How to quickly plot polygons in Matplotlib

Problem statement:

You need to plot a large collection of polygons in Matplotlib.

Solution:

A simple way to plot filled polygons in Matplotlib is to use the fill function. If you try to plot a collection of  polygons in Matplotlib using sequential calls to fill, it can take a lot of time to generate the graph. There are two ways to speed up the plotting. The first is to use Python’s extended call syntax and pass multiple polygons at one time. The other is to create a single list of points which are the corners of the polygon and separate each polygon by a ‘None’ entry. The extended call method yields an appreciable improvement in performance. Forming a single list of polygon corners separated by ‘None’ results in a significant time savings. For the test cases below, using extended call syntax decreased drawing time by about 50% and using a single list separated by ‘None’ decreased drawing time by about 99%.

Execution time for sequential plotting = 2.496564 sec
Execution time for extended call plotting = 1.172675 sec  
Execution time when using None = = 0.028234 sec

Sample Code:

'''
Code snippet to plot multiple triangle's in Matplotlib using different methods.
'''
import time
import sys
import matplotlib.pyplot as pp
import random

if sys.platform == "win32":
     # On Windows, the best timer is time.clock()
     default_timer = time.clock 
else:     
    # On most other platforms the best timer is time.time()     
    default_timer = time.time
    
    
# generate ends for the triangle line segments
xtris = []
ytris = []
for i in range(1000):
    x1 = random.random()
    x2 = x1 + random.random()/5.0
    x3 = x1 + random.random()/5.0
    xtips = [x1,x2,x3]
    y1 = random.random()
    y2 = y1 + random.random()/5.0
    y3 = y1 + random.random()/5.0
    ytips = [y1,y2,y3]
    xtris.append(xtips)
    ytris.append(ytips)

############################
# time sequential call to matplotlib
pp.figure()
pp.subplot(1,3,1)

t0 = default_timer()
for xtips,ytips in zip(xtris,ytris):
    pp.fill(xtips,ytips,
            facecolor='b',alpha=0.1, edgecolor='none')
t1 = default_timer()

pp.title('Sequential Plotting')

print 'Execution time for sequential plotting = %f sec' % (t1-t0)

# rebuild ends using none to separate polygons
xlist = []
ylist = []
for xtips,ytips in zip(xtris,ytris):
    xlist.extend(xtips)
    xlist.append(None)
    ylist.extend(ytips)
    ylist.append(None)

############################
# build argument list
call_list = []
for xtips,ytips in zip(xtris,ytris):
    call_list.append(xtips)
    call_list.append(ytips)
    call_list.append('-b')
    
############################
# time single call to matplotlib
pp.subplot(1,3,2)

t0 = default_timer()
pp.fill(*call_list,
            facecolor='b',alpha=0.1, edgecolor='none')

t1 = default_timer()

pp.title('Single Plot extended call')

print 'Execution time for extended call plotting = %f sec' % (t1-t0)

    
############################
# time single call to matplotlib
pp.subplot(1,3,3)

t0 = default_timer()
pp.fill(xlist,ylist,
        facecolor='b',alpha=0.1,edgecolor='none')
t1 = default_timer()

pp.title('Single Plot Using None')

print 'Execution time when using None = %f sec' % (t1-t0)

pp.show()

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Discussion:

   Sequential call and extended call syntax produce the same plot results. However, the single list of points using ‘None’ can generate a different image if transparency is used. In the sample code, ‘alpha=0.1’ was used. When sequential fill or extended call syntax is used, the color from each polygon is additive. When the polygon corners are combined into a single list of points, the polygon colors are not additive. This is only an issue when using transparency. If alpha is set to 1.0 (or not set at all), then all of the plots will be identical.

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Test Conditions:

• Win 7, 64 bit
• Intel i7
• PythonXY 2.5.6.2
  • Matplotlib 1.0.0

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Answers:

• How to speed up Matplotlib plots
• How to plot polygons in Matplotlib

This work is licensed under a Creative Commons Attribution By license.