Zoom in an Image using python-numpy

Question

I'm trying to zoom in an image.

 import numpy as np
 from scipy.ndimage.interpolation import zoom
 import Image 
 zoom_factor = 0.05 # 5% of the original image 
 img = Image.open(filename)
 image_array = misc.fromimage(img)
 zoomed_img = clipped_zoom(image_array, zoom_factor)
 misc.imsave('output.png', zoomed_img)

Clipped Zoom Reference:
Scipy rotate and zoom an image without changing its dimensions

This doesn't works and throws this error: ValueError: could not broadcast input array from shape

Any Help or Suggestions on this Is there a way to zoom an image given a zoom factor. And what's the problem ?

Traceback:

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/tornado/web.py", line 1443, in _execute
    result = method(*self.path_args, **self.path_kwargs)
  File "title_apis_proxy.py", line 798, in get
    image, msg = resize_image(image_local_file, aspect_ratio, image_url, scheme, radius, sigma)
  File "title_apis_proxy.py", line 722, in resize_image
    z = clipped_zoom(face, 0.5, order=0)
  File "title_apis_proxy.py", line 745, in clipped_zoom
    out[top:top+zh, left:left+zw] = zoom(img, zoom_factor, **kwargs)
ValueError: could not broadcast input array from shape (963,1291,2) into shape (963,1291,3)

I can imagine there is a problem with the basic data-format of the image. Why not read in the image with scipy's functions (which is also using PIL; but it may be better suited handling some assumptions about the data-structure)? And what is clipped_zoom? You are only importing zoom, which has just one function zoom in scipy 0.18.0. — sascha
– sascha, Commented Aug 7, 2016 at 18:28
I took help from this stackoverflow.com/questions/37119071/… this contains the clipped_zoom function — Abhi
– Abhi, Commented Aug 7, 2016 at 18:31
Well... this does not make it easier to debug. If you really want some help here, you should give us all the information. The clipped_zoom thing, how should we know? Also... you are posting only part of the error (it probably shows you, which reshape operation/numpy-operation is breaking -> where in code) — sascha
– sascha, Commented Aug 7, 2016 at 18:41

Community · Accepted Answer · 2017-05-23 12:22:56Z

5

The clipped_zoom function you're using from my previous answer was written for single-channel images only.

At the moment it's applying the same zoom factor to the "color" dimension as well as the width and height dimensions of your input array. The ValueError occurs because the the out array is initialized to the same number of channels as the input, but the result of zoom has fewer channels because of the zoom factor.

To make it work for multichannel images you could either pass each color channel separately to clipped_zoom and concatenate the results, or you could pass a tuple rather than a scalar as the zoom_factor argument to scipy.ndimage.zoom.

I've updated my previous answer using the latter approach, so that it will now work for multichannel images as well as monochrome.

edited May 23, 2017 at 12:22

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answered Aug 7, 2016 at 20:51

ali_m

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2 Comments

Abhi Over a year ago

Great it works fine, but sometimes it shows a warning

serWarning: From scipy 0.13.0, the output shape of zoom() is calculated with round() instead of int() - for these inputs the size of the returned array has changed.   "the returned array has changed.", UserWarning)

. Should I ignore this ?

ali_m Over a year ago

You should be fine, since my code also uses np.round to compute the dimensions of the output. By the way, scipy 0.13.0 came out ~3 years ago, so you might consider updating.

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