OpenCV 2.4.9 for Python, cannot find a chessboard (camera calibration guide)

Question

I am trying to calibrate the camera using OpenCV tools as follows in this guide .

The problem is that the findChessboardCorners function cannot find a single chessboard in the images I tried. I used a lot of them - even a simple checkerboard pattern. In any case, nothing was found.

Here is the code (almost the same as from the link above):

 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('*.png') 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, (7,6),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) cv2.destroyAllWindows() 

The only change I made was that I switched from .jpg to .png files - for some reason, the imread function cannot read jpg images (this is another strange problem for another topic).

Thanks in advance for your advice!

Image ref:

Answer 1

Just for other Python newbies who can go this route. Work code:

 import numpy as np import cv2 import glob # termination criteria criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 30, 0.001) # Arrays to store object points and image points from all the images. imgpoints = [] # 2d points in image plane. images = glob.glob('*.png') for fname in images: img = cv2.imread(fname) gray = cv2.cvtColor(img,cv2.COLOR_BGR2GRAY) ret = False # Find the chess board corners ret, corners = cv2.findChessboardCorners(gray, (7,7)) # If found, add object points, image points (after refining them) if ret == True: cv2.cornerSubPix(gray, corners, (11,11), (-1,-1), criteria) imgpoints.append(corners) # Draw and display the corners cv2.drawChessboardCorners(img, (7,7), corners, ret) cv2.imshow('img',img) cv2.waitKey(0) cv2.destroyAllWindows() 

Two main points:

• You must carefully calculate the size of your template. (7.7) for a regular chessboard.

• The line img = cv2.drawChessboardCorners(img, (7,6), corners2,ret) does not work, you must change it to cv2.drawChessboardCorners(img, (7,6), corners2,ret) (the function does not return an image).

Thanks AldurDisciple!