Thanks Ben for your answer.
I didn't know about that should_simplify attribute. It was effectively the
reason for the disappearing points. Thanks a lot.
I still have a few questions if you don't mind :
1. [low importance] Isn't it weird to have this attribute set to True by
default ? I would find it more natural if simplification had to be
explicitly requested.
2. You say "The first element of that list is the external vertexes, and
the rest of the elements are all vertex lists of the internal holes." I
like this a lot, but are you sure it is true ? As it comes in contradiction
with Ian Thomas explanation :
"The returned geometries are purposefully not documented. They are an
'implementation detail' and not considered part of the public interface.
and as such they could change at any time and hence should not be relied
upon. Of course you can choose to access them if you wish, as I do myself
sometimes, but we make no promises about what the order of the polygons is,
or that it won't change tomorrow."
http://matplotlib.1069221.n5.nabble.com/Structure-of-contour-object-returned-from-tricontourf-td44203.html
3. I have done a simple test and the output looks like this (2 rings):
[image: Images int?gr?es 1]
So I'm a bit confused as :
cs = plt.tricontourf(t, v, levels)
#cs.collections has 1 element (ok as there is only one level)
for i,collection in enumerate(cs.collections):
for path in collection.get_paths():
#collection.get_paths() has only 1 element, I would eventually have
expected 2 (1 for each ring)
polygons = path.to_polygons()
# polygons has 4 elements : the 4 rings are stored at the same place
# how can I recognise the exteriors from the interiors ?
Thanks for your help,
Francis
2015-11-30 19:37 GMT+01:00 Benjamin Root <ben.v.root at gmail.com>:
Francis,
I bet you that the inconsistency in the number of vertexes is due to path
simplification. The list of Path objects you get when you call get_paths()
on the collection object each have an attribute "should_simplify" and that
defaults to True. Set it to False, and you will have all of the vertexes.
Also, what you want to call is to_polygons() on the Path object after
setting "should_simplify" to False. That will return a list of lists. The
first element of that list is the external vertexes, and the rest of the
elements are all vertex lists of the internal holes.
I hope this description helps. I can't really give you more detailed
description due to the fact that I have developed software that does this
very thing for my employer, but what you want is certainly possible.
Also, as for whether or not we would want a geojson export function
available for matplotlib, it isn't really correct to have it in matplotlib
because we are a graphing library. However, it would make sense to make the
process of extracting the polygon information a bit easier, which would
make it easier for another package to be made that would export that
information into various data formats, not just geojson.
Cheers!
Ben Root
Hello,
I'm having some difficulties with the results of the tricontour function.
What I'm trying to achieve is fairly simple : I'd like to export the
results of the tricontour function as a geoJson. (I think a function doing
exactly this job would be nice to have in the library).
I wrote this :
cs = plt.tricontourf(t, v, levels)
for i,collection in enumerate(cs.collections):
for path in collection.get_paths():
Now I have this path object.
My first problem : when I check the number of vertices (via
len(path.vertices)) I get 732 vertices.
If I try to access those vertices with iter_segments as recommended in
the doc, I get only 125 vertices.
seg = path.iter_segments()
print len(list(seg))
==> 125
Am I doing something wrong, or is it possibly a bug ?
My second problem : geoJson works with interior and exterior rings. To
describe a polygon with a hole in it, we first declare a closed line (that
will be the exterior) and all the subsequent lines will be the "holes"
(interiors). It seems that what I get from iter_segments and to_polygons is
a bunch of lines, but there is no way to know which is an interior, which
is an exterior. But I guess this must be stored somewhere as MPL is able to
draw a graph from this information !
Any hints on how I should proceed ?
Let me know if you need additional information.
Thanks
ps : I got some of my infos from this thread :
http://matplotlib.1069221.n5.nabble.com/Structure-of-contour-object-returned-from-tricontourf-td44203.html
ps2 : If I can write this function I would be happy to integrate it in
the lib if you're interested.
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On Mon, Nov 30, 2015 at 12:46 PM, Francis Chabouis <fchabouis at gmail.com> > wrote: