<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def find_chessboard(self, sx=6, sy=9):
 """Finds the corners of an sx X sy chessboard in the image. Parameters sx : int Number of chessboard corners in x-direction. sy : int Number of chessboard corners in y-direction. Returns ------- :obj:`list` of :obj:`numpy.ndarray` A list containing the 2D points of the corners of the detected chessboard, or None if no chessboard found. """

        # termination criteria
        criteria = (cv2.TERM_CRITERIA_EPS +
                    cv2.TERM_CRITERIA_MAX_ITER, 30, 0.001)

        # prepare object points, like (0,0,0), (1,0,0), (2,0,0) ....,(6,5,0)
        objp = np.zeros((sx * sy, 3), np.float32)
        objp[:, :2] = np.mgrid[0:sx, 0:sy].T.reshape(-1, 2)

        # Arrays to store object points and image points from all the images.
        objpoints = []  # 3d point in real world space
        imgpoints = []  # 2d points in image plane.

        # create images
        img = self.data.astype(np.uint8)
        gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
        img_rgb = cv2.cvtColor(img, cv2.COLOR_BGR2RGB)

        # Find the chess board corners
        ret, corners = cv2.findChessboardCorners(gray, (sx, sy), None)

        # If found, add object points, image points (after refining them)
        if ret:
            objpoints.append(objp)
            cv2.cornerSubPix(gray, corners, (11, 11), (-1, -1), criteria)
            imgpoints.append(corners)

            if corners is not None:
                return corners.squeeze()
        return None