error: (-215) nimages > 0 in function calibrateCamera using Python and OpenCV

Question

I'm trying to calibrate my web-cam based on the example given in the opencv samples but when i run the code provided here:

def caliLeftCam():    
    args, img_mask = getopt.getopt(sys.argv[1:], '', ['save=', 'debug=', 'square_size='])
    args = dict(args)
    try: img_mask = img_mask[0]
    except: img_mask = '../cpp/img*.jpg'
    img_names = glob(img_mask)
    debug_dir = args.get('--debug')
    square_size = float(args.get('--square_size', 1.0))

    pattern_size = (7, 5)
    pattern_points = np.zeros( (np.prod(pattern_size), 3), np.float32 )
    pattern_points[:,:2] = np.indices(pattern_size).T.reshape(-1, 2)
    pattern_points *= square_size

    obj_points = []
    img_pointsL = []
    h, w = 0, 0
    for fn in img_names:
        print "processing %s..." % fn,
        imgL = cv2.imread(fn, 0)
        h, w = imgL.shape[:2]
        found, corners = cv2.findChessboardCorners(imgL, pattern_size)
        if found:
            term = ( cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_COUNT, 30, 0.1 )
            cv2.cornerSubPix(imgL, corners, (5, 5), (-1, -1), term)
        if debug_dir:
            vis = cv2.cvtColor(imgL, cv2.COLOR_GRAY2BGR)
            cv2.drawChessboardCorners(vis, pattern_size, corners, found)
            path, name, ext = splitfn(fn)
            cv2.imwrite('%s/%s_chess.bmp' % (debug_dir, name), vis)
        if not found:
            print "chessboard not found"
            continue
        img_pointsL.append(corners.reshape(-1, 2))
        obj_points.append(pattern_points)

        print 'ok'

    rmsL, cameraL_matrix, dist_coefsL, rvecsL, tvecsL = cv2.calibrateCamera(obj_points, img_pointsL, (w, h))

i got this error:

Traceback (most recent call last):
File "/home/sabrine/Downloads/opencv-2.4.9/samples/python2/Memo.py", line 293, in <module>
Img_pointsL, Cam_MatL, DisL = caliLeftCam()
File "/home/sabrine/Downloads/opencv-2.4.9/samples/python2/Memo.py", line 124, in caliLeftCam
rmsL, cameraL_matrix, dist_coefsL, rvecsL, tvecsL = cv2.calibrateCamera(obj_points, img_pointsL, (w, h))
error: /build/buildd/opencv-2.4.8+dfsg1/modules/calib3d/src/calibration.cpp:3415: 
error: (-215) nimages > 0 in function calibrateCamera

what does this error mean? and how can i solve it?

Answer 1

The error says that one of the vectors provided as arguments is empty.

The function has an assertion that prevents You from using it, if not all conditions are met. This time it checks if there are enough image points (nimages > 0 must be true).