Ipython cv2.imwrite() 不保存图像

Question

我在 python opencv 中编写了一个代码。我正在尝试将处理后的图像写回磁盘，但图像没有被保存并且没有显示任何错误（运行时和编译）代码是

"""
Created on Wed Oct 19 18:07:34 2016

@author: Niladri
"""

import numpy as np  
import cv2  

if __name__ == '__main__':  
import sys  

img = cv2.imread('C:\Users\Niladri\Desktop\TexturesCom_LandscapeTropical0080_2_S.jpg')
if img is None:  
    print 'Failed to load image file:'
    sys.exit(1)  

gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)  
h, w = img.shape[:2]  

eigen = cv2.cornerEigenValsAndVecs(gray, 15, 3)  
eigen = eigen.reshape(h, w, 3, 2)  # [[e1, e2], v1, v2]  
#flow = eigen[:,:,2] 
iter_n = 10
sigma = 5
str_sigma = 3*sigma
blend = 0.5
img2 = img
for i in xrange(iter_n):  
    print i,  

    gray = cv2.cvtColor(img2, cv2.COLOR_BGR2GRAY)  
    eigen = cv2.cornerEigenValsAndVecs(gray, str_sigma, 3)  
    eigen = eigen.reshape(h, w, 3, 2)  # [[e1, e2], v1, v2]  
    x, y = eigen[:,:,1,0], eigen[:,:,1,1] 
    print eigen

    gxx = cv2.Sobel(gray, cv2.CV_32F, 2, 0, ksize=sigma)  
    gxy = cv2.Sobel(gray, cv2.CV_32F, 1, 1, ksize=sigma)  
    gyy = cv2.Sobel(gray, cv2.CV_32F, 0, 2, ksize=sigma)  
    gvv = x*x*gxx + 2*x*y*gxy + y*y*gyy  
    m = gvv < 0  

    ero = cv2.erode(img, None)  
    dil = cv2.dilate(img, None)  
    img1 = ero  
    img1[m] = dil[m]  
    img2 = np.uint8(img2*(1.0 - blend) + img1*blend)  
#print 'done'
cv2.imshow('dst_rt', img2)
cv2.waitKey(0)
cv2.destroyAllWindows()

#cv2.imwrite('C:\Users\Niladri\Desktop\leaf_image_shock_filtered.jpg', img2)    



for i in xrange(iter_n):  
    print i,  

    gray = cv2.cvtColor(img2, cv2.COLOR_BGR2GRAY)  
    eigen = cv2.cornerEigenValsAndVecs(gray, str_sigma, 3)  
    eigen = eigen.reshape(h, w, 3, 2)  # [[e1, e2], v1, v2]  
    x, y = eigen[:,:,1,0], eigen[:,:,1,1] 
    print eigen

    gxx = cv2.Sobel(gray, cv2.CV_32F, 2, 0, ksize=sigma)  
    gxy = cv2.Sobel(gray, cv2.CV_32F, 1, 1, ksize=sigma)  
    gyy = cv2.Sobel(gray, cv2.CV_32F, 0, 2, ksize=sigma)  
    gvv = x*x*gxx + 2*x*y*gxy + y*y*gyy  
    m = gvv < 0  

    ero = cv2.erode(img, None)  
    dil = cv2.dilate(img, None)  
    img1 = dil  
    img1[m] = ero[m]  
    img2 = np.uint8(img2*(1.0 - blend) + img1*blend)  
print 'done'

#cv2.imwrite('D:\IP\tropical_image_sig5.bmp', img2)    


cv2.imshow('dst_rt', img2)
cv2.waitKey(0)
cv2.destroyAllWindows()

#cv2.imshow('dst_rt', img2)    

cv2.imwrite('C:\Users\Niladri\Desktop\tropical_image_sig5.bmp', img2)

谁能告诉我为什么它不起作用。 cv2.imshow 工作正常（因为它显示了正确的图像）。 谢谢并恭祝安康 尼拉德里

Answer 1

作为一般和绝对规则，您必须使用r 前缀或解释某些字符（例如：\n,\b,\v,\x aaaa 和\t，完整列表here):

所以当这样做时：

cv2.imwrite('C:\Users\Niladri\Desktop\tropical_image_sig5.bmp', img2)

您正在尝试保存到C:\Users\Niladri\Desktop<TAB>ropical_image_sig5.bmp

imread 和 imwrite 的烦人之处在于，这些函数不会在错误时抛出异常，而是默默地失败。 imwrite 返回False

>>> cv2.imread("D:/nonexisting.jpg")  # this returns None, no error
>>> s = cv2.imread("D:/sloth_book.jpg")  # this works
>>> s
array([[[250, 250, 250],
        [246, 246, 246],
        [255, 255, 255],
        ...,
>>> cv2.imwrite("inexistent_dir/file.jpg",s)  # dir doesn't exist, write fails
False

所以你必须检查这些函数的返回值。

这样做：

if not cv2.imwrite(r'C:\Users\Niladri\Desktop\tropical_image_sig5.bmp', img2):
     raise Exception("Could not write image")

注意：读取工作正常，因为“转义”大写字母在 python 2 中没有特殊含义（\U 和 \N 在 python 3 中有意义，所以它不会起作用）

如果出现错误，程序现在会大声抱怨。

Answer 2

正如 Jean 所建议的，错误是由于 \ 被解释为转义序列。因此使用os.path.join() 总是更安全，因为它更跨平台，您不必担心转义序列问题。例如，在您的情况下，您不必担心前几个参数，因为那是您的主目录

import os
cv2.imwrite(os.path.join(os.path.expanduser('~'),'Desktop','tropical_image_sig5.bmp'), img2)

os.path.expanduser('~') 将直接返回您的主目录。

Answer 3

如果您想让代码中的路径可移植，请查看 pathlib。

https://docs.python.org/3/library/pathlib.html