Re: [Matplotlib-users] Trouble with imshow

[Christoph G. <cg...@uc...>]

On 2/9/2011 4:29 PM, Benjamin Root wrote:
> On Wed, Feb 9, 2011 at 1:50 AM, Eric Firing <ef...@ha...
> <mailto:ef...@ha...>> wrote:
>
>     On 02/08/2011 02:39 PM, Christoph Gohlke wrote:
>     >
>
>     >
>     >  Please consider the attached patch for the _image.frombyte
>     function. It
>     >  avoids temporary copies in case of non-contiguous input arrays.
>     Copying
>     >  a 1024x1024 slice out of a contiguous 4096x4096 RGBA or RGB array is
>     >  about 7x faster (a common case for zooming/panning). Copying
>     contiguous
>     >  RGB input arrays is ~2x faster. Tested on win32-py2.7.
>     >
>     >  Christoph
>     >
>
>     Thank you!
>
>     Looks good, speeds up zooming and panning on large images as advertised.
>       An 8000x8000 image is actually manageable now.
>     interpolation='nearest' is still very slow until the image is
>     substantially zoomed, but everything is quite quick with other
>     interpolation styles.  The slowness of 'nearest' looks like a basic
>     characteristic of the implementation.
>
>     I committed the patch in 8966.
>
>     Before that I found and committed a big speed-up in Normalize.
>
>     Eric
>
>
> Bug Report:
>
> At some point between the recent revision and r8934, setting the alpha
> value to anythhing but None will cause the image to not show.  I suspect
> it has something to do with some of the recent revisions.  Maybe the
> alpha values were being converted into an integer, causing them to be
> zero?  Then again, even setting alpha to 1 will cause the image to
> disappear.
>
> Ideas?  Thoughts?  I included an example script below.
>
> Ben Root
>
>
> Example script:
>
>
> import numpy as np
> import matplotlib.pyplot as plt
>
> z = np.random.random((40, 50))
>
> fig = plt.figure()
> ax = fig.add_subplot(1, 2, 1)
> ax.imshow(z, alpha=1.0)
> ax.set_title('Blank!')
>
> ax = fig.add_subplot(1, 2, 2)
> ax.imshow(z, alpha=None)
> ax.set_title("Not Blank")
>
>
>
> plt.show()
>
>
>


This should fix it:

Index: lib/matplotlib/colors.py
===================================================================
--- lib/matplotlib/colors.py	(revision 8967)
+++ lib/matplotlib/colors.py	(working copy)
@@ -49,6 +49,7 @@
  'chartreuse' are supported.
  """
  import re
+import math
  import numpy as np
  from numpy import ma
  import matplotlib.cbook as cbook
@@ -547,6 +548,8 @@
          if alpha is not None:
              alpha = min(alpha, 1.0) # alpha must be between 0 and 1
              alpha = max(alpha, 0.0)
+            if bytes:
+                alpha = int(math.floor(alpha*255.9999999))
              if (lut[-1] == 0).all():
                  lut[:-1, -1] = alpha
                  # All zeros is taken as a flag for the default bad


Christoph