matplotlib-users

[Matplotlib-users] imshow without resampling

[Thomas R. <tho...@gm...>]

Hello,

I am using matplotlib to create postscript and SVG files. I am  
currently using imshow to show the contents of an array, but this  
means that when saving vector graphics files, matplotlib resamples the  
image/array onto a finer grid. What I would like, is for code such as  
this:

import matplotlib
matplotlib.use('PS')
from matplotlib.pyplot import *

import numpy as np

image = np.random.random((10,10))

fig = figure(figsize=(4,4))
ax = fig.add_subplot(111)
ax.imshow(image)
fig.savefig('example1.ps')

fig = figure(figsize=(8,8))
ax = fig.add_subplot(111)
ax.imshow(image)
fig.savefig('example2.ps')

to produce files that are the roughly the same size, rather than  
different by a factor of 4. In addition, both files should be very  
small since they should only contain a 10x10 bitmap in addition to the  
axes+labels. Postscript and SVG (as languages) both allow a bitmap of  
an arbitrary resolution to be scaled, translated, and rotated without  
resampling.

I have come across the figimage method which is meant to place an  
array in a plot without resampling, but I cannot figure out how to use  
it like imshow, i.e. to show the image inside the axes as before. I've  
also tried the pcolor functions, but it seems like they define each  
pixel as an individual polygon, which is inefficient.

I was wondering if anyone had a solution to this, or if there are  
plans to make matplotlib behave like this in future?

Thanks,

Thomas

Re: [Matplotlib-users] imshow without resampling

[John H. <jd...@gm...>]

On Tue, Mar 31, 2009 at 2:56 PM, Thomas Robitaille
<tho...@gm...> wrote:
> Hello,
>
> I am using matplotlib to create postscript and SVG files. I am
> currently using imshow to show the contents of an array, but this
> means that when saving vector graphics files, matplotlib resamples the
> image/array onto a finer grid. What I would like, is for code such as
> this:


I believe pcolor will be closer to what you are looking for.7

JDH

Re: [Matplotlib-users] imshow without resampling

[Thomas R. <tho...@gm...>]

The problem with pcolor is that it draws all the pixels as polygons,  
which is very inefficient in terms of file size. For example,

import matplotlib
matplotlib.use('PS')
from matplotlib.pyplot import *

import numpy as np

image = np.random.random((100,100))

fig = figure(figsize=(4,4))
ax = fig.add_subplot(111)
ax.pcolor(image)
fig.savefig('plot.ps')

produces a 2Mb+ file.

Thanks,

Thomas



On Mar 31, 2009, at 4:32 PM, John Hunter wrote:

> On Tue, Mar 31, 2009 at 2:56 PM, Thomas Robitaille
> <tho...@gm...> wrote:
>> Hello,
>>
>> I am using matplotlib to create postscript and SVG files. I am
>> currently using imshow to show the contents of an array, but this
>> means that when saving vector graphics files, matplotlib resamples  
>> the
>> image/array onto a finer grid. What I would like, is for code such as
>> this:
>
>
> I believe pcolor will be closer to what you are looking for.7
>
> JDH

Re: [Matplotlib-users] imshow without resampling

[Thomas R. <tho...@gm...>]

I've looked into this some more, and while pcolor draws pixels  
inefficiently as polygons, pcolorfast does appear to be writing the  
data as a bitmap into the eps file, so this is closer to what I need.

However, while this works great with the SVG driver, it seems that  
pcolorfast still produces quite large files with the PS driver. For  
example,

import matplotlib
matplotlib.use('Agg')
from matplotlib.pyplot import *
import numpy as np

image = np.random.random((100,100))

fig = figure(figsize=(4,4))
ax = fig.add_subplot(111)
ax.pcolorfast(image)
fig.savefig('plot.eps')

fig = figure(figsize=(4,4))
ax = fig.add_subplot(111)
ax.pcolorfast(image)
fig.savefig('plot.svg')

produces a 600k ps file and a 60k svg file. In fact, the PS file size  
remains the same if I change the size of the array.

So pcolorfast does appear to be the solution, but the output is not  
properly handled for PS files. Is this something that could be fixed  
in future?

Thanks,

Thomas

On Mar 31, 2009, at 4:32 PM, John Hunter wrote:

> On Tue, Mar 31, 2009 at 2:56 PM, Thomas Robitaille
> <tho...@gm...> wrote:
>> Hello,
>>
>> I am using matplotlib to create postscript and SVG files. I am
>> currently using imshow to show the contents of an array, but this
>> means that when saving vector graphics files, matplotlib resamples  
>> the
>> image/array onto a finer grid. What I would like, is for code such as
>> this:
>
>
> I believe pcolor will be closer to what you are looking for.7
>
> JDH