python：通过opencv显示numpy填充图像

Question

在 python 的 opencv 中，我加载了一张黑白图像。调整此图像的大小后，我在图像的每一侧添加了 5 个像素的填充：

resized_digit = cv2.resize(digit, (18,18))
cv2.imshow("resized Image", digit)

# Padding the digit with 5 pixels of black color (zeros) in each side to finally produce the image of (28, 28)
padded_digit = np.pad(resized_digit, ((5,5),(5,5)), "constant", constant_values=0)

如何通过 opencv 显示 padded_digit？因为当我尝试这样显示时：

cv2.imshow("Padded Image", padded_digit)

我收到了这个错误：

<class 'list'>
Traceback (most recent call last):

  File "<ipython-input-4-295bf6efa8c4>", line 39, in <module>
    padded_digit = np.pad(resized_digit, ((5,5),(5,5)), "constant", constant_values=0)

  File "<__array_function__ internals>", line 5, in pad

  File "/home/Alt/anaconda3/lib/python3.8/site-packages/numpy/lib/arraypad.py", line 746, in pad
    pad_width = _as_pairs(pad_width, array.ndim, as_index=True)

  File "/home/Alt/anaconda3/lib/python3.8/site-packages/numpy/lib/arraypad.py", line 521, in _as_pairs
    return np.broadcast_to(x, (ndim, 2)).tolist()

  File "<__array_function__ internals>", line 5, in broadcast_to

  File "/home/Alt/anaconda3/lib/python3.8/site-packages/numpy/lib/stride_tricks.py", line 180, in broadcast_to
    return _broadcast_to(array, shape, subok=subok, readonly=True)

  File "/home/Alt/anaconda3/lib/python3.8/site-packages/numpy/lib/stride_tricks.py", line 123, in _broadcast_to
    it = np.nditer(

ValueError: operands could not be broadcast together with remapped shapes [original->remapped]: (2,2) and requested shape (3,2)

Answer 1

您可以通过cv2.imread读取解决问题，然后将灰度（黑白）图像转换为cv2.COLOR_BGR2GRAY

import cv2
import numpy as np

img = cv2.imread("input.jpg")
gry = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
resized_digit = cv2.resize(gry, (18, 18))
padded_digit = np.pad(resized_digit, ((5, 5), (5, 5)), "constant", constant_values=0)
cv2.imshow("padded_digit", padded_digit)
cv2.waitKey(0)

imread以BGR方式加载图像，然后您可以使用COLOR_BGR2GRAY将其转换为灰度。

您也可以使用copyMakeBorder (source) 代替np.pad。使用np.pad并没有错，但是opencv更适合图像操作，因为你使用的是opencv函数，比如imshow、waitKey等。

例如：

import cv2

img = cv2.imread("input.jpg")
gry = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
resized_digit = cv2.resize(gry, (18, 18))
padded_digit = cv2.copyMakeBorder(resized_digit, 5, 5, 5, 5, cv2.BORDER_CONSTANT, value=0)
cv2.imshow("padded_digit", padded_digit)
cv2.waitKey(0)

您可以通过比较它们的形状来检查两个图像是否相等。他们俩print(padded_digit.shape) 将是(28, 28)