nameerror：找不到名字的错误，为什么全局变量会找不到名字呢？

import cv2
import numpy as np
import glob

# 找棋盘格角点
# 设置寻找亚像素角点的参数，采用的停止准则是最大循环次数30和最大误差容限0.001
criteria = (cv2.TERM_CRITERIA_EPS + cv2.TERM_CRITERIA_MAX_ITER, 30, 0.001) # 阈值
#棋盘格模板规格
w = 9   # 10 - 1
h = 6   # 7  - 1
# 世界坐标系中的棋盘格点,例如(0,0,0), (1,0,0), (2,0,0) ....,(8,5,0)，去掉Z坐标，记为二维矩阵
objp = np.zeros((w*h,3), np.float32)
objp[:,:2] = np.mgrid[0:w,0:h].T.reshape(-1,2)
objp = objp*18.1  # 18.1 mm

# 储存棋盘格角点的世界坐标和图像坐标对
objpoints = [] # 在世界坐标系中的三维点
imgpoints = [] # 在图像平面的二维点
#加载pic文件夹下所有的jpg图像
images = glob.glob('./*.jpg')  #   拍摄的十几张棋盘图片所在目录

i=0
for fname in images:

    img = cv2.imread(fname)
    # 获取画面中心点
    #获取图像的长宽
    h1, w1 = img.shape[0], img.shape[1] 
    global gray
    gray =cv2.cvtColor(img,cv2.COLOR_BGR2GRAY)
    u, v = img.shape[:2]
    # 找到棋盘格角点
    ret, corners = cv2.findChessboardCorners(gray, (w,h),None)
    # 如果找到足够点对，将其存储起来
    if ret == True:
        print("i:", i)
        i = i+1
        # 在原角点的基础上寻找亚像素角点
        cv2.cornerSubPix(gray,corners,(11,11),(-1,-1),criteria)
        #追加进入世界三维点和平面二维点中
        objpoints.append(objp)
        imgpoints.append(corners)
        # 将角点在图像上显示
        cv2.drawChessboardCorners(img, (w,h), corners, ret)
        cv2.namedWindow('findCorners', cv2.WINDOW_NORMAL)
        cv2.resizeWindow('findCorners', 640, 480)
        cv2.imshow('findCorners',img)
        cv2.waitKey(200)



cv2.destroyAllWindows()
#%% 标定
print('正在计算')
#标定
ret, mtx, dist, rvecs, tvecs = \
    cv2.calibrateCamera(objpoints, imgpoints, gray.shape[::-1], None, None)


print("ret:",ret  )
print("mtx:\n",mtx)      # 内参数矩阵
print("dist畸变值:\n",dist   )   # 畸变系数   distortion cofficients = (k_1,k_2,p_1,p_2,k_3)
print("rvecs旋转（向量）外参:\n",rvecs)   # 旋转向量  # 外参数
print("tvecs平移（向量）外参:\n",tvecs  )  # 平移向量  # 外参数
newcameramtx, roi = cv2.getOptimalNewCameraMatrix(mtx, dist, (u, v), 0, (u, v))
print('newcameramtx外参',newcameramtx)
#打开摄像机
camera=cv2.VideoCapture(1)
while True:
    (grabbed,frame)=camera.read()
    h1, w1 = frame.shape[:2]
    newcameramtx, roi = cv2.getOptimalNewCameraMatrix(mtx, dist, (u, v), 0, (u, v))
    # 纠正畸变
    dst1 = cv2.undistort(frame, mtx, dist, None, newcameramtx)
    #dst2 = cv2.undistort(frame, mtx, dist, None, newcameramtx)
    mapx,mapy=cv2.initUndistortRectifyMap(mtx,dist,None,newcameramtx,(w1,h1),5)
    dst2=cv2.remap(frame,mapx,mapy,cv2.INTER_LINEAR)
    # 裁剪图像，输出纠正畸变以后的图片
    x, y, w1, h1 = roi
    dst1 = dst1[y:y + h1, x:x + w1]

    #cv2.imshow('frame',dst2)
    #cv2.imshow('dst1',dst1)
    cv2.imshow('dst2', dst2)
    if cv2.waitKey(1) & 0xFF == ord('q'):  # 按q保存一张图片
        cv2.imwrite("../u4/frame.jpg", dst1)
        break

camera.release()
cv2.destroyAllWindows()

错误是

Traceback (most recent call last):
  File "e:\pyrscode\tempCodeRunnerFile.py", line 59, in <module>
    cv2.calibrateCamera(objpoints, imgpoints, gray.shape[::-1], None, None)
NameError: name 'gray' is not defined

我最初以为是gray在for循环内，是非全局变量，因此添加了global标识，后来依然有这个错误，初学者，求大家指教。