matplotlib-users

[Matplotlib-users] different behaviour in Windows and Linux

[Gary R. <gr...@bi...>]

I'm wondering whether someone can reproduce the following problem I'm 
seeing in Ubuntu Intrepid.

I often use matplotlib to save images created with imshow to take 
advantage of matplotlib's colour maps. I've noticed that the behaviour 
is different for 0.98.3 between Windows XP-32 and Ubuntu Intrepid. I 
don't remember seeing this problem with earlier versions. This minimal 
example demonstrates the problem:

--

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.cm as cm

px = 3
rcFig = {'figsize': (1, 1),
          'dpi': px,
          'subplot.bottom': 0,
          'subplot.left': 0,
          'subplot.right': 1,
          'subplot.top': 1,
          }
plt.rc('figure', **rcFig)

a = np.ones((px, px))
plt.axis('off')
plt.imshow(a, cmap=cm.gray)
plt.savefig('mpl_out.png', dpi=px)

--

In Windows I get the correct behaviour - in this case a 3x3 image with 
all black pixels:

bbb
bbb
bbb

However, in Linux the leftmost column of pixels is white

wbb
wbb
wbb

By the way, I think an imsave function that just saved an array as an 
image with a specified colourmap and clims would be a nice addition to 
matplotlib.image. Is there another way to achieve the same 1-to-1 array 
element-to-pixel image saving applying colourmaps and clims?

thanks,
Gary R.

Re: [Matplotlib-users] different behaviour in Windows and Linux

[Gary R. <gr...@bi...>]

I just realised that the example I gave may not be the best since it's 
not obvious what the autoscaling will do when all array values are 
equal. Nevertheless, even when the array contains a range of values and 
I use a greyscale colourmap, I'm seeing the leftmost pixel column set to 
all white when the array values in that column are all zeros and the 
image is written in Linux, whereas it's black when written in Windows.

Gary

Re: [Matplotlib-users] different behaviour in Windows and Linux

[Sunnje L B. <Sun...@nf...>]

Hi Gary,
I just tested your example, also on Intrepid with 0.98.3, and I get the
exact same behaviour as you, with white pixels in the left column. But I
have no idea why, sorry.
Sünnje

-----Original Message-----
From: Gary Ruben [mailto:gr...@bi...]
Sent: Tue 12/2/2008 11:13
To: mat...@li...
Subject: [Matplotlib-users] different behaviour in Windows and Linux
 
I'm wondering whether someone can reproduce the following problem I'm 
seeing in Ubuntu Intrepid.

I often use matplotlib to save images created with imshow to take 
advantage of matplotlib's colour maps. I've noticed that the behaviour 
is different for 0.98.3 between Windows XP-32 and Ubuntu Intrepid. I 
don't remember seeing this problem with earlier versions. This minimal 
example demonstrates the problem:

--

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.cm as cm

px = 3
rcFig = {'figsize': (1, 1),
          'dpi': px,
          'subplot.bottom': 0,
          'subplot.left': 0,
          'subplot.right': 1,
          'subplot.top': 1,
          }
plt.rc('figure', **rcFig)

a = np.ones((px, px))
plt.axis('off')
plt.imshow(a, cmap=cm.gray)
plt.savefig('mpl_out.png', dpi=px)

--

In Windows I get the correct behaviour - in this case a 3x3 image with 
all black pixels:

bbb
bbb
bbb

However, in Linux the leftmost column of pixels is white

wbb
wbb
wbb

By the way, I think an imsave function that just saved an array as an 
image with a specified colourmap and clims would be a nice addition to 
matplotlib.image. Is there another way to achieve the same 1-to-1 array 
element-to-pixel image saving applying colourmaps and clims?

thanks,
Gary R.

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Re: [Matplotlib-users] different behaviour in Windows and Linux

[Gary R. <gr...@bi...>]

Thanks for verifying this Sünnje. It looks like an Agg bug. I just tried 
changing to the GTK backend in Linux and the problem disappears.

Gary

Sunnje L Basedow wrote:
> Hi Gary,
> I just tested your example, also on Intrepid with 0.98.3, and I get the
> exact same behaviour as you, with white pixels in the left column. But I
> have no idea why, sorry.
> Sünnje

Re: [Matplotlib-users] different behaviour in Windows and Linux

[Michael D. <md...@st...>]

There is an explicit offset of one pixel on the left when it sets up a 
clip box in Agg.  I don't know why this is there, but it dates back to 
0.98.0, and earlier versions did something completely different.  I can 
only guess it was to compensate for an earlier bug in the precise 
drawing of the axes rectangle.  I can't explain why it would have 
different behavior on Windows vs. Linux, though.

I have fixed this in SVN r6465 and am including a patch below (which 
unfortunately requires a recompile).

Cheers,
Mike


Modified: trunk/matplotlib/src/_backend_agg.cpp
===================================================================
--- trunk/matplotlib/src/_backend_agg.cpp	2008-12-01 19:35:39 UTC (rev 6465)
+++ trunk/matplotlib/src/_backend_agg.cpp	2008-12-02 15:27:23 UTC (rev 6466)
@@ -312,8 +312,8 @@
 
   double l, b, r, t;
   if (py_convert_bbox(cliprect.ptr(), l, b, r, t)) {
-    rasterizer.clip_box(int(mpl_round(l)) + 1, height - int(mpl_round(b)),
-                        int(mpl_round(r)),     height - int(mpl_round(t)));
+    rasterizer.clip_box(int(mpl_round(l)), height - int(mpl_round(b)),
+                        int(mpl_round(r)), height - int(mpl_round(t)));
   }
 
   _VERBOSE("RendererAgg::set_clipbox done");

Re: [Matplotlib-users] different behaviour in Windows and Linux

[Gary R. <gr...@bi...>]

Thanks for the rapid fix Mike.

regards,
Gary

Michael Droettboom wrote:
> There is an explicit offset of one pixel on the left when it sets up a 
> clip box in Agg.  I don't know why this is there, but it dates back to 
> 0.98.0, and earlier versions did something completely different.  I can 
> only guess it was to compensate for an earlier bug in the precise 
> drawing of the axes rectangle.  I can't explain why it would have 
> different behavior on Windows vs. Linux, though.
> 
> I have fixed this in SVN r6465 and am including a patch below (which 
> unfortunately requires a recompile).
> 
> Cheers,
> Mike