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对$Ubuntu18.04$环境下的$\mathrm{ROS}$的$\mathrm{melodic}$版本安装$\mathrm{usb}-\mathrm{cam}$过程出现的两个常见问题提出解决方案。

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1.问题1：usb-cam功能包编译时出现"未定义的引用"的问题

问题描述：把$\mathrm{usb}-\mathrm{cam}$功能包下载到工作空间进行编译时，出现很多的"未定义的引用"的错误，具体问题见下图所示：

2.解决方案

查看$\mathrm{usb}-\mathrm{cam}$功能包下的$CMakeLists.txt$文件，其中$CMakeLists.txt$文件具体内容如下代码块所示：

cmake_minimum_required(VERSION 2.8.12)
project(usb_cam)## Find catkin macros and libraries
## if COMPONENTS list like find_package(catkin REQUIRED COMPONENTS xyz)
## is used, also find other catkin packages
find_package(catkin REQUIRED COMPONENTS cv_bridge image_transport roscpp std_msgs std_srvs sensor_msgs camera_info_manager)# Find OpenCV
find_package(OpenCV REQUIRED)## pkg-config libraries
find_package(PkgConfig REQUIRED)
pkg_check_modules(avcodec libavcodec REQUIRED)
pkg_check_modules(avutil libavutil REQUIRED)
pkg_check_modules(swscale libswscale REQUIRED)###################################################
## Declare things to be passed to other projects ##
##################################################### LIBRARIES: libraries you create in this project that dependent projects also need
## CATKIN_DEPENDS: catkin_packages dependent projects also need
## DEPENDS: system dependencies of this project that dependent projects also need
catkin_package(INCLUDE_DIRS includeLIBRARIES ${PROJECT_NAME}
)###########
## Build ##
###########include_directories(include${catkin_INCLUDE_DIRS}${avcodec_INCLUDE_DIRS}${swscale_INCLUDE_DIRS}${avutil_INCLUDE_DIRS}${OpenCV_INCLUDE_DIRS}
)## Build the USB camera library
add_library(${PROJECT_NAME} src/usb_cam.cpp)
target_link_libraries(${PROJECT_NAME}${avcodec_LINK_LIBRARIES}${avutil_LINK_LIBRARIES}${swscale_LINK_LIBRARIES}${catkin_LIBRARIES}${OpenCV_LIBS}
)## Declare a cpp executable
add_executable(${PROJECT_NAME}_node nodes/usb_cam_node.cpp)
target_link_libraries(${PROJECT_NAME}_node${PROJECT_NAME}${avcodec_LINK_LIBRARIES}${avutil_LINK_LIBRARIES}${swscale_LINK_LIBRARIES}${catkin_LIBRARIES}${OpenCV_LIBS}
)#############
## Install ##
############### Mark executables and/or libraries for installation
install(TARGETS ${PROJECT_NAME}_node ${PROJECT_NAME}RUNTIME DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION}LIBRARY DESTINATION ${CATKIN_PACKAGE_LIB_DESTINATION}
)## Copy launch files
install(DIRECTORY launch/DESTINATION ${CATKIN_PACKAGE_SHARE_DESTINATION}/launchFILES_MATCHING PATTERN "*.launch"
)install(DIRECTORY include/${PROJECT_NAME}/DESTINATION ${CATKIN_PACKAGE_INCLUDE_DESTINATION}FILES_MATCHING PATTERN "*.h" PATTERN "*.hpp"
)

分析$CMakeLists.txt$文件可知在编译内容上，所依赖的库：$\mathrm{avcodec}、\mathrm{swscale}、\mathrm{avutil}$，使用$\mathrm{locate}$命令找出路径：

###########
## Build ##
###########include_directories(include${catkin_INCLUDE_DIRS}${avcodec_INCLUDE_DIRS}${swscale_INCLUDE_DIRS}${avutil_INCLUDE_DIRS}${OpenCV_INCLUDE_DIRS}
)## Build the USB camera library
add_library(${PROJECT_NAME} src/usb_cam.cpp)
target_link_libraries(${PROJECT_NAME}${avcodec_LINK_LIBRARIES}${avutil_LINK_LIBRARIES}${swscale_LINK_LIBRARIES}${catkin_LIBRARIES}${OpenCV_LIBS}
)## Declare a cpp executable
add_executable(${PROJECT_NAME}_node nodes/usb_cam_node.cpp)
target_link_libraries(${PROJECT_NAME}_node${PROJECT_NAME}${avcodec_LINK_LIBRARIES}${avutil_LINK_LIBRARIES}${swscale_LINK_LIBRARIES}${catkin_LIBRARIES}${OpenCV_LIBS}
)

# 在终端下分别locate出三个库的具体路径
locate libavcodec
locate libavutil
locate libswscale

得出具体路径后，在$CMakeLists.txt$文件写入路径，见下面代码块：

## Declare a cpp executable
add_executable(${PROJECT_NAME}_node nodes/usb_cam_node.cpp)# CMakeLists.txt文件新增路径部分
# ====================================================================
set(avcodec_LINK_LIBRARIES /usr/lib/x86_64-linux-gnu/libavcodec.so)
set(avutil_LINK_LIBRARIES /usr/lib/x86_64-linux-gnu/libavutil.so)
set(swscale_LINK_LIBRARIES /usr/lib/x86_64-linux-gnu/libswscale.so)
# ====================================================================target_link_libraries(${PROJECT_NAME}_node${PROJECT_NAME}${avcodec_LINK_LIBRARIES}${avutil_LINK_LIBRARIES}${swscale_LINK_LIBRARIES}${catkin_LIBRARIES}${OpenCV_LIBS}
)

修改$CMakeLists.txt$文件后，返回工作空间进行编译，如下图所示：

3.问题2：usb-cam安装好后出现"找不到video0"的问题

问题描述：解决问题1后，成功进行编译，但在打开摄像头的时候出现找不到$Video0$的问题，具体如下图所示：

4.解决方案

# 1.关闭虚拟机，进入编辑虚拟机设置；
# 2.在"USB控制器选项"的"显示所有USB设备"前打上√；
# 3.启动虚拟机；
# 4.在虚拟机选项-->可移动设备-->USB 2.0 HD UVC Web Cam-->点击"连接";
# 5.查看是否显示video0;
cd /dev/
ls