(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`:

```python
>>> 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`:

```python
>>> 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:

```python
>>> 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.