Python - converting to binary a colour image using intermediate values

Question

I would like to take a colour image and convert it to a binary image where what was close to either black or white return False, and all the intermediate values return True.

What is the proper syntax to impose the two conditions below simultaneously?

binary = color.rgb2gray(img) > 0.05 
binary = color.rgb2gray(img) < 0.95

If I used this:

%matplotlib inline
import numpy as np
import matplotlib.pyplot as plt
from skimage import color
import requests
from PIL import Image
from StringIO import StringIO

url = 'https://mycarta.files.wordpress.com/2014/03/spectrogram_jet.png'
r = requests.get(url)
img = np.asarray(Image.open(StringIO(r.content)).convert('RGB'))

and then:

binary = color.rgb2gray(img)  < 0.95

I would get a proper binary image that I can plot with:

fig = plt.figure(figsize=(10,10))
ax = fig.add_subplot(111)
plt.imshow(binary, cmap='gray')
ax.xaxis.set_ticks([])
ax.yaxis.set_ticks([])
plt.show()

Similarly with this:

color.rgb2gray(img) < 0.95

But if I tried them together like this:

binary = color.rgb2gray(img) > 0.05 and color.rgb2gray(img) < 0.95

I got this message:

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

Same as suggested in @caenyon's answer: binary = color.rgb2gray(img) > 0.05 and color.rgb2gray(img) < 0.95 — MyCarta
– MyCarta, Commented Mar 6, 2016 at 23:33
what is the img variable? an complete example would be easier (imports etc) — Felix
– Felix, Commented Mar 6, 2016 at 23:48

Zach Gates · Accepted Answer · 2016-03-07 00:00:58Z

2

Your code isn't working because the rgb2gray method of skimage.color returns an array. The skimage module utilizes the numpy module which refuses to perform boolean comparisons on arrays. This is where your ValueError comes from.

When using a comparison operator like and on arrays, numpy will not comply. Instead, you should use np.logical_and or the binary operator &.

answered Mar 7, 2016 at 0:00

Zach Gates

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1 Comment

MyCarta Over a year ago

your suggestion worked, thank you for the explanation too

Felix · Accepted Answer · 2016-03-06 23:20:51Z

-1

you can use 'and' to combine these two conditions

binary = color.rgb2gray(img) > 0.05 and color.rgb2gray(img) < 0.95

or you can just write them as one condition

binary = 0.05 < color.rgb2gray(img) < 0.95

answered Mar 6, 2016 at 23:20

Felix

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6 Comments

MyCarta Over a year ago

When I try those two options you suggest, in both cases (the first one is what I tried before) I get the error message

Felix Over a year ago

what exactly does the color.rgb2gray function return? are you sure that it's a number and not a list?

Felix Over a year ago

how does the rest of your program look like? i'd assume that the problem is somewhere else...

MyCarta Over a year ago

color.rgb2gray(img) just applies the formula Y = (0.2125 R + 0.7154 G + 0.0721 B) to convert to gray

MyCarta Over a year ago

The code is very simple. This works: binary = color.rgb2gray(img) < 0.95 fig = plt.figure(figsize=(10,10)) ax1 = fig.add_subplot(111) plt.imshow(binary, cmap='gray') ax1.xaxis.set_ticks([]) ax1.yaxis.set_ticks([]) plt.show()

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