Android平台实时推流的架构经过调研多以直播平台的方式为主，与目前的应用场景不一样；主要是设备间的低延迟（百毫秒）摄像头预览的推流。基本上RTSP/RTMP 的流式框架太重，且都进行了多次转码，对机器的性能要求较高。
采用嵌入式推流框架的mjpg-streamer框架进行Android平台适配：
优点：
- 低延迟
- 对机器性能要求低，占用机器资源少（必经在嵌入式设备上都可以跑）
缺点：
- 占用带宽要高于推流RTMP等架构
- 直接适配后需要root权限进行启动推流，集成到app还待验证
适配过程
原地址仓库：https://github.com/jacksonliam/mjpg-streamer.git
因为作者不再维护了，fork一个版本，适配Android，后续自己维护，仓库：https://github.com/braincs/mjpg-streamer
编译脚本
采用cmake进行编译的，发现已有CMakeLists.txt，直接进行cmake编译即可
便携生成Android执行文件shell脚本
#/bin/bash
export ANDROID_NDK=/opt/android-ndk-r21e
rm -r build
mkdir build && cd build
cmake -DCMAKE_TOOLCHAIN_FILE=$ANDROID_NDK/build/cmake/android.toolchain.cmake \
-DANDROID_ABI="arm64-v8a" \
-DANDROID_NDK=$ANDROID_NDK \
-DANDROID_PLATFORM=android-26 \
..
make #&& make install
cd ..
注意修改自己的NDK路径 export ANDROID_NDK=<你的ndk路径>
jpeg库的适配
发现cmake中有对jpeg库的依赖，注释掉后，自己进行适配；所以注释掉这行如下：
# find_library(JPEG_LIB jpeg)
将编译好的libjepg.so 和头文件放入到thirdparty 路径下，并进行cmake配置链接libjpeg库
set(THIRDPARTY ${CMAKE_SOURCE_DIR}/thirdparty)
include_directories(${THIRDPARTY}/libjpeg/include)
pthread库适配
报错：
/opt/android-ndk-r21e/toolchains/llvm/prebuilt/linux-x86_64/lib/gcc/aarch64-linux-android/4.9.x/../../../../aarch64-linux-android/bin/ld: cannot find -lpthread
clang: error: linker command failed with exit code 1 (use -v to see invocation)
在Linux平台下引入pthread库如下
target_link_libraries(mjpg_streamer pthread dl)
但在Android上已经默认集成了pthread库，所以采用以下方式进行适配
target_link_libraries(mjpg_streamer
-pthread
dl)
CMakeFiles/input_uvc.dir/input_uvc.c.o: In function `input_init':
/home/changshuai/cworkspace/mjpg-streamer/mjpg-streamer-experimental/plugins/input_uvc/input_uvc.c:254: undefined reference to `parse_resolution_opt'
/home/changshuai/cworkspace/mjpg-streamer/mjpg-streamer-experimental/plugins/input_uvc/input_uvc.c:(.text.input_init+0x8e4): undefined reference to `resolutions_help'
CMakeFiles/input_uvc.dir/input_uvc.c.o: In function `help':
/home/changshuai/cworkspace/mjpg-streamer/mjpg-streamer-experimental/plugins/input_uvc/input_uvc.c:531: undefined reference to `resolutions_help'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
plugins/input_uvc/CMakeFiles/input_uvc.dir/build.make:173: recipe for target 'plugins/input_uvc/input_uvc.so' failed
make[2]: *** [plugins/input_uvc/input_uvc.so] Error 1
CMakeFiles/Makefile2:286: recipe for target 'plugins/input_uvc/CMakeFiles/input_uvc.dir/all' failed
make[1]: *** [plugins/input_uvc/CMakeFiles/input_uvc.dir/all] Error 2
Makefile:129: recipe for target 'all' failed
make: *** [all] Error 2
plugins库适配
mjpg-streamer采用插件式的模式，所以对plugin插件很重要，plugins插件库中主要问题有2个：
- Android的pthread库中没有pthread_cancel 方法，需要进行替换，替换成 pthread_kill
- uvc的插件中有对外层的utils.c和utils.h 有依赖，需要调用对输入参考的解析，会报undefined symbol的错误。
