如何在opencv中改变轮廓形成的区域(阈值)

我正在尝试建立OCR来从图像中提取文本,我正在使用轮廓来形成文本字符的边界,

经过几次改变cv2.threshold的尝试,我得到了形成文本字符边界的最佳轮廓拟合。

#files = os.listdir(r'letters/harry.jpeg',0)
file = r'/home/naga/Documents/Naga/Machine Learning/Data_extract/letters/Harry/Harry Potter and the Sorcerer s Stone-page-006.jpg'
im1 = cv2.imread(file,0)
im = cv2.imread(file)

# ret,thresh1 = cv2.threshold(im1,180,278,cv2.THRESH_BINARY)
# _,contours, hierarchy = cv2.findContours(thresh1,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

ret,thresh1 = cv2.threshold(im1,180,278,cv2.THRESH_BINARY)
kernel = np.ones((5,5),np.uint8)
dilated = cv2.dilate(im1,kernel,iterations = 1)
_,contours, hierarchy = cv2.findContours(dilated,cv2.RETR_TREE,cv2.CHAIN_APPROX_SIMPLE)

for cnt in contours:
    x,y,w,h = cv2.boundingRect(cnt)
    #bound the images
    cv2.rectangle(im,(x,y),(x+w,y+h),(0,255,0),1)

cv2.namedWindow('BindingBox', cv2.WINDOW_NORMAL)
cv2.imwrite('output2/BindingBox4.jpg',im)

我对opencv还不熟悉,我一直都在坚持 cv2 threshold 但我不明白怎么用它。请给你的输入形成轮廓的话。