为什么用OpenCV保存图像会导致黑色图像？

Question

所以，我想用pythonnumpy库创建一个500x500的白色图像，尽管我可以在photoshop中轻松地完成。下面的代码是有效的，图像是白色的（因为我用cv2.imsave函数保存了图像，后来我用windows photos viewer打开了它）。但是当我尝试使用cv2.imshow函数显示它时，会显示一个黑色图像。为什么？这是cv2的缺点吗？在

import cv2
import numpy as np

img = np.arange(500*500*3)
for i in range(500*500*3):
    img[i] = 255
img = img.reshape((500, 500, 3))
cv2.imwrite("fff.jpg", img)
cv2.imshow('', img)

Answer 1

注意到，{{CD1>}模块是围绕C++ OpenCV包的一个薄薄包装。这是它的文档，并且与它们接口的python包装器函数的签名不会改变。从documentation-

void cv::imshow   (const String &winname,
       InputArray     mat 
 )        

Displays an image in the specified window.

The function imshow displays an image in the specified window. [...]

• If the image is 8-bit unsigned, it is displayed as is.
• If the image is 16-bit unsigned or 32-bit integer, the pixels are divided by 256. That is, the value range [0,255*256] is mapped to [0,255].
• If the image is 32-bit floating-point, the pixel values are multiplied by 255. That is, the value range [0,1] is mapped to [0,255].

默认情况下，numpy数组初始化为np.int32或np.int64类型（这取决于您的计算机）。如果希望数组不做任何更改地显示，则应确保将它们作为8位无符号传递。对你来说，像这样-

cv2.imshow('', img.astype(np.uint8))

或者，在初始化数组时，请执行以下操作-

^{pr2}$