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.. _limitations:
Limitations
-----------
Duplicate contour points
^^^^^^^^^^^^^^^^^^^^^^^^
It is possible for calculated contour lines and filled polygons to contain identical adjacent
points. This can occur when contouring a z-level that exactly matches the value at a grid point.
As an example, consider calculating contour lines at ``z=0.1`` on a grid of a single quad with one
corner at ``z=0`` and the other three at ``z=1``:
>>> from contourpy import contour_generator
>>> cont_gen = contour_generator(z=[[0, 1], [1, 1]])
>>> cont_gen.lines(0.1)
[array([[0. , 0.1],
[0.1, 0. ]])]
The returned contour line has two points, from ``(0, 0.1)`` to ``(0.1, 0)``.
If you contour a z-value of ``z=0.01``:
>>> cont_gen.lines(0.1)
[array([[0. , 0.01],
[0.01, 0. ]])]
the contoured points are correspondingly closer to the ``z=0`` corner.
If the contoured z-level is ``0``, exactly matching the z-value of the corner:
>>> cont_gen.lines(0)
[array([[0., 0.],
[0., 0.]])]
then the returned contour line still has two points but they are identical, exactly the same as the
corner's ``(x, y)`` location.
Similar applies to filled contours so that it is possible for a filled contour polygon to have
multiple duplicate points and zero area.
Depending on what you are doing with the calculated contours, this may or may not be a problem.
If you are using them with `Shapely`_ for example, they might be considered invalid; see
:ref:`shapely_invalid` for how to deal with this.
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