阈值输入图像

Question

I am trying to create a basic thresholding program which checks to see if the pixel value is > the threshold. (In my case i set the threshold as 128) if it is greater than 128 I want to set that pixel value as 128 else i set it to 0. I am having an issue trying to get this logic down. I get an error message IndexError: invalid index to scalar variable. Where am i going wrong?

import pylab as plt
import matplotlib.image as mpimg
import numpy as np

  img = np.uint8(mpimg.imread('abby.jpg'))


 img = np.uint8((0.2126* img[:,:,0]) + \
       np.uint8(0.7152 * img[:,:,1]) +\
         np.uint8(0.0722 * img[:,:,2]))



threshold = 128

for row in img:
    for col in row:
    if col[0] > threshold:
        col[0] = threshold
    else:
          col[0] = 0


plt.xlim(0, 255)
plt.hist(img,10)
plt.show()

Answer 1

The problem is that you're trying to index a numpy.uint8 (8 bit unsigned integer), which is not a numpy array. Just throw some print statements in your code and you'll easily find the bug. Here's what I did.

In [24]: for row in img:
    ...:     # print(row)
    ...:     for col in row:
    ...:         print(type(col))
    ...:         break
    ...:     break
    ...: 
<class 'numpy.uint8'>

Just change col[0] to col . Also, I usually use plt.imshow(x) to plot 2d numpy arrays as images.

import pylab as plt
import matplotlib.image as mpimg
import numpy as np

img = np.uint8(mpimg.imread("test.png"))

img = np.uint8((0.2126* img[:,:,0]) + \
    np.uint8(0.7152 * img[:,:,1]) +\
    np.uint8(0.0722 * img[:,:,2]))

threshold = 128

for row in img:
    for col in row:
        if col > threshold:
            col = threshold
        else:
            col = 0

plt.imshow(img)
plt.show()