Sample code doesn't run: OpenCV Error: Assertion failed (ni > 0 && ni == ni1) in collectCalibrationData

[abidrahmank]

Sample code doesn't run: OpenCV Error: Assertion failed (ni > 0 && ni == ni1) in collectCalibrationData

[SaberD]

I had the same problem. I used a different size checkered board image. Mine was size 6,9 and not 7,6.
I got this error when i did not change the dimension in some, but not all the necessary places.
Below is the code that worked for me, with the checkered board image saved as "test.png" in the same folder as the python file. link 🏁 to the image i used

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

objp = np.zeros((6*9,3), np.float32)
objp[:,:2] = np.mgrid[0:9,0:6].T.reshape(-1,2)

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

img = cv2.imread("test2.png")

gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)

# Find the chess board corners
ret, corners = cv2.findChessboardCorners(gray, (6,9),None)

# If found, add object points, image points (after refining them)
if ret == True:
    objpoints.append(objp)

    corners2 = cv2.cornerSubPix(gray,corners,(11,11),(-1,-1),criteria)
    imgpoints.append(corners2)

# Draw and display the corners
cv2.drawChessboardCorners(img, (6,9), corners2,ret)
cv2.imshow('img',img)
cv2.waitKey(500)

[mgmacias95]

Hi @alexblack

I had the same issue and after a little research I found the problem in stackoverflow

I resolved it changing the shape of objp:

objp = objp.reshape(-1,1,3)

Also I had another problem: the number of corners found by findChessboardCorners can be less than 7*6 (pattern size) so I just kept number of corners found 3D points:

corners2 = cv2.cornerSubPix(image=gray, corners=corners, winSize=(11,11), zeroZone=(-1,-1),
                            criteria=(cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 30, 0.001))
imgpoints.append(corners2)
objpoints.append(objp[0:corners2.shape[0]])

after that the code worked fine 😄

[rabiafyz]

Hi @mgmacias95

I did not understand what you mean by doing reshape obj.
Do you mean reshap it at the time of initailization?

[mgmacias95]

Hi @rabiafyz

Yes, just before the for loop:

# 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((6*7,3), np.float32)
objp[:,:2] = np.mgrid[0:7,0:6].T.reshape(-1,2)
objp = objp.reshape(-1,1,3)

# 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.

images = glob.glob('*.jpg')

for fname in images:
    print 'Filename: ', fname
    .
    .
    .

[petered]

I had a similar problem (error: (-215) ni >= 0 in function collectCalibrationData). For me it was that my objp points were float64 instead of float32, which was not exactly obvious from that message.

[ujwalaparupalli]

Hi @mgmacias95 @petered @alexblack ,

I am also having the same problem. I tried the above suggested changes but still of no help and getting the same error. can someone please help me.

Thank you.