Re: [matplotlib-devel] Misalignment imshow vs. grid lines

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Thanks for making this so easy to reproduce. It is so much easier when
there is a small, self contained, correct example!

Initially I was thinking that the problem was some kind of snapping issue
with the ticks, but it turns out, if you change the interpolation scheme to
"none", you get perfect alignment:

import numpy as np
import matplotlib.pyplot as plt

n = 16
fig = plt.figure(figsize=(10,10))

Z = np.zeros((n, n))
Z[::2, ::2] = 1
Z[1::2, 1::2] = 1

# pick the interpolation you want:
interp = 'nearest'
interp = 'none'

plt.imshow(Z, interpolation=interp,
                cmap='gray', extent=[0, n, 0, n],
                alpha=0.25)

plt.xticks(np.arange(n))
plt.yticks(np.arange(n))
plt.grid(ls='solid')

plt.show()

I haven't considered where to go from this point, but I wanted to let you
know about this option.

Thanks,

Phil

On 30 October 2012 07:10, Nicolas Rougier <Nic...@in...> wrote:

>
>
> Sorry, I was too fast in my reply.
>
> When I save the figure, the png output is ok while the pdf is displaying
> some kind of interpolation with the image.
>
> import numpy as np
> import matplotlib
> matplotlib.use('Agg')
> import matplotlib.pyplot as plt
>
> n = 16
> fig = plt.figure(figsize=(6,6))
> Z = np.array(([0,1]*(n//2) + [1,0]*(n//2))*(n//2)).reshape(n,n)
> plt.imshow(Z, interpolation='none', cmap=plt.cm.gray, extent=[0,n,0,n],
> alpha=.25)
> plt.xticks(np.arange(0,n), [])
> plt.yticks(np.arange(0,n), [])
> plt.grid(ls='solid')
> plt.savefig('pylab-grid.png')
> plt.savefig('pylab-grid.pdf')
> plt.show()
>
>
>
>
>
> Just for the record, "Skim", "Preview" and "Adobe Reader" on OSX do not
> give the same output on the saved pdf.
> "Adobe Reader" displays the expected result (same as saved png) while
> "Preview" and "Skim" are apparently trying to make some (bad) interpolation
> of the checkboard image.
>
>
> Nicolas
>
>
> On Oct 30, 2012, at 7:52 , Nicolas Rougier wrote:
>
> >
> >
> > You're right. Using 'none' interpolation seems to solve the problem.
> Good to know !
> >
> > One last question, why is the 'none' interpolation restricted to
> Agg/PS/pdf ?
> >
> >
> > Nicolas
> >
> >
> >
> > On Oct 30, 2012, at 6:53 , Jae-Joon Lee wrote:
> >
> >> On Tue, Oct 30, 2012 at 12:25 AM, Nicolas Rougier
> >> <Nic...@in...> wrote:
> >>>
> >>>
> >>> Thanks for testing.
> >>>
> >>> If I zoom at any line cross, the lines are definitely at the wrong
> place for me.
> >>
> >> As  jules hummon commented, I see lines in right places when I zoom in.
> >>
> >>> As for screen aliasing I'm not sure since both the png and pdf seems
> to be wrong in my case.
> >>
> >> It still can be some aliasing-related issue. Note that with
> >> interpolation="nearest", the images are rasterized with given dpi even
> >> if you save the figure as pdf.
> >> The agg backend tries to adjust the location of lines and images so
> >> that they are well-aligned with the pixels, and the issue seems to be
> >> related with that behavior.
> >>
> >> In my case, using interpolation="none" worked out okay. So give it a
> try.
> >>
> >> Regards,
> >>
> >> -JJ
> >>
> >>
> >>> Weird !
> >>>
> >>>
> >>> Nicolas
> >>>
> >>>
> >>> On Oct 29, 2012, at 15:40 , jules hummon wrote:
> >>>
> >>>> Nicolas
> >>>>
> >>>> I get that too, (with your script and various things in my work).
> >>>> But if you zoom in, the lines are in the right place.   Is it
> >>>> some kind of screen aliasing?
> >>>>
> >>>> Jules
> >>>>
> >>>>
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