title: Draw on webcam using OpenCV
body: <p>I want to draw/paint on a webcam screen using OpenCV. Since I'm reading from a cam, the frames are constantly changing, so I'm trying to figure out a way to keep or save the drawing on the current frame and use it for the next frame. The code below allows you to draw on the screen but when it gets the next frame, the drawing is gone and it starts over.</p>
  
  <p>Could someone please help me ... Thanks.</p>
  
  <pre><code>          CvCapture *input;
            input = cvCaptureFromCAM( 0 );
  
            cvSetMouseCallback("Demo",&amp;on_mouse, 0);
  
                   for(;;)
                      {
                          frame = cvQueryFrame(input);
  
                          if(!image)
                          {
                              image = cvCreateImage( cvSize(frame-&gt;width, frame-&gt;height), IPL_DEPTH_8U, 3);
                              screenBuffer = cvCreateImage( cvSize(frame-&gt;width, frame-&gt;height), IPL_DEPTH_8U, 3);
                          }
  
                          cvCopy(frame, image, 0);
  
                          if(drawing) //drawing is a global variable
                          { 
                             cvCircle(image, cvPoint(last_x,last_y), 10,CV_RGB(red,green,blue), -1, CV_AA, 0);
                             cvCopy(image, screenBuffer, 0);
                          }
  
                          cvShowImage( "Demo", screenBuffer );
                  }
  
  
          void on_mouse( int event, int x, int y, int flags, void* param )
          {
              last_x = x;
              last_y = y;
  
              if(event==CV_EVENT_LBUTTONDOWN)
                  {
                      drawing = 1;
                  }
          }
  </code></pre>