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Assertion failed when use opencv in python

I am doing Camera Calibration in opencv in python and I followed the tutorials on this page. My code is completely copied from the page with tiny adjustment on the parameters.
Code:
import numpy as np
import cv2
import glob
# 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)
# 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('../easyimgs/*.jpg')
print('...loading')
for fname in images:
print(f'processing img:{fname}')
img = cv2.imread(fname)
gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
print('grayed')
# Find the chess board corners
ret, corners = cv2.findChessboardCorners(gray, (8, 11),None)
# If found, add object points, image points (after refining them)
if ret == True:
print('chessboard detected')
objpoints.append(objp)
corners2 = cv2.cornerSubPix(gray,corners,(11,11),(-1,-1),criteria)
imgpoints.append(corners2)
# Draw and display the corners
img = cv2.drawChessboardCorners(img, (8,11), corners2,ret)
cv2.namedWindow('img',0)
cv2.resizeWindow('img', 500, 500)
cv2.imshow('img',img)
cv2.waitKey(500)
cv2.destroyAllWindows()
img2 = cv2.imread("../easyimgs/5.jpg")
print(f"type objpoints:{objpoints[0].shape}")
print(f"type imgpoints:{imgpoints[0].shape}")
ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(objpoints, imgpoints, gray.shape[::-1],None,None)
h, w = img2.shape[:2]
newcameramtx, roi=cv2.getOptimalNewCameraMatrix(mtx,dist,(w,h),1,(w,h))
dst = cv2.undistort(img2, mtx, dist, None, newcameramtx)
# crop the image
x,y,w,h = roi
dst = dst[y:y+h, x:x+w]
cv2.namedWindow('result', 0)
cv2.resizeWindow('result', 400, 400)
cv2.imshow('result',dst)
cv2.destroyAllWindows()
but when I run it, an error shows up as:
Traceback (most recent call last):
File "*/undistortion.py", line 51, in <module>
ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(objpoints, imgpoints, gray.shape[::-1],None,None)
cv2.error: OpenCV(3.4.2) C:\projects\opencv-python\opencv\modules\calib3d\src\calibration.cpp:3143: error: (-215:Assertion failed) ni == ni1 in function 'cv::collectCalibrationData'
Here is my image.
I have searched on the Internet that many people are also confronted with this problem. But most of the solution on blog is saying that this is caused by the type of the first and second parameters of calibrateCamera() which is objpoints and imgpoints. But those are all solution for opencv on c++.
Could any one tell me how to solve it in python?

The number of entries in objpoints and imgpoints must be the same. This assertion means they aren't. It looks like you're creating a set of 6*7=42 objpoints, which is intended for a chessboard of 6x7 intersections, but your actual chessboard has 8*11=88 intersections. So as you process images, your objpoints and imgpoints lists get different lengths. You need to modify your creation/initialization of objp to have 8*11=88 points, whose coordinates correspond to the real physical coordinates on your chessboard.
To do this, you'll need to really understand the code you are using. Putting in more debug statements will help you to trace through what is happening in your code.
And note that the Python API to OpenCV is just a wrapper around the C++ API, so any solution for using OpenCV with C++ are (usually) relevant in Python too.

Camera calibration Open CV-Python

I'm trying to do camera calibration, I have taken the code from open cv documentation. Here is my code -
import numpy as np
import cv2
import glob
criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 30, 0.001)
objp = np.zeros((6*7,3), np.float32)
objp[:,:2] = np.mgrid[0:7,0:6].T.reshape(-1,2)
objpoints = []
imgpoints = []
images = glob.glob('/usr/local/share/OpenCV/samples/cpp/chess*.jpg')
img = cv2.imread("2.jpg")
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
ret = False
ret, corners = cv2.findChessboardCorners(gray, (7, 6))
print (ret)
if ret == True:
objpoints.append(objp)
cv2.cornerSubPix(gray, corners, (11,11), (-1,-1), criteria)
imgpoints.append(corners)
# Draw and display the corners
cv2.drawChessboardCorners(img, (7,6), corners, ret)
cv2.imshow('img',img)
cv2.imwrite('Corners_detected.jpg', img, None)
cv2.waitKey(0)
cv2.destroyAllWindows()
ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(objpoints, imgpoints,
gray.shape[::-1],None,None)
img = cv2.imread('2.jpg')
h, w = img.shape[:2]
newcameramtx, roi=cv2.getOptimalNewCameraMatrix(mtx,dist,(w,h),1,(w,h))
# undistort
dst = cv2.undistort(img, mtx, dist, None, newcameramtx)
cv2.imwrite('calibration_result.png',dst)
In this code image 2.jpg is taken for calibration,
This is the image considered for understanding of calibration
My code is detecting corners for only this image. It is not working fine with other checker board image.It is not able to detect corners. Why is it so ?

Unfortunately, I do not have enough reputation to comment and clarify some points. However, I will try to answer anyway. Given you have added the print(ret) I assume this is where your problem lies.
It looks like you are using the wrong checkerboard size in cv2.findChessboardCorners(gray, (7, 6)). I have found this function returns False given the wrong input dimension values.
This is als the case for the objp object.
Given the image, you are showing this should be n-1 and m-1 (where n and m are the checkboard dimensions).
For your given image, this should be cv2.findChessboardCorners(gray, (9, 6))
Notice on the opencv calibration example the checkerboard is an 8x7, hence the given 7x6 input value.

