New contourf() drawing polygon boundaries for some reason?

I can't reproduce this error with the current code in SVN trunk, but I remember seeing this bug a while ago. So I'm guessing that this bug has already been fixed in SVN. Zane, could you try installing the latest matplotlib from trunk and see if you still see this bug?



--- On Mon, 6/15/09, Michael Droettboom <mdroe@...86...> wrote:

From: Michael Droettboom <mdroe@...86...>
Subject: Re: [Matplotlib-users] New contourf() drawing polygon boundaries for some reason?
To: zane@...2552..., "matplotlib-users" <>, "Michiel de Hoon" <mjldehoon@...9...>
Date: Monday, June 15, 2009, 6:08 PM

Thanks for the thorough investigation.

Michiel: can you look into why the macosx backend is
drawing the
strokes around the polygons? Probably as simple as
ignoring an argument.

I'll look into the path simplification-related issues.



On 06/15/2009 03:29 PM, Zane Selvans wrote:

  Yes. By far the worst of these behaviors is the
macosx GUI output. I
could see the other ones just being the way it's
supposed to look.
Here's a summary:

backend: macosx; path.simplify: (false|true) GUI =>
black borders to
drawn polygons (incl. contour region crossing lines, very

backend: agg (PNG output) path.simplify: (true|false) =>
visible borders between polygons (esp. adjacent to
contrasting colors)

backend: pdf (PDF output) path.simplify: true =>
reliably visible
irregularities (but probably this is somewhat expected with

backend: pdf (PDF output) path.simplify: false =>
thin lines of background color visible between contour
filled regions.

And here's what I used to generate them:

def broken_contourf():
    Simple demonstration that filled contour plots are

    Looking at the PDF and PNG output using the macosx
backend, everything is
    fine. It's only the GUI output which is for some
reason outlining the
    polygons in the filled contours.

    from numpy.random import uniform, seed
    from matplotlib.mlab import griddata
    import matplotlib.pyplot as plt
    import numpy as np
    # make up data.
    #npts = int(raw_input('enter # of random points to
    npts = 200
    x = uniform(-2,2,npts)
    y = uniform(-2,2,npts)
    z = x*np.exp(-x**2-y**2)
    # define grid.
    xi = np.linspace(-2.1,2.1,100)
    yi = np.linspace(-2.1,2.1,100)
    # grid the data.
    zi = griddata(x,y,z,xi,yi)
    # contour the gridded data, plotting dots at the
nonuniform data points.
    CS = plt.contourf(xi,yi,zi,15,
    # plot data points.
    plt.title('griddata() and contourf() test')

On Fri, Jun 12, 2009 at 3:02 PM, Zane Selvans<zane@...1923...> > wrote:
    I switched back to using the macosx backend, and
it turns out that the
thin black lines surrounding the polygons (including
crossing the
filled contour regions from one closed contour to another)
only get
displayed in the GUI. PDF and PNG output look fine.


On Fri, Jun 12, 2009 at 2:27 PM, Zane Selvans<zane@...1923...> > wrote:
      If I set path.simplify: False, the shape of the
gaps between the
filled polygons does change. Instead of being irregular,
it becomes
an infinitessimally thin gap of uniform width, allowing the
(in this
case white) background to show through.

In both of these cases (path.simplify: True|False), the PNG
version of
the same figures also show representations of these gaps
which are
identical to those which appear in the PDF (though
pixelated), so I don't think it's something
that's wrong in the vector
graphics code per se.


On Fri, Jun 12, 2009 at 11:46 AM, Michael Droettboom<mdroe@...86...> > wrote:
        Shot in the dark here, but what if you set the
rcParam "path.simplify" to
False? There have been recent changes to that code.

Also, since the Agg backend doesn't have an associated
GUI, you need to use
the savefig() command and provide a filename, rather than
using show().


Zane Selvans wrote:
          Um, yeah. So my response got bounced
because of the attachment. Take 2:

For some reason my script bombed when I switched to the Agg
trying to display to the screen (it said Figure has no
method show())

So I output the plot as both a PDF and a PNG (still having
agg in my rcfile) and in both of those cases, irregular
gaps are
visible between the polygons making up the filled contours.
wasn't the case with my previously installed setup.
It looks as if
for some reason the vertices of the filled polygons are
calculated differently from different sides of the same
leading to overlap in some places, and gaps in others.
You can download
the PDF version (in which the exact geometry is much


On Fri, Jun 12, 2009 at 5:51 AM, Michael Droettboom<mdroe@...86...> > wrote:

            So you see this behavior if you switch to
the Agg backend? That's the
backend used to generate the images in the gallery. If
there's a
there, that would seem to suggest some tweaking of the
macosx backend
is still relatively new) is in order.


Zane Selvans wrote:

              I just installed the latest SciPy
Superpack in order to get access to
the scipy.spatial.KDTree class, and discovered that for
some reason
now when I use contourf() lines get drawn at the boundaries
the filled contours. Additionally, there is always a
single vertical
line crossing from each contour boundary to the next.
I'm guessing
that these are the edges of the filled polygons which are
drawn. This behavior doesn't seem to be consistent
with the
contourf() documentation and when I run code in it
doesn't come out looking like the picture in the

Is anyone else seeing this behavior? Is there a keyword I
can use to
force the edges of the polygons not to get drawn?

This is on Mac OS X 10.5.7, with
scipy.__version__ = 0.8.0.dev5635
matplotlib.__version__ = 0.98.6svn

As installed by

backend: macosx


Michael Droettboom
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA

Michael Droettboom
Science Software Branch
Operations and Engineering Division
Space Telescope Science Institute
Operated by AURA for NASA

Zane A. Selvans
Amateur Earthling
+1 303 815 6866

Zane A. Selvans
Amateur Earthling
+1 303 815 6866