Using mouse to select image region (OpenCV and C++)

main.cpp

#include <opencv2/opencv.hpp>

using namespace cv;
using namespace std;

struct SelectionState {
  Point startPt, endPt, mousePos;
  bool started = false, done = false;

  Rect toRect() {
    return Rect (
      min(this->startPt.x, this->mousePos.x),
      min(this->startPt.y, this->mousePos.y),
      abs(this->startPt.x-this->mousePos.x),
      abs(this->startPt.y-this->mousePos.y));
  }
};

void onMouse(int event, int x, int y, int, void *data) {
  SelectionState *state = (SelectionState*) data;

  switch(event) {
    case EVENT_LBUTTONDOWN:
      state->startPt.x = x;
      state->startPt.y = y;
      state->mousePos.x = x;
      state->mousePos.y = y;
      state->started = true;
      break;

    case EVENT_LBUTTONUP:
      state->endPt.x = x;
      state->endPt.y = y;
      state->done = true;
      break;

    case EVENT_MOUSEMOVE:
      state->mousePos.x = x;
      state->mousePos.y = y;
      break;
  }
}

Rect selectRect(Mat image, Scalar color=Scalar(255,0,0), int thickness=2) {
  const string window = "rect";
  SelectionState state;
  namedWindow(window, WINDOW_NORMAL);
  setMouseCallback(window, onMouse, &state);

  while(!state.done) {
    waitKey(100);

    if(state.started) {
      Mat copy = image.clone();
      Rect selection = state.toRect();
      rectangle(copy, selection, color, thickness);
      imshow(window, copy);
    } else {
      imshow(window, image);
    }
  }

  return state.toRect();
}

int main(int argc, char *argv[]) {
  Mat img = imread("image.png", CV_LOAD_IMAGE_COLOR);
  Rect rect = selectRect(img);

  cout << "Selected " << rect << endl;
}