[Matplotlib-devel] Use of contourf changed?

[Viktoria Schubert]

Hello,

Yeah, this helps indeed with "corner_mask='legacy' the result is
identical, even though I get a warning:
/usr/lib/python3.5/site-packages/matplotlib/cbook.py:136:
MatplotlibDeprecationWarning: The corner_mask='legacy' attribute was
deprecated in version 1.5. Use corner_mask=False or True instead.
  warnings.warn(message, mplDeprecation, stacklevel=1)

I tried creating a minimal example which recreates the problem with
simpler data. The result does not exactly represent 100% of the
problem, but it is screwed anyway. Again, with the option
corner_mask='legacy' option, it looks just fine.
You can find my example here:
https://paste.kde.org/poyn7upbi

Thank you very much!
Viktoria



2015-11-12 17:09 GMT+01:00 Benjamin Root <ben.v.root at gmail.com>:
> Interesting, but it is hard to tell what is going on without any code or
> data. In particular, matplotlib's contourf polygons has always been
> "unstacked". In other words, the polygons representing a contour level does
> not overlap with any other polygon of another contour level, so I am not
> exactly sure how you are getting the behavior you are seeing in either
> image.
>
> Now, the default algorithm for contouring did change in v1.5. You can access
> the old algorithm by passing `corner_mask='legacy'` as a keyword argument to
> contourf() in v1.5. Could you try that and see if at least you get identical
> results for v1.5 and v1.4?  Note, that keyword argument is not available in
> v1.4. This should help us narrow down the source of your issue, but I
> wouldn't treat it as a solution because the old algorithm is slated for
> removal at some point, and it still doesn't explain the difference you are
> seeing with the new contouring algorithm.
>
> Ben
>
>
> On Thu, Nov 12, 2015 at 8:11 AM, Viktoria Schubert via Matplotlib-devel
> <matplotlib-devel at python.org> wrote:
>>
>> Hello together,
>>
>> Since the 1.5 version of matplotlib together with python 3.5, I get
>> strange results from using the contourf function. What I'm actually
>> doing is plotting a lot of points in a 2D-space. The density equals the
>> likelihood of the data, so what I want is to plot the 1Sigma, 2Sigma and
>> 3Sigma areas of the data.
>> For this purpose, I was using the following line of code:
>> cs = plt.contourf(hist2D, extent=extent, levels=[L1,L2,L3,Lmin],
>> linestyles=['-','-','-','-'], colors=['blue','blue','blue'], alpha=0.25)
>>
>> The problem now is, that with matplotlib 1.4, the resulting image shows
>> the most dense area in white instead of darker blue, which was the
>> previous behavior. I did not change my code, I can get the old result
>> simply by moving back to 1.4.
>> I link to the two pictures I created, so maybe one of you can explain
>> me, what changed with the version so that the plot is getting messed up.
>> V1.4:
>> http://i.imgur.com/HH7jKBE.png
>> V1.5:
>> http://i.imgur.com/0LZ9Tso.png
>>
>> Thank you very much,
>> Viktoria
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>
>