Thread: [Matplotlib-users] export of a specific zone of a figure

Hi,

I would like to export a zone of a Figure in .png.
Something like figure.savefig("mypicture.png", box = (0,0,5,5))
How may I proceed, without drawing all the plots again ?
I use wxagg.
Thanks,

Nicolas
       
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Le jeudi 28 juin 2007, Nicolas a =C3=A9crit=C2=A0:
> Hi,
>
> I would like to export a zone of a Figure in .png.
> Something like figure.savefig("mypicture.png", box =3D (0,0,5,5))
> How may I proceed, without drawing all the plots again ?
> I use wxagg.
> Thanks,
>
> Nicolas
>
> ---------------------------------
>  Ne gardez plus qu'une seule adresse mail ! Copiez vos mails vers Yahoo!
> Mail

Look at PIL, you have normaly a such function there.

=2D-=20
Lionel Roubeyrie - lro...@li...
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http://www.limair.asso.fr

Thanks for your reply.

However, I don't want to had a PIL dependency.

Is there any other method, using only matplotlib or wx ?

Nicolas

On 6/28/07, Nicolas <nic...@ya...> wrote:
> Thanks for your reply.
>
> However, I don't want to had a PIL dependency.
>
> Is there any other method, using only matplotlib or wx ?

agg offers methods to convert the image pixel buffer to strings or
buffers, which you could then convert to numpy arrays, so a slice
extraction, and reconvert back to a buffer and ultimately a PNG. I
don't have time right now to write some example code, but you may want
to poke around in backend_agg to see if you can figure it out, and if
not remind me next week.

Thanks,
JDH

Hi,
I can figure the first steps :
something like :

            matrix = []
            buffer = self.get_renderer().tostring_argb()
            l, h = self.GetSize()
            for ligne in xrange(h):
                matrix.append([])
                for colonne in xrange(l):
                    i = 4*(ligne*h + colonne)
                    pixel = buffer[i:i+4]
                    matrix[-1].append(pixel)
            zone_to_export = array(matrix)[pixely0:pixely1, pixelx0:pixelx1]
            new_buffer = buffer("".join("".join(elt for elt in ligne) for
ligne in zone_to_export ))

But then I don't know what to with this new buffer.
I tried to create a new RenderAgg instance, so as to use its png export
facilities.
r = RendererAgg(pixelx1 - pixelx0, pixely1 - pixely0, Value(dpi))
...
r._renderer.write_png(nom)

But I don't know what to put between the two previous lines.
How may I load a buffer content into a RenderAgg instance ?
I suppose I may use something like :
r.draw_image(0, 0, im)
but what is the correct format for im ? Is there an Image class in
matplotlib (I looked for, but didn't find).
How may I convert my buffer ?

Thanks a lot,

Nicolas



How may I transform my buffer into an image ?
On 6/28/07, John Hunter <jd...@gm...> wrote:
>
> On 6/28/07, Nicolas <nic...@ya...> wrote:
> > Thanks for your reply.
> >
> > However, I don't want to had a PIL dependency.
> >
> > Is there any other method, using only matplotlib or wx ?
>
> agg offers methods to convert the image pixel buffer to strings or
> buffers, which you could then convert to numpy arrays, so a slice
> extraction, and reconvert back to a buffer and ultimately a PNG. I
> don't have time right now to write some example code, but you may want
> to poke around in backend_agg to see if you can figure it out, and if
> not remind me next week.
>
> Thanks,
> JDH
>

I don't know how to do it with the MPL agg back-end, but I think you 
mentioned wx, and you can do it there instead. a wxImage can be 
constructed from a buffer object, then saved as a PNG. You may need to 
set the rgb and alpha portions separately. See the wxPython wiki and 
search for "Image".

Also:

>             matrix = []
>             buffer = self.get_renderer().tostring_argb()
>             l, h = self.GetSize()
>             for ligne in xrange(h):
>                 matrix.append([])
>                 for colonne in xrange(l):
>                     i = 4*(ligne*h + colonne)
>                     pixel = buffer[i:i+4]
>                     matrix[-1].append(pixel)

This is a very slow way to create the numpy array!

