Path effects guide

Defining paths that objects follow on a canvas.

Matplotlib’s patheffects module provides functionality to
apply a multiple draw stage to any Artist which can be rendered via a
Path.

Artists which can have a path effect applied to them include Patch,
Line2D, Collection and even
Text. Each artist’s path effects can be controlled via the
set_path_effects method (set_path_effects), which takes
an iterable of AbstractPathEffect instances.

The simplest path effect is the Normal effect, which simply
draws the artist without any effect:

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import matplotlib.pyplot as plt
import matplotlib.patheffects as path_effects

fig = plt.figure(figsize=(5, 1.5))
text = fig.text(0.5, 0.5, 'Hello path effects world!\nThis is the normal '
                          'path effect.\nPretty dull, huh?',
                ha='center', va='center', size=20)
text.set_path_effects([path_effects.Normal()])
plt.show()

sphx_glr_patheffects_guide_001

Whilst the plot doesn’t look any different to what you would expect without any path
effects, the drawing of the text now been changed to use the path effects
framework, opening up the possibilities for more interesting examples.

Adding a shadow

A far more interesting path effect than Normal is the
drop-shadow, which we can apply to any of our path based artists. The classes
SimplePatchShadow and
SimpleLineShadow do precisely this by drawing either a filled
patch or a line patch below the original artist:

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import matplotlib.patheffects as path_effects

text = plt.text(0.5, 0.5, 'Hello path effects world!',
                path_effects=[path_effects.withSimplePatchShadow()])

plt.plot([0, 3, 2, 5], linewidth=5, color='blue',
         path_effects=[path_effects.SimpleLineShadow(),
                       path_effects.Normal()])
plt.show()

sphx_glr_patheffects_guide_002

Notice the two approaches to setting the path effects in this example. The
first uses the with* classes to include the desired functionality automatically
followed with the “normal” effect, whereas the latter explicitly defines the two path
effects to draw.

Making an artist stand out

One nice way of making artists visually stand out is to draw an outline in a bold
color below the actual artist. The Stroke path effect
makes this a relatively simple task:

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fig = plt.figure(figsize=(7, 1))
text = fig.text(0.5, 0.5, 'This text stands out because of\n'
                          'its black border.', color='white',
                          ha='center', va='center', size=30)
text.set_path_effects([path_effects.Stroke(linewidth=3, foreground='black'),
                       path_effects.Normal()])
plt.show()

sphx_glr_patheffects_guide_003

It is important to note that this effect only works because we have drawn the text
path twice; once with a thick black line, and then once with the original text
path on top.

You may have noticed that the keywords to Stroke and
SimplePatchShadow and SimpleLineShadow are not the usual Artist
keywords (such as facecolor and edgecolor etc.). This is because with these
path effects we are operating at lower level of matplotlib. In fact, the keywords
which are accepted are those for a matplotlib.backend_bases.GraphicsContextBase
instance, which have been designed for making it easy to create new backends - and not
for its user interface.

Greater control of the path effect artist

As already mentioned, some of the path effects operate at a lower level than most users
will be used to, meaning that setting keywords such as facecolor and edgecolor
raise an AttributeError. Luckily there is a generic PathPatchEffect path effect
which creates a PathPatch class with the original path.
The keywords to this effect are identical to those of PathPatch:

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fig = plt.figure(figsize=(8, 1))
t = fig.text(0.02, 0.5, 'Hatch shadow', fontsize=75, weight=1000, va='center')
t.set_path_effects([path_effects.PathPatchEffect(offset=(4, -4), hatch='xxxx',
                                                 facecolor='gray'),
                    path_effects.PathPatchEffect(edgecolor='white', linewidth=1.1,
                                                 facecolor='black')])
plt.show()

sphx_glr_patheffects_guide_004

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