cv2.imread:检查图像是否正在被读取 [英] cv2.imread: checking if image is being read

问题描述

我正在用 python 编写 OpenCV 程序，在某些时候我有类似的东西

I'm writing an OpenCV program in python, and at some point I have something like

import cv2
import numpy as np
... 
img = cv2.imread("myImage.jpg")

# do stuff with image here 

问题是我必须在继续之前检测图像文件是否被正确读取.cv2.imread 返回 False 如果无法打开图像，所以我想这样做:

The problem is that I have to detect if the image file is being correctly read before continuing. cv2.imread returns False if not able to open the image, so I think of doing something like:

if (img):
   #continue doing stuff

如果图像没有打开(例如，如果文件不存在)img 等于 None(正如预期的那样)，会发生什么情况.但是，当 imread 工作时，条件会中断:

What happens is that if the image is not opened (e.g. if the file does not exist) img is equal to None (as expected). However, when imread works, the condition, breaks:

ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

即返回的 numpy.ndarray 不能用作布尔值.问题似乎是 imread 如果成功则返回 numpy.ndarray，否则返回 False (boolean).

i.e. the returned numpy.ndarray cannot be used as a boolean. The problem seems to be that imread returns numpy.ndarray if success and False (boolean) otherwise.

到目前为止，我的解决方案涉及使用返回值的 type 如下:

My solution so far involves using the type of the returned value as follows:

if (type(img) is np.ndarray): 
     #do stuff with image

但我想知道:没有更好的解决方案，更接近初始检查 if(img): #do stuff 吗?

But I was wondering: isn't there a nicer solution, closer to the initial check if(img): #do stuff ?

推荐答案

如果您确定 img 的值为 None 在您的情况下，您可以简单地使用 if not img is None，或者等效地，if img is not None.您不需要显式检查类型.

If you're sure that the value of img is None in your case, you can simply use if not img is None, or, equivalently, if img is not None. You don't need to check the type explicitly.

请注意，None 和 False 是不同的值.但是，bool(None)==False，这就是 if None 失败的原因.

Note that None and False are not the same value. However, bool(None)==False, which is why if None fails.

imread 的文档(包括 OpenCV 2 和 3)指出，在出错时应该返回一个空矩阵.您可以使用 if img.size ==0

The documentation for imread, both for OpenCV 2 and 3, states, however, that a empty matrix should be returned on error. You can check for that using if img.size ==0

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