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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

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<head>

<title>Path Tutorial — Matplotlib 1.3.1 documentation</title>

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<h3><a href="../contents.html">Table Of Contents</a></h3>

<ul>

<li><a class="reference internal" href="#">Path Tutorial</a><ul>

<li><a class="reference internal" href="#bezier-example">Bézier example</a></li>

<li><a class="reference internal" href="#compound-paths">Compound paths</a></li>

</ul>

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<h1>Path Tutorial<a class="headerlink" href="#path-tutorial" title="Permalink to this headline">¶</a></h1>

The object underlying all of the <tt class="xref py py-mod docutils literal">matplotlib.patch</tt> objects is

the <a class="reference internal" href="../api/path_api.html#matplotlib.path.Path" title="matplotlib.path.Path"><tt class="xref py py-class docutils literal">Path</tt></a>, which supports the standard set of

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

consisting of line segments and splines. The <tt class="docutils literal">Path</tt> 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

<div class="highlight-python"><div class="highlight"><pre>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.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()

</pre></div>

</div>

(<a class="reference external" href="../users/path_tutorial-1.py">Source code</a>, <a class="reference external" href="../users/path_tutorial-1.png">png</a>, <a class="reference external" href="../users/path_tutorial-1.hires.png">hires.png</a>, <a class="reference external" href="../users/path_tutorial-1.pdf">pdf</a>)

</div>

The following path codes are recognized

</colgroup>

<th class="head">Vertices</th>

<th class="head">Description</th>

</tr>

</thead>

<td>1 (ignored)</td>

<td>A marker for the end of the entire path (currently not required and ignored)</td>

</tr>

<tr class="row-odd"><td><tt class="docutils literal">MOVETO</tt></td>

<td>Pick up the pen and move to the given vertex.</td>

</tr>

<tr class="row-even"><td><tt class="docutils literal">LINETO</tt></td>

<td>Draw a line from the current position to the given vertex.</td>

</tr>

<tr class="row-odd"><td><tt class="docutils literal">CURVE3</tt></td>

<td>2 (1 control point, 1 endpoint)</td>

<td>Draw a quadratic Bézier curve from the current position, with the given control point, to the given end point.</td>

</tr>

<tr class="row-even"><td><tt class="docutils literal">CURVE4</tt></td>

<td>3 (2 control points, 1 endpoint)</td>

<td>Draw a cubic Bézier curve from the current position, with the given control points, to the given end point.</td>

</tr>

<tr class="row-odd"><td><tt class="docutils literal">CLOSEPOLY</tt></td>

<td>1 (point itself is ignored)</td>

<td>Draw a line segment to the start point of the current polyline.</td>

</tr>

</tbody>

</table>

<h2>Bézier example<a class="headerlink" href="#bezier-example" title="Permalink to this headline">¶</a></h2>

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

for example CURVE 3 is a <a class="reference external" href="http://en.wikipedia.org/wiki/B%C3%A9zier_curve">bézier</a> 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 Bézier spline – the bézier curve will be contained in the

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

point

<div class="highlight-python"><div class="highlight"><pre>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,

]

fig = plt.figure()

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

ax.add_patch(patch)

xs, ys = zip(*verts)

ax.plot(xs, ys, 'x--', lw=2, color='black', ms=10)

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()

</pre></div>

</div>

(<a class="reference external" href="../users/path_tutorial-2.py">Source code</a>, <a class="reference external" href="../users/path_tutorial-2.png">png</a>, <a class="reference external" href="../users/path_tutorial-2.hires.png">hires.png</a>, <a class="reference external" href="../users/path_tutorial-2.pdf">pdf</a>)

</div>

<h2>Compound paths<a class="headerlink" href="#compound-paths" title="Permalink to this headline">¶</a></h2>

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

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

like <a class="reference internal" href="../api/axes_api.html#matplotlib.axes.Axes.hist" title="matplotlib.axes.Axes.hist"><tt class="xref py py-meth docutils literal">hist()</tt></a> and

<a class="reference internal" href="../api/axes_api.html#matplotlib.axes.Axes.bar" title="matplotlib.axes.Axes.bar"><tt class="xref py py-meth docutils literal">bar()</tt></a>, which create a number of

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

efficiently using a compound path. The reason <tt class="docutils literal">bar</tt> creates a list

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

<a class="reference internal" href="../api/path_api.html#matplotlib.path.Path" title="matplotlib.path.Path"><tt class="xref py py-class docutils literal">Path</tt></a> code is comparatively new and <tt class="docutils literal">bar</tt>

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 <tt class="docutils literal">bins</tt> is 1 greater than the length of <tt class="docutils literal">n</tt> in the

example below:

<div class="highlight-python"><div class="highlight"><pre># histogram our data with numpy

data = np.random.randn(1000)

n, bins = np.histogram(data, 100)

</pre></div>

</div>

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

<tt class="docutils literal">left</tt>, <tt class="docutils literal">bottom</tt>, etc, arrays below is <tt class="docutils literal">len(n)</tt>, where <tt class="docutils literal">n</tt> is

the array of counts for each histogram bar:

<div class="highlight-python"><div class="highlight"><pre># get the corners of the rectangles for the histogram

left = np.array(bins[:-1])

right = np.array(bins[1:])

bottom = np.zeros(len(left))

top = bottom + n

</pre></div>

</div>

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

series of <tt class="docutils literal">MOVETO</tt>, <tt class="docutils literal">LINETO</tt> and <tt class="docutils literal">CLOSEPOLY</tt> for each rectangle.

For each rectangle, we need 5 vertices: 1 for the <tt class="docutils literal">MOVETO</tt>, 3 for

the <tt class="docutils literal">LINETO</tt>, and 1 for the <tt class="docutils literal">CLOSEPOLY</tt>. 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:

<div class="highlight-python"><div class="highlight"><pre>nverts = nrects*(1+3+1)

verts = np.zeros((nverts, 2))

codes = np.ones(nverts, int) * path.Path.LINETO

codes[0::5] = path.Path.MOVETO

codes[4::5] = path.Path.CLOSEPOLY

verts[0::5,0] = left

verts[0::5,1] = bottom

verts[1::5,0] = left

verts[1::5,1] = top

verts[2::5,0] = right

verts[2::5,1] = top

verts[3::5,0] = right

verts[3::5,1] = bottom

</pre></div>

</div>

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

<tt class="xref py py-class docutils literal">PathPatch</tt>, and add it to our axes:

<div class="highlight-python"><div class="highlight"><pre>barpath = path.Path(verts, codes)

patch = patches.PathPatch(barpath, facecolor='green',

edgecolor='yellow', alpha=0.5)

ax.add_patch(patch)

</pre></div>

</div>

Here is the result

(<a class="reference external" href="../pyplots/compound_path_demo.py">Source code</a>, <a class="reference external" href="../pyplots/compound_path_demo.png">png</a>, <a class="reference external" href="../pyplots/compound_path_demo.hires.png">hires.png</a>, <a class="reference external" href="../pyplots/compound_path_demo.pdf">pdf</a>)

</div>

</div>

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>index</a></li>

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>previous</a> |</li>

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Last updated on Oct 10, 2013.

Created using <a href="http://sphinx-doc.org/">Sphinx</a> 1.2b2.

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