[Matplotlib-users] exporting tricontour function results

Francis Chabouis fchabouis at gmail.com
Thu Dec 3 15:54:49 EST 2015

Thank you to both of you, it's getting clearer. I understand only now that
tricontourf is not "hiding"
some information about who is an exterior and who is an interior...it
doesn't have this information in the first place (If I get it right).
I have managed to sort all those polygons and group them correctly.
Probably not robust nor efficient, but good enough for the moment !

2015-12-02 21:48 GMT+01:00 Benjamin Root <ben.v.root at gmail.com>:

> Ian,
> I see now how tricontourf()'s polygons are fundamentally different from
> contourf()'s polygons (and explains the discrepancy the user is seeing),
> and I see I misread your comments. That said, any sort of modification that
> would change the output of to_polygons() in a substantial way would
> certainly need major justification. The functions have been there for a
> long time and people have come to expect them to behave in a certain way.
> Ben Root
> On Wed, Dec 2, 2015 at 3:00 PM, Ian Thomas <ianthomas23 at gmail.com> wrote:
>> Francis,
>> Your question about interior and exterior polygons is already answered in
>> the thread you keep referring to.  I'll repeat it here.  tricontouf does
>> not return any information about which interior polygons are located inside
>> which exterior polygons.  All you get is a collection of polygons, composed
>> of one or more exteriors and zero or more interiors, and they can be in any
>> order.  The backends take these arbitrary collections of exterior and
>> interior polygons and render them correctly.  As all of the backends are
>> capable of calculating the exterior/interior containment themselves, there
>> is no need for tricontourf to do it as well.
>> contourf produces different output, grouping each exterior polygon with
>> its contained interior polygons.  This is because it dates from before all
>> the backends were capable of calculating polygon containment, so contourf
>> had to do it.  The recent rewrite of the contourf C++ code still does this
>> so that it produces output consistent with the legacy code.
>> If you want take the output of tricontourf and calculate the
>> exterior/interior containment, you'll either have to find some other
>> library to do it, or write the code yourself.  I have not looked into
>> libraries that do this as I do not need this functionality.  Writing the
>> code to do it yourself is pretty easy, but making it robust and efficient
>> is much harder.
>> Ben,
>> I think you have misunderstood my comments from last year.  When I was
>> talking about what I consider private, I was referring to the segs and
>> kinds that are passed from C++ to python to make up the various Path
>> objects.  I didn't refer to the function to_polygons() being private, in
>> fact I didn't refer to it at all.
>> Ian
>> On 1 December 2015 at 21:57, Francis Chabouis <fchabouis at gmail.com>
>> wrote:
>>> Ben,
>>> 1. Exporting data like this was never an intended use => I understand
>>> your point. In fact if calling the underlying C++ code for tricontour had
>>> been easy I would happily have skipped the call to MPL. All I'm interested
>>> in is the polygons coordinates really.
>>> 2. good news :)
>>> 3. please find the script attached.
>>> thanks
>>> Francis
>>> 2015-12-01 22:33 GMT+01:00 Benjamin Root <ben.v.root at gmail.com>:
>>>> Francis,
>>>> 1. Keep in mind, matplotlib is a plotting library first. Path
>>>> simplification takes into account the resolution of the output device when
>>>> used for drawing and essentially simplifies out any unresolvable features,
>>>> which greatly reduces drawing time for complex plots. Exporting data like
>>>> this was never an intended use.
>>>> 2. I will have to respectfully disagree with Ian on this point.
>>>> to_polygons() is not a private method and it is perfectly reasonable to
>>>> expect it to be used by people outside the matplotlib codebase. As a
>>>> developer, I would reject any patches that changes the output semantics of
>>>> to_polygons() without going through a lengthy deprecation cycle. Plus, the
>>>> primary use of this method is for easy input to Polygon artist objects,
>>>> which has a constructor that isn't going to change, so why should
>>>> to_polygons() change?
>>>> 3. Without the source example data, I am at a bit of a loss for what is
>>>> happening here. I could come up with all sorts of guesses, but I can't tell
>>>> you for sure without having something I can run myself.
>>>> Cheers!
>>>> Ben Root
>>>> On Tue, Dec 1, 2015 at 4:18 PM, Francis Chabouis <fchabouis at gmail.com>
>>>> wrote:
>>>>> 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 intégrées 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
>>>>>> On Mon, Nov 30, 2015 at 12:46 PM, Francis Chabouis <
>>>>>> fchabouis at gmail.com> wrote:
>>>>>>> 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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>>>>>>> Matplotlib-users at python.org
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