[Matplotlib-users] Bug: Image transparency varies periodically for imshow(img, aspect='auto') at high magnification

[Joel H. <yo...@if...>]

Hi!

I believe I've found a bug in matplotlib which manifests after=20
imshow(img, aspect=3D'auto') when only a small portion of the image is=20
displayed, e.g: when (xmax-xmin) << img.shape[1]. The effect of the bug=20
is that the transparency for the entire image seems to vary, seemingly=20
as a periodic funtion of (xmax-xmin). Explicitly setting the alpha=20
channel for each pixel has no effect.

The attached script generates a random image of a certain size, and=20
plots it in four different subplots. The only thing that differs between=20
the subplots are slight variations in x_max. The four subplots are labele=
d:

1) The background image is visible.
2) The background image is not visible!
3) The background image is faded!
4) The background image is again visible!

which reflects the actual result on my system. As the subplots are=20
scaled (for example by resizing the window), the intensity then varies=20
in a periodic manner in each of the subplots.

The expected result is of course that the background should be fully=20
visible in all subplots.

Additional details:

$ python test.py --verbose-helpful
matplotlib data path /usr/lib/python2.4/site-packages/matplotlib/mpl-data
$HOME=3D/home/bioinfo/yohell
CONFIGDIR=3D/home/bioinfo/yohell/.matplotlib
loaded rc file=20
/usr/lib/python2.4/site-packages/matplotlib/mpl-data/matplotlibrc
matplotlib version 0.87.7
verbose.level helpful
interactive is False
platform is linux2
numerix numpy 1.0
font search path ['/usr/lib/python2.4/site-packages/matplotlib/mpl-data']
loaded ttfcache file /home/bioinfo/yohell/.matplotlib/ttffont.cache
backend WXAgg version 2.6.1.2pre

In this example I used the WXAgg backend, but GTKAgg shows the same=20
results. I've also reproduced this bug on a WinXP machine.

Thanks for an excellent plotting package!
/Joel Hedlund
Link=F6ping University