example_09-02.cpp

// Toy program for using a mouse to draw boxes on the screen
//
#include <opencv2/opencv.hpp>

using namespace std;

// Define our callback which we will install for
// mouse events
//
void my_mouse_callback(
  int event, int x, int y, int flags, void* param
);

cv::Rect box;
bool drawing_box = false;

// A little subroutine to draw a box onto an image
//
void draw_box( cv::Mat& img, cv::Rect box ) {
  cv::rectangle(
    img,
    box.tl(),
    box.br(),
    cv::Scalar(0x00,0x00,0xff) /* red */
  );
}

void help() {
  std::cout
    << "Call: ./ch4_ex4_1\n"
    << " shows how to use a mouse to draw regions in an image."
    << std::endl;
}

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

  help();

  box = cv::Rect(-1,-1,0,0);
  cv::Mat image(200, 200, CV_8UC3), temp;
  image.copyTo(temp);

  box = cv::Rect(-1,-1,0,0);
  image = cv::Scalar::all(0);

  cv::namedWindow( "Box Example" );

  // Here is the crucial moment where we actually install
  // the callback. Note that we set the value of 'params' to
  // be the image we are working with so that the callback
  // will have the image to edit.
  //
  cv::setMouseCallback(
    "Box Example",
    my_mouse_callback,
    (void*)&image
  );
  // The main program loop. Here we copy the working image
  // to the temp image, and if the user is drawing, then
  // put the currently contemplated box onto that temp image.
  // Display the temp image, and wait 15ms for a keystroke,
  // then repeat.
  //
  for(;;) {

    image.copyTo(temp);

    if( drawing_box ) draw_box( temp, box );

    cv::imshow( "Box Example", temp );

    if( cv::waitKey( 15 ) == 27 ) break;

  }
  return 0;
}

// This is our mouse callback. If the user
// presses the left button, we start a box.
// When the user releases that button, then we
// add the box to the current image. When the
// mouse is dragged (with the button down) we
// resize the box.
//
void my_mouse_callback(
  int event,
  int x,
  int y,
  int flags,
  void* param
) {

  cv::Mat& image = *(cv::Mat*) param;

  switch( event ) {

    case cv::EVENT_MOUSEMOVE: {
      if( drawing_box ) {
        box.width = x-box.x;
        box.height = y-box.y;
      }
    }
    break;

    case cv::EVENT_LBUTTONDOWN: {
      drawing_box = true;
      box = cv::Rect( x, y, 0, 0 );
    }
    break;

    case cv::EVENT_LBUTTONUP: {
      drawing_box = false;
      if( box.width < 0 ) {
        box.x += box.width;
        box.width *= -1;
      }
      if( box.height < 0 ) {
        box.y += box.height;
        box.height *= -1;
      }
      draw_box( image, box );
    }
    break;

  }

}