Why doesn't the shape of my numpy array change?

Question

I have made a numpy array out of data from an image. I want to convert the numpy array into a one-dimensional one.

import numpy as np
import matplotlib.image as img

if __name__ == '__main__':

  my_image = img.imread("zebra.jpg")[:,:,0]
  width, height = my_image.shape
  my_image = np.array(my_image)
  img_buffer = my_image.copy()
  img_buffer = img_buffer.reshape(width * height)
  print str(img_buffer.shape)

The 128x128 image is here.

enter image description here

However, this program prints out (128, 128). I want img_buffer to be a one-dimensional array though. How do I reshape this array? Why won't numpy actually reshape the array into a one-dimensional array?

It looks like you have a color image, but you're only reading the red channel for each pixel of the image. Is that your intention? — John Vinyard
– John Vinyard, Commented Nov 8, 2012 at 5:24
Thanks! My original intention is to read all of RGB. Why am I reading just the red channel? — dangerChihuahua007
– dangerChihuahua007, Commented Nov 8, 2012 at 8:03
@DavidFaux as wim said, since apparently you had quickly edited the question to have correct code, can you please change it back. Its utterly confusing to have a question "Why does this not work" with code that works. — seberg
– seberg, Commented Nov 8, 2012 at 10:09
@DavidFaux, matplotlib.image.imread returns a three-dimensional array. The first two dimensions are the width and height of the image, and the last are the red, green, blue, and alpha channels for each pixel. I think your code, specifically img[:,:,0], is choosing every pixel, but only the red channel. Check out matplotlib.org/users/image_tutorial.html for more info. — John Vinyard
– John Vinyard, Commented Nov 8, 2012 at 15:01

wim · Accepted Answer · 2012-11-08 05:10:43Z

8

.reshape returns a new array, rather than reshaping in place.

By the way, you appear to be trying to get a bytestring of the image - you probably want to use my_image.tostring() instead.

answered Nov 8, 2012 at 5:10

wim

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John Vinyard · Accepted Answer · 2012-11-08 05:49:52Z

5

reshape doesn't work in place. Your code isn't working because you aren't assigning the value returned by reshape back to img_buffer.

If you want to flatten the array to one dimension, ravel or flatten might be easier options.

>>> img_buffer = img_buffer.ravel()
>>> img_buffer.shape
(16384,)

Otherwise, you'd want to do:

>>> img_buffer = img_buffer.reshape(np.product(img_buffer.shape))
>>> img_buffer.shape
(16384,)

Or, more succinctly:

>>> img_buffer = img_buffer.reshape(-1)
>>> img_buffer.shape
(16384,)

edited Nov 8, 2012 at 5:49

answered Nov 8, 2012 at 5:10

John Vinyard

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