链接器遇到重名符号时优先链接本地符号[^1]。否则将会出现错误：
+
+```text
+libavcodec.a(vc1dsp_mmx.o): relocation R_X86_64_PC32 against symbol `ff_pw_9' can not be used when making a shared object; recompile with -fPIC
+```
+
+# 编译OpenCV[^2]
 
 ```shell
+yum install wget unzip
+wget https://github.com/opencv/opencv/archive/4.5.3.zip
+wget https://github.com/opencv/opencv_contrib/archive/refs/tags/4.5.3.zip -O 4.5.3_contrib.zip
+unzip 4.5.3.zip
+unzip 4.5.3_contrib.zip
+cp -frap opencv_contrib-4.5.3/ opencv-4.5.3/
+cd opencv-4.5.3/
 mkdir build && cd build
-PKG_CONFIG_PATH="$HOME/ffmpeg_build/lib/pkgconfig" cmake -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=OFF -DWITH_FFMPEG=ON -DCMAKE_SHARED_LINKER_FLAGS=-Wl,-Bsymbolic ../
+cmake -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED_LIBS=OFF -DWITH_FFMPEG=ON -DCMAKE_SHARED_LINKER_FLAGS=-Wl,-Bsymbolic ../
 make -j6 && make install
 ```
 
-[^1]: Installation in Linux https://docs.opencv.org/master/d7/d9f/tutorial_linux_install.html
+OpenCV 4.5.3 的FFmpeg的视频模块需要修改一处源代码，将OpenCV源码目录下的`modules/videoio/src/cap_ffmpeg_hw.hpp`文件939行`s_stream_ = std::istringstream(accel_list);`修改为`s_stream_.str(accel_list);`。
+
+[^1]: 编译安装ffmpeg+OpenCV的时候的注意点 https://www.licc.tech/article?id=78
+
+[^2]: Installation in Linux https://docs.opencv.org/master/d7/d9f/tutorial_linux_install.html