undefined reference to `cv::calcOpticalFlowFarneback'

　　因为calcOpticalFlowFarneback函数是包含于<opencv2/video/trating.hpp>的，而caffe工程一般调试的opencv不包含video的头文件，所以，由此引出当编译报出此类跟opencv　video库相关的问题时解决方法。

　　１．Qt调试caffe工程，编译报出此类错误信息时，按照以下方法解决：

打开Denpendencies.cmake然后找到Opencv库中按照下列方法添加video
 

# ---[ OpenCV
if(USE_OPENCV)
  find_package(OpenCV QUIET COMPONENTS core highgui imgproc imgcodecs video)

２．终端编译caffe方式编译报出此类错误

１）打开MakeFile(注意不是MakeFile.config)，如下方式添加opencv_videoio，然后一定要先make clean，再重新make all，一般情况按照如下方式即能成功；如果还是报错，继续第二步操作

ifeq ($(USE_OPENCV), 1)
	LIBRARIES += opencv_core opencv_highgui opencv_imgproc

	ifeq ($(OPENCV_VERSION), 3)
		LIBRARIES += opencv_imgcodecs opencv_videoio
	endif
		
endif

2)打开MakeFile.config，将USE_PKG_CONFIG := 1前的注释符"#"去除，再make clean, make all

# Uncomment to use `pkg-config` to specify OpenCV library paths.
# (Usually not necessary -- OpenCV libraries are normally installed in one of the above $LIBRARY_DIRS.)
USE_PKG_CONFIG := 1