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 Invalid geometry for how to deal with this.