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Problem showing images

asked 2012-08-20 04:27:05 -0500

mcirri gravatar image

I am quite new to SAGE so I am probably asking something silly but some days of google search did not provide a simple answer to my question. Using instructions from this website I created a small sage program:

from numpy import *
import scipy.ndimage
import matplotlib.image
import matplotlib.pyplot
print 'Image dtype: %s'%(img.dtype)
print 'Image size: %6d'%(img.size)
print 'Image shape: %3dx%3d'%(img.shape[0],img.shape[1])
print 'Max value %1.2f at pixel %6d'%(img.max(),img.argmax())
print 'Min value %1.2f at pixel %6d'%(img.min(),img.argmin())
print 'Variance: %1.5f\nStandard deviation: %1.5f'%(img.var(),img.std())

where dummy.png is an existing PNG image that I loaded into the worksheet. The script works perfectly and I get the required info out of it:

Image dtype: float32
Image size: 17335905
Image shape: 2305x2507
Max value 1.00 at pixel   1767
Min value 0.00 at pixel 2032962
Variance: 0.06310
Standard deviation: 0.25120

now if I want to actually see the picture everything fails. I tried to type:


My final goal was to overlay the image, a photo, with the plot resulting from an analysis to show the accuracy of the prediction. Does anyone know how to do that?

thanks, mcirri

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answered 2012-08-25 03:53:57 -0500

Volker Braun gravatar image

In fact, img is a numpy array containing (r,g,b) values. In Sage, you can plot it with

sage: matrix_plot(img, origin='lower')

Colors may be inverted (is 1.0 white or black?), you can try

sage: matrix_plot(1.0-img, origin='lower')

to flip it around.

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Considering how often I use `matrix_plot`, I should have tried this... but I got stuck in mpl mode, which I don't really get that well :)

kcrisman gravatar imagekcrisman ( 2012-08-27 03:39:19 -0500 )edit

answered 2012-08-24 10:27:38 -0500

kcrisman gravatar image

img isn't a graphic, as the blog post indicates.

sage: type(img) 
<type 'numpy.ndarray'>

The "right" thing to do is use imshow like here, but I can never figure out how to actually get it to show up in the notebook. But this might give you enough hints if you've used mpl in the past.

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answered 2012-10-05 09:22:54 -0500

this post is marked as community wiki

This post is a wiki. Anyone with karma >750 is welcome to improve it.

You can do

import Image
img ="filename") # if you load the image from disk
img = Image.fromarray(imgarray) # if the image is numpy array"test.png")

It will show up in the notebook. But it will also save a copy of the image to your disk. If I find out how to write the image to /dev/null, then maybe I could avoid writing to disk.

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Asked: 2012-08-20 04:27:05 -0500

Seen: 614 times

Last updated: Oct 05 '12