matplotlib-users Mailing List for matplotlib

On Sun, May 25, 2008 at 5:19 AM, Gregor Thalhammer
<gre...@gm...> wrote:

> matplotlib is _not_ a dedicated image manipulation package, so please
> don't blame it that you don't manage to easily use it as such one.
> However, matplotlib indeed offers high quality image scaling abilities
> (at the moment not for downscaling) due to the use of Agg.

While mpl it is not a general purporse image library as you say, it
would be nice if we exposed a minimal set of image functionality so
users could take advantage of agg's nice rescaling.  Easy stuff should
be easy....  I did add support for PIL loading in imread, so if you
have it you will use it.  Thus in svn you can do, if PIL is installed,

   X =  imread('somefile.jpg') # or any other format supported by PIL

This will help reduce the boilerplate.

I would like to expose at least a resize and rotation from agg, with a
choice of interpolation schemes, exposed at the array level and not
just the display level.  I think agg2.4 also added some new
interpolation schemes designed to support down-sampling which is
useful for large mega-pixel images, and these haven't been exposed
since I wrote the original interpolation options against agg2.3.

In testing this stuff, I found a bug bug in image handling on the
trunk -- if you zoom to part of an image with zoom-to-rect, the part
that you get zoomed to is not the part you select.  This is most
apparent if you load an image with easily recognizable features, eg a
picture of a person.  This problem is on the trunk but not the branch
-- here is some example code:

In [8]: fig = plt.figure()

In [9]: ax = fig.add_subplot(111)

In [10]: ax.set_aspect('auto')

In [11]: X = imread('/Users/jdhunter/Desktop/IMG_0907.JPG')

In [12]: ax.imshow(X, origin='lower', aspect='auto')
Out[12]: <matplotlib.image.AxesImage object at 0x112586b0>

In [13]: ax.figure.canvas.draw()

In [14]: ax.cla()

In [15]: ax.set_aspect('auto')

In [16]: ax.imshow(X, origin='upper', aspect='auto')
Out[16]: <matplotlib.image.AxesImage object at 0x11258690>

In [17]: fig.canvas.draw()

thanks for your answer, i have come to understand that matplotlib is not 
dedicated to raster image manipulation. a nice person pointed out to me 
how to do that in cairo:

img = cairo.ImageSurface.create_from_png( 'c:/temp/tel-hotline.png' )
width = img.get_width()
height = img.get_height()
imgpat = cairo.SurfacePattern(img)
scaler = cairo.Matrix()
#1 = 100%; 2 = 50%;0.5 = 200%
scaler.scale(3,0.5)
imgpat.set_matrix(scaler)
#set resampling filter
imgpat.set_filter(cairo.FILTER_BEST)
canvas = cairo.ImageSurface(cairo.FORMAT_ARGB32,320,240)
ctx = cairo.Context(canvas)
ctx.set_source(imgpat)
ctx.paint()
canvas.write_to_png( 'c:/temp/helo-modified.png' )

that's a bit of code but i'm writing a wrapper `def scale_image()` 
anyhow. the data flow seems reasonable. alas, it only works for *.png, 
no *.jpg support. still i guess i'll go with cairo as it's seemingly 
used inside firefox3 and does impressively good scaling.

cheers & ~flow


Gregor Thalhammer wrote:
> > [had to delete an endless amount of stuff not related to matplotlib]
>
> matplotlib is _not_ a dedicated image manipulation package, so please 
> don't blame it that you don't manage to easily use it as such one. 
> However, matplotlib indeed offers high quality image scaling abilities 
> (at the moment not for downscaling) due to the use of Agg.
>
> If you want a simpler solution, try this:
>
> import wx
> img = wx.Image('your image.jpg')
> scale = 2
> img.Rescale(img.Width * scale, img.Height * scale, wx.IMAGE_QUALITY_HIGH)
> img.SaveFile('your scaled image.jpg')
>
> Gregor
>
>

 > [had to delete an endless amount of stuff not related to matplotlib]

matplotlib is _not_ a dedicated image manipulation package, so please 
don't blame it that you don't manage to easily use it as such one. 
However, matplotlib indeed offers high quality image scaling abilities 
(at the moment not for downscaling) due to the use of Agg.

If you want a simpler solution, try this:

import wx
img = wx.Image('your image.jpg')
scale = 2
img.Rescale(img.Width * scale, img.Height * scale, wx.IMAGE_QUALITY_HIGH)
img.SaveFile('your scaled image.jpg')

Gregor

Hi John,
   Thanks for the hint.  I am able to pass the rect returned by
Subplot.get_position().

Bing

On Sat, May 24, 2008 at 9:27 PM, John Hunter <jd...@gm...> wrote:

> On Sat, May 24, 2008 at 4:08 PM, Bing <bin...@gm...> wrote:
> > Hello,
> >    Could anyone tell me how to use plot3D in a subplot?
> > I looked at the examples at
> >   http://www.scipy.org/Cookbook/Matplotlib/mplot3D
> > But seems to me that matplotlib.axes3d.Axes3D can
> > be only constructed from a Figure instance but
> > not from a Subplot instance.
>
> You should be able to set the rect keyword argument of the Axes3D
> constructor:
>
>  fig=p.figure()
>  ax = pylab.Axes3D(fig, rect=[0.2, 0.6, 0.7, 0.3])
>
> where rect is [left, bottom, width, height] in fractions of the figure 0..1
>
> A subplot is just an axes with a rect set so that it lives on a regular
> grid.
>
> Note though that the 3D stuff is experimental, slow, somewhat buggy
> and not supported, so we won't be able to help much with real
> problems.
>
> JDH
>

On Sat, May 24, 2008 at 4:08 PM, Bing <bin...@gm...> wrote:
> Hello,
>    Could anyone tell me how to use plot3D in a subplot?
> I looked at the examples at
>   http://www.scipy.org/Cookbook/Matplotlib/mplot3D
> But seems to me that matplotlib.axes3d.Axes3D can
> be only constructed from a Figure instance but
> not from a Subplot instance.

You should be able to set the rect keyword argument of the Axes3D constructor:

  fig=p.figure()
  ax = pylab.Axes3D(fig, rect=[0.2, 0.6, 0.7, 0.3])

where rect is [left, bottom, width, height] in fractions of the figure 0..1

A subplot is just an axes with a rect set so that it lives on a regular grid.

Note though that the 3D stuff is experimental, slow, somewhat buggy
and not supported, so we won't be able to help much with real
problems.

JDH