Perspective transformation OpenCV C++

This tutorial will show how OpenCV Perspective transformation can use to transform image. Here I used two OpenCV function, getPerspectiveTransform and warpPerspective. Below code demonstrate how the PerspectiveTransform work with mouse. First we need to get 4 points which is the source point to transform. This done by right clicking the mouse on each corner of the card. After selecting four point drag the mouse to draw the rectangle which will used to get the transformed point.

The Video demonstrate the usage of the application and how to select the points.

Demo Video

C++ Code

Mat src, img, ROI;
Rect cropRect(0, 0, 0, 0);
Point P1(0, 0);
Point P2(0, 0);
std::vector < Point2f > quad_pts;
std::vector < Point2f > rect_pts;

const char *winName = "Crop Image";
bool clicked = false;
int i = 0;
char imgName[15];

void prspectiveTransform() {

  Mat transmtx = getPerspectiveTransform(quad_pts, rect_pts);
  int offsetSize = 150;
  Mat transformed = Mat::zeros(cropRect.height + offsetSize,
                               cropRect.width + offsetSize, CV_8UC3);
  warpPerspective(src, transformed, transmtx, transformed.size());

  imshow("quadrilateral", transformed);
}
void checkBoundary() {
  // check croping rectangle exceed image boundary
  if (cropRect.width > img.cols - cropRect.x)
    cropRect.width = img.cols - cropRect.x;

  if (cropRect.height > img.rows - cropRect.y)
    cropRect.height = img.rows - cropRect.y;

  if (cropRect.x < 0)
    cropRect.x = 0;

  if (cropRect.y < 0)
    cropRect.height = 0;
}

void showImage() {
  img = src.clone();
  checkBoundary();

  if (quad_pts.size() == 1)
    circle(img, quad_pts[0], 5, Scalar(0, 0, 255), 1.5, LINE_AA, 0);
  if (quad_pts.size() > 1) {
    for (int i = 0; i < quad_pts.size() - 1; i++) {
      line(img, quad_pts[i], quad_pts[i + 1], Scalar(0, 0, 255), 1, LINE_AA, 0);
      circle(img, quad_pts[i], 5, Scalar(0, 0, 255), 1.5, LINE_AA, 0);
      circle(img, quad_pts[i + 1], 5, Scalar(0, 0, 255), 1.5, LINE_AA, 0);
    }
  }

  if (quad_pts.size() == 4)
    line(img, quad_pts[0], quad_pts[3], Scalar(0, 0, 255), 1, LINE_AA, 0);

  rectangle(img, cropRect, Scalar(0, 255, 0), 1, 8, 0);
  imshow(winName, img);
}

void onMouse(int event, int x, int y, int f, void *) {

  switch (event) {

  case EVENT_RBUTTONDOWN:
    if (quad_pts.size() < 4)
      quad_pts.push_back(Point(x, y));
    else
      cout << "Maximum allowd point is 4, Try to remove by pressing 'r' "
           << endl;
    break;

  case EVENT_LBUTTONDOWN:
    clicked = true;

    P1.x = x;
    P1.y = y;
    P2.x = x;
    P2.y = y;
    break;

  case EVENT_LBUTTONUP:
    P2.x = x;
    P2.y = y;
    clicked = false;
    break;

  case EVENT_MOUSEMOVE:
    if (clicked) {
      P2.x = x;
      P2.y = y;
    }
    break;

  default:
    break;
  }

  if (clicked) {
    if (P1.x > P2.x) {
      cropRect.x = P2.x;
      cropRect.width = P1.x - P2.x;
    } else {
      cropRect.x = P1.x;
      cropRect.width = P2.x - P1.x;
    }

    if (P1.y > P2.y) {
      cropRect.y = P2.y;
      cropRect.height = P1.y - P2.y;
    } else {
      cropRect.y = P1.y;
      cropRect.height = P2.y - P1.y;
    }

    rect_pts.clear();
    rect_pts.push_back(Point(cropRect.x, cropRect.y));
    rect_pts.push_back(Point(cropRect.x + cropRect.width, cropRect.y));
    rect_pts.push_back(
        Point(cropRect.x + cropRect.width, cropRect.y + cropRect.height));
    rect_pts.push_back(Point(cropRect.x, cropRect.y + cropRect.height));
  }

  if ((rect_pts.size() == 4) && (quad_pts.size() == 4))
    prspectiveTransform();
  showImage();
}
int main() {

  cout << "Right click each corner of card to select the point to be "
          "trasnformed."
       << endl
       << endl;
  cout << "Click and drag for Selection" << endl << endl;
  cout << "------> Press 's' to save" << endl << endl;

  cout << "------> Press '8' to move up" << endl;
  cout << "------> Press '2' to move down" << endl;
  cout << "------> Press '6' to move right" << endl;
  cout << "------> Press '4' to move left" << endl << endl;

  cout << "------> Press 'w' increas top" << endl;
  cout << "------> Press 'x' increas bottom" << endl;
  cout << "------> Press 'd' increas right" << endl;
  cout << "------> Press 'a' increas left" << endl << endl;

  cout << "------> Press 't' decrease top" << endl;
  cout << "------> Press 'b' decrease bottom" << endl;
  cout << "------> Press 'h' decrease right" << endl;
  cout << "------> Press 'f' decrease left" << endl << endl;

  cout << "------> Press 'r' to reset quad points" << endl;
  cout << "------> Press 'Esc' to quit" << endl << endl;

  src = imread("card.png", 1);

  namedWindow(winName, 1);
  setMouseCallback(winName, onMouse, NULL);
  imshow(winName, src);

  while (1) {
    char c = waitKey();
    if (c == 's' && ROI.data) {
      sprintf(imgName, "%d.jpg", i++);
      imwrite(imgName, ROI);
      cout << "  Saved " << imgName << endl;
    }
    if (c == '6')
      cropRect.x++;
    if (c == '4')
      cropRect.x--;
    if (c == '8')
      cropRect.y--;
    if (c == '2')
      cropRect.y++;

    if (c == 'w') {
      cropRect.y--;
      cropRect.height++;
    }
    if (c == 'd')
      cropRect.width++;
    if (c == 'x')
      cropRect.height++;
    if (c == 'a') {
      cropRect.x--;
      cropRect.width++;
    }

    if (c == 't') {
      cropRect.y++;
      cropRect.height--;
    }
    if (c == 'h')
      cropRect.width--;
    if (c == 'b')
      cropRect.height--;
    if (c == 'f') {
      cropRect.x++;
      cropRect.width--;
    }

    if (c == 27)
      break;
    if (c == 'r') {
      if (quad_pts.size() > 0) {
        quad_pts.pop_back();
        showImage();
      }
    }
  }

  return 0;
}