The thing about the Camera Calibration method is that it sometimes will not recognize a Checkerboard grid that isn't the maximum size. You could most likely get away with 8,6 or 9,5 as the size. However, with 6,7 there is too much of a difference and so the method won't recognize it.
I don't have any research sources but I've tested this myself before.

Python + OpenCV name not defined

I'm using a source code example from Open CV for Python documentation as follows:
import numpy as np
import cv2
import glob
# termination criteria in this, 30 max number of iterations, 0.001 minimum accuracy
# CV_TERMCRIT_ITER or CV_TERMCRIT_EPS, tells the algorithm that we want to terminate either after some number of iterations or when the convergence metric reaches some small value (respectively).
# The next two arguments set the values at which one, the other, or both of these criteria should terminate the algorithm.
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*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.
objpoints = [] # 3d point in real world space
imgpoints = [] # 2d points in image plane.
images = glob.glob('*.jpg')
# fname= 'C:\\Users\\Bender\\Desktop\\fotospayloads\\'
for fname in images:
img = cv2.imread(fname)
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
# Find the chess board corners
ret, corners = cv2.findChessboardCorners(gray, (9,6), 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
img = cv2.drawChessboardCorners(img, (9,6), corners2, ret)
cv2.imshow('img', img)
cv2.waitKey(500)
cv2.destroyAllWindows()
rms, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(objpoints, imgpoints, gray.shape[::-1], None, None)
Unfortunately when I run the source code I get the following error:
"NameError: name 'gray' is not defined" (line 50).
Any help would be very much appreciated.
Thanks
Isaac

There are no images in folder where your script is located and that is why glob.glob('.jpg') does not return any files and grey object is not created.

Opencv Pythonprogram

I am trying opencv with python now.
What mean this error?
OpenCV Error: Assertion failed (m.dims >= 2) in Mat, file /build/opencv-ISmtkH/opencv-2.4.9.1+dfsg/modules/core/src/matrix.cpp, line 269
Traceback (most recent call last):
File "sabun5.py", line 16, in <module>
img_m = cv2.threshold(img_df, 50, 255, cv2.THRESH_BINARY)[1]
cv2.error: /build/opencv-ISmtkH/opencv-2.4.9.1+dfsg/modules/core/src/matrix.cpp:269: error: (-215) m.dims >= 2 in function Mat

You can see in the OpenCV documentation, that the threshold function just allow single-channel images.
If your image is a color one, it won't work. If it's grayscale but you are loading it with imread, it might be possible that OpenCV load it as a 3-channel one. You can add the flag to load it as a single-channel with CV_8UC1 (supposing it is an 8 bit unsigned one, which is the more common for a grayscale image).
For example:
img_df = cv2.imread("image/path", cv2.CV_8UC1)

you have to convert the image into grayscale before thresholding your image has more then two dimentions i.e (height,width,color-channel) gray scale image has only two dimention(height,width)
it might help
import cv2
img = cv.imread('x.png',0)
# where 0 converts the image in grayscale or gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
img_m = cv2.threshold(img, 50, 255, cv2.THRESH_BINARY)[1]
cv2.waitKey(0)

OpenCV 3.1: calibrateCamera "assertion failed" - Raspberry Pi / Python

Morning!
I've read several other topics about this error, but none seem to quite apply. I'm learning about OpenCV and I don't usually program so please feel free to point out better resources and apologies if I've missed something.
I'm running OpenCV 3.1 and Python 2.7 on a Raspberry Pi 2 B running Jessie. I installed OpenCV according to this article on pyimagesearch and it works fine otherwise.
I'm now trying to follow the calibrateCamera tutorial here. I've gotten to the point where I can acquire and save all my images (or use the provided sample ones) and see the overlay generated, but when I reach the line with calibrateCamera, I get the following error:
OpenCV Error: Assertion failed (ni == ni1) in collectCalibrationData, file /home/pi/opencv-3.1.0/modules/calib3d/src/calibration.cpp, line 3064
Traceback (most recent call last):
File "findcorners.py", line 38, in
ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(objpoints, imgpoints, gray.shape[::-1], None, None)
cv2.error: /home/pi/opencv-3.1.0/modules/calib3d/src/calibration.cpp:3064: error: (-215) ni == ni1 in function collectCalibrationData
Below is the complete code I am using (currently testing on provided sample images):
import numpy as np
import cv2
import glob
# 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)
# 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:
img = cv2.imread(fname)
gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
# Find the chess board corners
ret, corners = cv2.findChessboardCorners(gray, (8,6),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
img = cv2.drawChessboardCorners(img, (7,6), corners2,ret)
cv2.imshow('img',img)
cv2.waitKey(500)
ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(objpoints, imgpoints, gray.shape[::-1], None, None)
np.savez("webcam_calibration_ouput", ret=ret, mtx=mtx, dist=dist, rvecs=rvecs, tvecs=tvecs)
cv2.destroyAllWindows()