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Apr 13, 2020
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DOCS: add examples of how one "should" use Bbox #17090

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82 changes: 76 additions & 6 deletions lib/matplotlib/transforms.py
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Original file line number Diff line number Diff line change
Expand Up @@ -682,6 +682,82 @@ def intersection(bbox1, bbox2):
class Bbox(BboxBase):
"""
A mutable bounding box.

Examples
--------
**Create from known bounds**

The default constructor takes the boundary "points" ``[[xmin, ymin],
[xmax, ymax]]``.

>>> Bbox([[1, 1], [3, 7]])
Bbox([[1.0, 1.0], [3.0, 7.0]])

Alternatively, a Bbox can be created from the flattened points array, the
so-called "extents" ``(xmin, ymin, xmax, ymax)``

>>> Bbox.from_extents(1, 1, 3, 7)
Bbox([[1.0, 1.0], [3.0, 7.0]])

or from the "bounds" ``(xmin, ymin, width, height)``.

>>> Bbox.from_bounds(1, 1, 2, 6)
Bbox([[1.0, 1.0], [3.0, 7.0]])

**Create from collections of points**

The "empty" object for accumulating Bboxs is the null bbox, which is a
stand-in for the empty set.

>>> Bbox.null()
Bbox([[inf, inf], [-inf, -inf]])

Adding points to the null bbox will give you the bbox of those points.

>>> box = Bbox.null()
>>> box.update_from_data_xy([[1, 1]])
>>> box
Bbox([[1.0, 1.0], [1.0, 1.0]])
>>> box.update_from_data_xy([[2, 3], [3, 2]], ignore=False)
>>> box
Bbox([[1.0, 1.0], [3.0, 3.0]])

Setting ``ignore=True`` is equivalent to starting over from a null bbox.

>>> box.update_from_data_xy([[1, 1]], ignore=True)
>>> box
Bbox([[1.0, 1.0], [1.0, 1.0]])

.. warning::

It is recommended to always specify ``ignore`` explicitly. If not, the
default value of ``ignore`` can be changed at any time by code with
access to your Bbox, for example using the method `~.Bbox.ignore`.

**Properties of the ``null`` bbox**

.. note::

The current behavior of `Bbox.null()` may be surprising as it does
not have all of the properties of the "empty set", and as such does
not behave like a "zero" object in the mathematical sense. We may
change that in the future (with a deprecation period).

The null bbox is the identity for intersections

>>> Bbox.intersection(Bbox([[1, 1], [3, 7]]), Bbox.null())
Bbox([[1.0, 1.0], [3.0, 7.0]])

except with itself, where it returns the full space.

>>> Bbox.intersection(Bbox.null(), Bbox.null())
Bbox([[-inf, -inf], [inf, inf]])

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Is this a reasonable behavior? Seems to contradict the semantics of update_from_data_xy.

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Yeah part of why I'm documenting this is because this behavior was very unexpected. I expected Bbox.null() to be the "zero" object of the intersection/union ring over the Bbox's.

But it's not technically a bug per se since it's not documented, so I didn't open an issue.

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@brunobeltran brunobeltran Apr 10, 2020 •
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That said, I'd be happy to make a separate PR to fix this "bug", if other people also agree that it shouldn't be this way, but don't want to get bogged down talking about deprecation timelines, etc. here, would rather open a separate issue.

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This and below clearly look like bugs, but I'm fine documenting these here and changing them later.

A union containing null will always return the full space (not the other
set!)

>>> Bbox.union([Bbox([[0, 0], [0, 0]]), Bbox.null()])
Bbox([[-inf, -inf], [inf, inf]])
"""

def __init__(self, points, **kwargs):
Expand All @@ -690,12 +766,6 @@ def __init__(self, points, **kwargs):
----------
points : ndarray
A 2x2 numpy array of the form ``[[x0, y0], [x1, y1]]``.

Notes
-----
If you need to create a `Bbox` object from another form
of data, consider the static methods :meth:`unit`,
:meth:`from_bounds` and :meth:`from_extents`.
"""
BboxBase.__init__(self, **kwargs)
points = np.asarray(points, float)
Expand Down