Hello,
I am also interested in masking polygons and defining the polygon by 'clicking' on the image... but I do not know anything about GUI.... does anyone can help? Is there already something implemented?
Thanks!
Chiara

> Date: Wed, 23 Jan 2008 13:50:15 +1300
> From: amcmorl@gmail.com
> To: matplotlib-users@lists.sourceforge.net
> Subject: Re: [Matplotlib-users] Polygon masking possible?
>
> Hi Søren,
>
> I've put this back on the list in case it's useful to anyone else, or
> if there are better suggestions or improvements around. Hope you don't
> mind.
>
> On 22/01/2008, Søren Nielsen <soren.skou.nielsen@gmail.com> wrote:
> > Yeah i'd like to see your code if I can..
>
> import numpy as n
>
> def get_poly_pts(x, y, shape):
> """Creates convex polygon mask from list of corners.
>
> Parameters
> ----------
> x : array_like
> x co-ordinates of corners
> y : array_like
> y co-ordinates of corners, in order corresponding to x
> shape : array_like
> dimension sizes of result
>
> Returns
> -------
> build : ndarray
> 2-D array of shape shape with values True inside polygon
>
> Notes
> -----
> Code is constrained to convex polygons by "inside"
> assessment criterion.
>
> """
> x = n.asarray(x)
> y = n.asarray(y)
> shape = n.asarray(shape)
> npts = x.size # should probably assert x.size == y.size
> inds = n.indices( shape )
> xs = inds[0]
> ys = inds[1]
> xav = n.round(x.mean()).astype(int)
> yav = n.round(y.mean()).astype(int)
> for i in xrange(npts): # iterate over pairs of co-ordinates
> j = (i + 1) % npts
> m = (y[j] - y[i])/(x[j] - x[i])
> c = (x[j] * y[i] - x[i] * y[j])/(x[j] - x[i])
> thisone = ( ys > m * xs + c )
> if thisone[xav, yav] == False:
> thisone = ~thisone
> if i == 0:
> build = thisone
> else:
> build &= thisone
> return build
>
> (released under BSD licence)
>
> > I just needed the push over the edge to know how to draw on the canvas,
> > mapping clicks etc. since i'm still fairly new to matplotlib, so I think
>
> I hope so. As you can see this code doesn't do any of the drawing or
> click collecting, but the cookbook page should be able to guide you
> there. Ask again on the list if you have any further questions and
> we'll see if we can help.
>
> Also, the code assumes that the average co-ordinate is inside the
> shape - that's true for convex polygons, but not necessarily for
> arbitrary ones. I use if after taking a convex hull of a greater list
> of points (using the delaunay module in scipy (now in scikits, I
> hear)), which ensures convexity. You just need to be aware of that
> limitation.
>
> Cheers,
>
> A.
> --
> AJC McMorland, PhD candidate
> Physiology, University of Auckland
>
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