Option a: first create an empty array:

matrix = numpy.empty((l,h,4), numpy.byte)

then fill that in. but even better:

you can build the array directly from the buffer string:

matrix = numpy.fromstring(buffer, dtype=numpy.byte)
lotlib-users

-- 
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Chr...@no...

Thank you very much.
I know very little about numpy in fact.

If I don't find a pure matplotlib method, I will use your suggestion with
wx.
I think however matplotlib may be used only (and it will be even better as I
plan to make a Qt version in the future)

So, in :
>>> from matplotlib.transforms import Value
>>> from matplotlib.backends.backend_agg import RendererAgg
>>> r = RendererAgg(50, 50, Value(72))
>>> r.draw_image(0, 0, im)

What is the correct format for im ?

Thanks,

Nicolas

On 7/2/07, Christopher Barker <Chr...@no...> wrote:
>
> I don't know how to do it with the MPL agg back-end, but I think you
> mentioned wx, and you can do it there instead. a wxImage can be
> constructed from a buffer object, then saved as a PNG. You may need to
> set the rgb and alpha portions separately. See the wxPython wiki and
> search for "Image".
>
> Also:
>
> >             matrix = []
> >             buffer = self.get_renderer().tostring_argb()
> >             l, h = self.GetSize()
> >             for ligne in xrange(h):
> >                 matrix.append([])
> >                 for colonne in xrange(l):
> >                     i = 4*(ligne*h + colonne)
> >                     pixel = buffer[i:i+4]
> >                     matrix[-1].append(pixel)
>
> This is a very slow way to create the numpy array!
>
> Option a: first create an empty array:
>
> matrix = numpy.empty((l,h,4), numpy.byte)
>
> then fill that in. but even better:
>
> you can build the array directly from the buffer string:
>
> matrix = numpy.fromstring(buffer, dtype=numpy.byte)
> lotlib-users
>
> --
> Christopher Barker, Ph.D.
> Oceanographer
>
> Emergency Response Division
> NOAA/NOS/OR&R            (206) 526-6959   voice
> 7600 Sand Point Way NE   (206) 526-6329   fax
> Seattle, WA  98115       (206) 526-6317   main reception
>
> Chr...@no...
>

Nicolas wrote:
> I think however matplotlib may be used only (and it will be even better 
> as I plan to make a Qt version in the future)

good idea.

> So, in :
>  >>> from matplotlib.transforms import Value
>  >>> from matplotlib.backends.backend_agg import RendererAgg
>  >>> r = RendererAgg(50, 50, Value(72))
>  >>> r.draw_image (0, 0, im)
> 
> What is the correct format for im ?

I'm no expert, but probably a string that's the same format as what 
tostring_argb() returns, so something like this should work (untested!):


buffer = self.get_renderer().tostring_argb()
l, h = self.GetSize()
matrix = numpy.fromstring(buffer, dtype=numpy.byte)
matrix.shape = (l,h,4) # 4 for a,r,g,b
sub_matrix = matrix[min_x:max_x, min:y_max_y, :]
r.draw_image (0, 0, sub_matrix.tostring())

-Chris



-- 
Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

Chr...@no...

Nicolas wrote:
>      > as I plan to make a Qt version in the future)
>     good idea.
> You mean, to use Qt instead of Wx ?

No, I'm a big wx fan. What I meant was that it's a good idea to keep 
this functionality GUI toolkit independent.

> I've  found a possible solution :

looks good to me.

>             buffer = self.get_renderer().buffer_rgba(0,0) # better 
> because alpha should be the last value

Why is that? Is it the "native" backend_agg format?

-Chris

-- 
Christopher Barker, Ph.D.
Oceanographer

Emergency Response Division
NOAA/NOS/OR&R            (206) 526-6959   voice
7600 Sand Point Way NE   (206) 526-6329   fax
Seattle, WA  98115       (206) 526-6317   main reception

Chr...@no...