OpenCV4开发入门教程147：帧差图absdiff()

如果摄像机是固定的，那么我们可以认为场景（背景）大多数情况下是不变的，而只有前景（被跟踪的目标）会运动，这样就可以建立背景模型。通过比较当前帧和背景模型，就能轻松地跟踪目标运动情况了。这里，最容易想到的比较方式就是当前帧减去背景模型了

函数声明

void cv::absdiff 	( 	InputArray  	src1,
		InputArray  	src2,
		OutputArray  	dst 
	)

输入1
输入2
输出

计算两个数组之间或数组与标量之间的每元素绝对差。

可以简单的理解为结果是画面中动的部分

// FRAME DIFFERENCING

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/video/background_segm.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include <iostream>
#include <sstream>

using namespace cv;
using namespace std;

Mat frameDiff(Mat prevFrame, Mat curFrame, Mat nextFrame)
{
    Mat diffFrames1, diffFrames2, output;

    // Compute absolute difference between current frame and the next frame
    absdiff(nextFrame, curFrame, diffFrames1);

    // Compute absolute difference between current frame and the previous frame
    absdiff(curFrame, prevFrame, diffFrames2);

    // Bitwise "AND" operation between the above two diff images
    bitwise_and(diffFrames1, diffFrames2, output);

    return output;
}

Mat getFrame(VideoCapture cap, float scalingFactor)
{
    //float scalingFactor = 0.5;
    Mat frame, output;

    // Capture the current frame
    cap >> frame;

    // Resize the frame
    resize(frame, frame, Size(), scalingFactor, scalingFactor, INTER_AREA);

    // Convert to grayscale
    cvtColor(frame, output, CV_BGR2GRAY);

    return output;
}

int main(int argc, char* argv[])
{
    Mat frame, prevFrame, curFrame, nextFrame;
    char ch;

    // Create the capture object
    // 0 -> input arg that specifies it should take the input from the webcam
    VideoCapture cap(0);

    // If you cannot open the webcam, stop the execution!
    if( !cap.isOpened() )
        return -1;

    //create GUI windows
    namedWindow("Frame");

    // Scaling factor to resize the input frames from the webcam
    float scalingFactor = 0.75;

    prevFrame = getFrame(cap, scalingFactor);
    curFrame = getFrame(cap, scalingFactor);
    nextFrame = getFrame(cap, scalingFactor);

    // Iterate until the user presses the Esc key
    while(true)
    {
        // Show the object movement
        imshow("Object Movement", frameDiff(prevFrame, curFrame, nextFrame));

        // Update the variables and grab the next frame
        prevFrame = curFrame;
        curFrame = nextFrame;
        nextFrame = getFrame(cap, scalingFactor);

        // Get the keyboard input and check if it's 'Esc'
        // 27 -> ASCII value of 'Esc' key
        ch = waitKey( 30 );
        if (ch == 27) {
            break;
        }
    }

    // Release the video capture object
    cap.release();

    // Close all windows
    destroyAllWindows();

    return 1;
}