三帧法

三帧差法是在相邻帧差法基础上改进的算法，在一定程度上优化了运动物体双边，粗轮廓的现象，相比之下，三帧差法比相邻帧差法更适用于物体移动速度较快的情况，比如道路上车辆的智能监控。

三帧差法基本实现步骤：

1. 前两帧图像做灰度差

2. 当前帧图像与前一帧图像做灰度差

3. 1和2的结果图像按位做“与”操作

#include "core/core.hpp"  
#include "highgui/highgui.hpp"  
#include "imgproc/imgproc.hpp"  

using namespace cv;

int main()
{
	VideoCapture videoCap;
	videoCap.open(0);
	if (!videoCap.isOpened())
	{
		return -1;
	}
	Mat framePrePre; //上上一帧  
	Mat framePre; //上一帧  
	Mat frameNow; //当前帧  
	Mat frameDet; //运动物体  
	videoCap >> framePrePre;
	videoCap >> framePre;
	cvtColor(framePrePre, framePrePre, CV_RGB2GRAY);
	cvtColor(framePre, framePre, CV_RGB2GRAY);
	int save = 0;
	while (true)
	{
		videoCap >> frameNow;
		cvtColor(frameNow, frameNow, CV_RGB2GRAY);
		Mat Det1;
		Mat Det2;
		absdiff(framePrePre, framePre, Det1);  //帧差1  
		absdiff(framePre, frameNow, Det2);     //帧差2  
		threshold(Det1, Det1, 0, 255, CV_THRESH_OTSU);  //自适应阈值化  
		threshold(Det2, Det2, 0, 255, CV_THRESH_OTSU);
		Mat element = getStructuringElement(0, Size(3, 3));  //膨胀核  
		dilate(Det1, Det1, element);    //膨胀  
		dilate(Det2, Det2, element);
		bitwise_and(Det1, Det2, frameDet);
		framePrePre = framePre;
		framePre = frameNow;
		imshow("Video", frameNow);
		imshow("Detection", frameDet);
		if (cvWaitKey(27) == 27)
			break;
	}
	return 0;
}