docs: private void calc() {
          int hMin = (int) ((this.cImage.getHeight()) / 4.0);
          int hMax = (int) ((this.cImage.getHeight()) * 3.0 / 4.0);
          init();
  
          for (int y = hMin; y < hMax; y++) {
              for (int x = 1; x < (this.cImage.getWidth() - 2); x++) {
                  // only lower edges are considered
                  if (ImageUtil.isBlack(this.cImage, x, y)) {
                      if (!ImageUtil.isBlack(this.cImage, x, y + 1)) {
                          calc(x, y);
                      }
                  }
              }
          }
  
      }