向现有图形Opencv添加另一个点

Question

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So far I know that there are limitation regarding to Opencv Plotting real time graph. So I currently modifying the code from histogram graph to plot a real time graph. Below will be my code. But the result is not as I expected as using loop to draw the graph, every time it will plot a NEW graph instant of plot on existing graph. Again, the code below work just fine.

int hist_w = 700; int hist_h = 700; //image size
void show_histogram(std::string const& name, cv::Mat1b const& image, cv::Mat &histImage)
{
double seconds = difftime( time(0), start);


  /// Establish the number of bins
  int histSize = 1000;

  /// Set the ranges 
  float range[] = { 0, 1000 } ;
  const float* histRange = { range };

  bool uniform = true; bool accumulate = false;

  Mat hist;

  /// Compute the histograms:
  calcHist( &image, 1, 0, Mat(), hist, 1, &histSize, &histRange, uniform, accumulate );

  //Draw the histograms
  //int bin_w = cvRound( (double) hist_w/histSize );
   int bin_w = cvRound( (double) seconds);//x-axis



  /// Normalize the result to [ 0, histImage.rows ]
  normalize(hist, hist, 0, histImage.rows, NORM_MINMAX, -1, Mat() );

  /// Draw for points,for y-axis
  for( int i = 1; i < histSize; i++ )
  {
    Point ptPrev =  Point( bin_w*(seconds-1), hist_h - cvRound(hist.at<float>(0,255)));
    Point ptNew =    Point( bin_w*(seconds), hist_h - cvRound(hist.at<float>(0,255)));
    line( histImage,  ptPrev,ptNew,cv::Scalar::all(255), 5, 8, 0  );
    ptPrev = ptNew;
  }

  /// Display
   imshow(name, histImage);

 }

int main( int argc, char** argv )
{

//On spot video 
VideoCapture cap(0); // open the default camera
if(!cap.isOpened())  // check if we succeeded
    return -1;

Mat edges,gray;
  namedWindow("edges",1);
  Mat histImage( hist_h, hist_w, CV_8UC3, Scalar( 0,0,0) );
  for(;;)
{
    Mat frame;
    cap >> frame; // get a new frame from camera    
    imshow("Frame",frame);        
    cvtColor(frame, gray, CV_BGR2GRAY);
    GaussianBlur(gray, edges, Size(7,7), 1.5, 1.5);
    Canny(edges, edges, 0, 30, 3);
    cv::namedWindow("edges",0);  
    cv::resizeWindow("edges", 100, 100);       
    imshow("edges", edges);
    show_histogram("White Pixel",edges);
    if(waitKey(30) >= 0) break;
}

cv::waitKey();
return 0;

}

Answer 1

Your problem is that every iteration of the for(;;) you mentioned in the comments you are creating a new Mat histImage . You need to create it outside of the function, and pass it as an argument(by reference) to the function, make the changes and show the image.

Delete these lines from the body of the function, and put them outside the for(;;) loop:

int hist_w = 700; int hist_h = 700; //image size
Mat histImage( hist_h, hist_w, CV_8UC3, Scalar( 0,0,0) );

Change the function signature to be:

void show_histogram(std::string const& name, cv::Mat1b const& image, cv::Mat &histImage)