在灰度图像中标记特定的像素值

Question

I have a grayscale image and a threshold. The pixel values exceeding the threshold should be marked as either blue color or "+" sign.

thresh_img = np.zeros((r,c))
thresh_img[:,:] = img[:,:]
thresh_img[thresh_img > 40] = 0

How to do this in python?

Answer 1

Use boolean indexing to identify the values, then use numpy.nonzero or numpy.where to get their indices. For images or matrices the indices can be directly used as positions. Then use matplotlib.plot(x, y, 'b+')

Answer 2

A way to do this is using the PIL library ( http://www.pythonware.com/products/pil/ ). First, you create an array to store values. Then you open the image and using a for loop, you iterate over all the pixels it has. Depending on the color of the pixel, you store a '+' or something else (you don't specify what so I'll suppose it's the color, which you can store as a single number since grays have similar RG and B values). So, I might do something this way:

from PIL import Image
cols = []
im = Image.open("dead_parrot.jpg") #Can be many different formats.
pix = im.load()
w = im.size[0]
h = im.size[1]
for i in range(w):
    row = []
    for j in range(h):
        red = pix[i,j][0]
        if red > threshold:
            row.append('+')
        else:
            row.append(str(red))
    cols.append(row)
print(cols)

I believe this should do the job. Could you try it?