Undefined name problem in camera calibration - python-3.x

Question

I am using the same code that is provided by the OpenCv tutorial, it was working few weeks ago, today I was trying to run it is says that gray name is not defined!! can some one find me the error?
import numpy as np
#import matplotlib.pyplot as plt
import cv2
import glob
import os
def draw(img, corners, imgpts):
corner = tuple(corners[0].ravel())
img = cv2.line(img, corner, tuple(imgpts[0].ravel()), (255,0,0), 5)
img = cv2.line(img, corner, tuple(imgpts[1].ravel()), (0,255,0), 5)
img = cv2.line(img, corner, tuple(imgpts[2].ravel()), (0,0,255), 5)
return img
# 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((7*7,3), np.float32)
objp[:,:2] = np.mgrid[0:7,0:7].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.
img_dir = "C:\\Hungary\\Biblography\\Rotating Solitary Wave\\My Work\\Final Work\\Experiment1111 \\Camera Calibration\\Image Processing\\chess"
data_path = os.path.join(img_dir,'*bmp')
images = glob.glob(data_path)
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, (7,7),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, (7,7), corners2,ret)
cv2.imshow('img',img)
cv2.waitKey(500)
cv2.destroyAllWindows()
ret, mtx, dist, rvecs, tvecs = cv2.calibrateCamera(objpoints, imgpoints, gray.shape [::-1],None,None)
print('Rotation Vector, or the Angles For Each Photo: ', rvecs, '\n')
R = cv2.Rodrigues(rvecs[0])
print('The Rotation Matrix is: ', R)
print('Translation Vector: ', tvecs, '\n')
print(mtx, '\n')
print('Distortion Coefficients ', dist, '\n')
img = cv2.imread('00000274.bmp')
h, w = img.shape[:2]
newcameramtx, roi=cv2.getOptimalNewCameraMatrix(mtx,dist,(w,h),1,(w,h))
print('Camera Matrix', newcameramtx, '\n')
# undistort
dst = cv2.undistort(img, mtx, dist) #, None, newcameramtx)
p = np.ones_like(dst)
# crop the image
x,y,w,h = roi
dst = dst[y:y+h, x:x+w]
# undistort
mapx,mapy = cv2.initUndistortRectifyMap(mtx,dist,None,newcameramtx,(w,h),5)
dst = cv2.remap(img,mapx,mapy,cv2.INTER_LINEAR)
# crop the image
x,y,w,h = roi
dst = dst[y:y+h, x:x+w]
cv2.imwrite('calibresult.png',dst)
mean_error = 0
for i in range(len(objpoints)):
imgpoints2, _ = cv2.projectPoints(objpoints[i], rvecs[i], tvecs[i], mtx, dist)
error = cv2.norm(imgpoints[i],imgpoints2, cv2.NORM_L2)/len(imgpoints2)
mean_error += error
print("total error: ", mean_error/len(objpoints))
If you read the opencv document you will find that I did little changes on the code and it was working but today it is raising this error about the gray name is not defined!

Answer 1

Check your path once, and see if images is an empty list. In that case, for loop will not be executed where the gray variable is defined.