解决办法
1替换 pthread_cancel
以 input_uvc.c为例
/******************************************************************************
Description.: Stops the execution of worker thread
Input Value.: -
Return Value: always 0
******************************************************************************/
int input_stop(int id)
{
input * in = &pglobal->in[id];
context *pctx = (context*)in->context;
DBG("will cancel camera thread #%02d\n", id);
pthread_kill(pctx->threadID, 0); //替换这行
return 0;
}
替换的文件如下
modified: plugins/input_control/input_uvc.c
modified: plugins/input_file/input_file.c
modified: plugins/input_http/input_http.c
modified: plugins/input_ptp2/input_ptp2.c
modified: plugins/input_uvc/input_uvc.c
modified: plugins/output_autofocus/output_autofocus.c
modified: plugins/output_file/output_file.c
modified: plugins/output_http/output_http.c
modified: plugins/output_rtsp/output_rtsp.c
modified: plugins/output_udp/output_udp.c
modified: plugins/output_viewer/output_viewer.c
2增加对libjpeg库的连接
只有uvc库对libjpeg有依赖，所以在uvc插件库的CMakeLists.txt中进行修改
plugins/input_uvc/CMakeLists.txt
check_include_files(linux/videodev2.h HAVE_LINUX_VIDEODEV2_H)
MJPG_STREAMER_PLUGIN_OPTION(input_uvc "Video 4 Linux input plugin"
ONLYIF HAVE_LINUX_VIDEODEV2_H)
if (PLUGIN_INPUT_UVC)
add_definitions(-DLINUX -D_GNU_SOURCE)
find_library(V4L2_LIB v4l2)
# message("${THIRDPARTY}/libjpeg/lib")
find_library(JPEG_LIB NAMES jpeg PATHS ${THIRDPARTY}/libjpeg/lib NO_DEFAULT_PATH)
if (V4L2_LIB)
message("---V4L2_LIB")
add_definitions(-DUSE_LIBV4L2)
endif (V4L2_LIB)
if (NOT JPEG_LIB)
message("---not JPEG_LIB")
add_definitions(-DNO_LIBJPEG)
endif (NOT JPEG_LIB)
message("----------${JPEG_LIB}")
MJPG_STREAMER_PLUGIN_COMPILE(input_uvc dynctrl.c
input_uvc.c
jpeg_utils.c
v4l2uvc.c
utils.c)
if (V4L2_LIB)
target_link_libraries(input_uvc ${V4L2_LIB})
endif (V4L2_LIB)
if (JPEG_LIB)
message("---JPEG_LIB, link ${THIRDPARTY}/libjpeg/lib/libjpeg.so")
target_link_libraries(input_uvc ${JPEG_LIB})
endif (JPEG_LIB)
target_link_libraries(input_uvc ${THIRDPARTY}/libjpeg/lib/libjpeg.so) # 这行是关键
endif()
发现用find_library的方式适配有点问题，直接强行在最后加入链接了
使用
测试机器：RK3399的一块开发板，带USB摄像头
脚本：将使用的程序+so库+www资源文件推到设备上
# 将build产物推到机器上
adb shell mkdir /data/local/tmp/mstreamer
adb push ./build/mjpg_streamer /data/local/tmp/mstreamer/
adb push ./build/plugins/input_uvc/input_uvc.so /data/local/tmp/mstreamer/
adb push ./build/plugins/output_http/output_http.so /data/local/tmp/mstreamer/
adb push ./mjpg-streamer-experimental/www /data/local/tmp/mstreamer/
adb 进入机器执行：
adb shell
cd /data/local/tmp/mstreamer/
export LD_LIBRARY_PATH="$(pwd):$LD_LIBRARY_PATH"
./mjpg_streamer -i "./input_uvc.so" -o "./output_http.so -w ./www"
