Re: [Matplotlib-users] imshow and pcolor differences

[Eric F. <ef...@ha...>]

Richard McMahon wrote:
> Hello,
> 
> I want to display some data as an image and also as a contour.
> 
> I have been looking at imshow and pcolor and find that contour
> and imshow are behaving differently than pcolor. In the example below I
> have a 5x5 image. pshow displays the pixels but imshow and contour shows
> resampling artifacts since they resample offset by 0.5pixels.
> 
> The advantage of imshow is that the pixels are square which is what I
> want. I also want to use contour which also seems to show the same
> type of resampling as imshow.

This is not a matter of resampling (unless I am misunderstanding 
you)--it is a difference in the grid.  For imshow and contour, the data 
points and the grid intersections coincide; for pcolor (and pcolormesh), 
the grid gives the boundaries of colored quadrilaterals. Therefore, if 
the data array is MxN, then the the grid X dimension should be N+1 and 
the grid Y dimension should be M+1.

For an example of how to use contours with images, see:

http://matplotlib.sourceforge.net/examples/pylab_examples/contour_image.html

Also, it sounds like maybe you don't want imshow to interpolate: try 
using the interpolation='nearest' kwarg.

If you do want to use pcolor or pcolormesh or Axes.pcolorfast, then you 
can still get square pixels by suitable choice of aspect ratio.  Try 
axis('equal') or axis('scaled'), or axis('image'),  or use the 
set_aspect() method of the Axes instance.

Eric

> 
> import numpy as np
> import matplotlib as mpl
> import matplotlib.pyplot as plt
> 
> image = np.random.rand(5,5)
> 
> plt.figure()
> plt.pcolor(image)
> plt.title('pcolor defaults')
> 
> plt.figure()
> plt.imshow(image, origin='lower')
> plt.title('imshow defaults with origin=lower')
> 
> plt.show()
> 
> Is there a method to force imshow to not resample the image
> It is not obvius to me from reading the help for imshow and pcolor.
> 
> 
> Thanks, richard
> 
> -------------------------------------------------------------------
>  Dr. Richard G. McMahon    | Phone (office)     44-(0)-1223-337519
>  University of Cambridge   |       (switchboard)       1223-337548
>  Institute of Astronomy    |       (secretary)         1223-337516
>  Madingley Rd              | FAX                       1223-337523
>  Cambridge, CB3 OHA, UK.   | mobile                    7885-409019
>  Office: Hoyle 18          | home                      1223-359770
> -------------------------------------------------------------------
>  email: rg...@as...  | WWW:    http://www.ast.cam.ac.uk/~rgm
>  ric...@gm... | skype:                richardgmcmahon
> -------------------------------------------------------------------
> 
> ------------------------------------------------------------------------------
> Let Crystal Reports handle the reporting - Free Crystal Reports 2008 30-Day 
> trial. Simplify your report design, integration and deployment - and focus on 
> what you do best, core application coding. Discover what's new with 
> Crystal Reports now.  http://p.sf.net/sfu/bobj-july
> _______________________________________________
> Matplotlib-users mailing list
> Mat...@li...
> https://lists.sourceforge.net/lists/listinfo/matplotlib-users