path_tutorial.rst

.. _path_tutorial:

Path Tutorial

The object underlying all of the :mod:`matplotlib.patch` objects is

the :class:`~matplotlib.path.Path`, which supports the standard set of

moveto, lineto, curveto commands to draw simple and compoud outlines

consisting of line segments and splines. The ``Path`` is instantiated

with a (N,2) array of (x,y) vertices, and a N length array of path

codes. For example to draw the unit rectangle from (0,0) to (1,1), we

could use this code

.. plot::

:include-source:

import matplotlib.pyplot as plt

from matplotlib.path import Path

import matplotlib.patches as patches

verts = [

(0., 0.), # left, bottom

(0., 1.), # left, top

(1., 1.), # right, top

(1., 0.), # right, bottom

(0., 0.), # ignored

]

codes = [Path.MOVETO,

Path.LINETO,

Path.LINETO,

Path.LINETO,

Path.CLOSEPOLY,

]

path = Path(verts, codes)

fig = plt.figure()

ax = fig.add_subplot(111)

patch = patches.PathPatch(path, facecolor='orange', lw=2)

ax.add_patch(patch)

ax.set_xlim(-2,2)

ax.set_ylim(-2,2)

plt.show()

The following path codes are recognized

Code Vertices Description

``STOP`` 1 (ignored) A marker for the end of the entire path (currently not required and ignored)

``MOVETO`` 1 Pick up the pen and move to the given vertex.

``LINETO`` 1 Draw a line from the current position to the given vertex.

``CURVE3`` 2 (1 control point, 1 endpoint) Draw a quadratic Bezier curve from the current position, with the given control point, to the given end point.

``CURVE4`` 3 (2 control points, 1 endpoint) Draw a cubic Bezier curve from the current position, with the given control points, to the given end point.

``CLOSEPOLY`` 1 (ignored) Draw a line segment to the start point of the current polyline.

Bezier example

Some of the path components require multiple vertices to specify them:

for example CURVE 3 is a `bezier

<http://en.wikipedia.org/wiki/B%C3%A9zier_curve>`_ curve with one

control point and one end point, and CURVE4 has three vertices for the

two control points and the end point. The example below shows a

CURVE4 Bezier spline -- the bezier curve will be contained in the

convex hul of the start point, the two control points, and the end

point

.. plot::

:include-source:

import matplotlib.pyplot as plt

from matplotlib.path import Path

import matplotlib.patches as patches

verts = [

(0., 0.), # P0

(0.2, 1.), # P1

(1., 0.8), # P2

(0.8, 0.), # P3

]

codes = [Path.MOVETO,

Path.CURVE4,

Path.CURVE4,

Path.CURVE4,

]

path = Path(verts, codes)

fig = plt.figure()

ax = fig.add_subplot(111)

patch = patches.PathPatch(path, facecolor='none', lw=2)

ax.add_patch(patch)

xs, ys = zip(*verts)

ax.text(-0.05, -0.05, 'P0')

ax.text(0.15, 1.05, 'P1')

ax.text(1.05, 0.85, 'P2')

ax.text(0.85, -0.05, 'P3')

ax.set_xlim(-0.1, 1.1)

ax.set_ylim(-0.1, 1.1)

plt.show()

Compound paths

All of the simple patch primitives in matplotlib, Rectangle, Circle,

Polygon, etc, are implemented with simple path. Plotting functions

like :meth:`~matplotlib.axes.Axes.hist` and

:meth:`~matplotlib.axes.Axes.bar`, which create a number of

primitives, eg a bunch of Rectangles, can usually be implemented more

efficiently using a compund path. The reason ``bar`` creates a list

of rectangles and not a compound path is largely historical: the

:class:`~matplotlib.path.Path` code is comparatively new and ``bar``

predates it. While we could change it now, it would break old code,

so here we will cover how to create compound paths, replacing the

functionality in bar, in case you need to do so in your own code for

efficiency reasons, eg you are creating an animated bar plot.

We will make the histogram chart by creating a series of rectangles

for each histogram bar: the rectangle width is the bin width and the

rectangle height is the number of datapoints in that bin. First we'll

create some random normally distributed data and compute the

histogram. Because numpy returns the bin edges and not centers, the

length of ``bins`` is 1 greater than the length of ``n`` in the

example below::

We'll now extract the corners of the rectangles. Each of the

``left``, ``bottom``, etc, arrays below is ``len(n)``, where ``n`` is

the array of counts for each histogram bar::

Now we have to construct our compound path, which will consist of a

series of ``MOVETO``, ``LINETO`` and ``CLOSEPOLY`` for each rectangle.

For each rectangle, we need 5 vertices: 1 for the ``MOVETO``, 3 for

the ``LINETO``, and 1 for the ``CLOSEPOLY``. As indicated in the

table above, the vertex for the closepoly is ignored but we still need

it to keep the codes aligned with the vertices::

All that remains is to create the path, attach it to a

:class:`~matplotlib.patch.PathPatch`, and ad it to our axes::

Here is the result

.. plot:: pyplots/compound_path_demo.py