svgpathtools

svgpathtools is a collection of tools for manipulating and analyzing SVG Path objects and Bézier curves.

Features

svgpathtools contains functions designed to easily read, write and display SVG files as well as a large selection of geometrically-oriented tools to transform and analyze path elements.

Additionally, the submodule bezier.py contains tools for for working with general nth order Bezier curves stored as n-tuples.

Some included tools:

• read, write, and display SVG files containing Path (and other) SVG elements
• convert Bézier path segments to numpy.poly1d (polynomial) objects
• convert polynomials (in standard form) to their Bézier form
• compute tangent vectors and (right-hand rule) normal vectors
• compute curvature
• break discontinuous paths into their continuous subpaths.
• efficiently compute intersections between paths and/or segments
• find a bounding box for a path or segment
• reverse segment/path orientation
• crop and split paths and segments
• smooth paths (i.e. smooth away kinks to make paths differentiable)
• transition maps from path domain to segment domain and back (T2t and t2T)
• compute area enclosed by a closed path
• compute arc length
• compute inverse arc length
• convert RGB color tuples to hexadecimal color strings and back

Note on Python 3

While I am hopeful that this package entirely works with Python 3, it was born from a larger project coded in Python 2 and has not been thoroughly tested in Python 3. Please let me know if you find any incompatibilities.

Prerequisites

• numpy
• svgwrite

Setup

If not already installed, you can install the prerequisites using pip.

$ pip install numpy

$ pip install svgwrite

Then install svgpathtools:

$ pip install svgpathtools

Alternative Setup

You can download the source from Github and install by using the command (from inside the folder containing setup.py):

$ python setup.py install

Credit where credit's due

Much of the core of this module was taken from the svg.path (v2.0) module. Interested svg.path users should see the compatibility notes at bottom of this readme.

Also, a big thanks to the author(s) of A Primer on Bézier Curves, an outstanding resource for learning about Bézier curves and Bézier curve-related algorithms.

Basic Usage

Classes

The svgpathtools module is primarily structured around four path segment classes: Line, QuadraticBezier, CubicBezier, and Arc. There is also a fifth class, Path, whose objects are sequences of (connected or disconnected1) path segment objects.

• Line(start, end)
• Arc(start, radius, rotation, large_arc, sweep, end) Note: See docstring for a detailed explanation of these parameters
• QuadraticBezier(start, control, end)
• CubicBezier(start, control1, control2, end)
• Path(*segments)

See the relevant docstrings in path.py or the official SVG specifications for more information on what each parameter means.

1 Warning: Some of the functionality in this library has not been tested on discontinuous Path objects. A simple workaround is provided, however, by the Path.continuous_subpaths() method. ↩

from __future__ import division, print_function

# Coordinates are given as points in the complex plane
from svgpathtools import Path, Line, QuadraticBezier, CubicBezier, Arc
seg1 = CubicBezier(300+100j, 100+100j, 200+200j, 200+300j)  # A cubic beginning at (300, 100) and ending at (200, 300)
seg2 = Line(200+300j, 250+350j)  # A line beginning at (200, 300) and ending at (250, 350)
path = Path(seg1, seg2)  # A path traversing the cubic and then the line

# We could alternatively created this Path object using a d-string
from svgpathtools import parse_path
path_alt = parse_path('M 300 100 C 100 100 200 200 200 300 L 250 350')

# Let's check that these two methods are equivalent
print(path)
print(path_alt)
print(path == path_alt)

# On a related note, the Path.d() method returns a Path object's d-string
print(path.d())
print(parse_path(path.d()) == path)

Path(CubicBezier(start=(300+100j), control1=(100+100j), control2=(200+200j), end=(200+300j)),
     Line(start=(200+300j), end=(250+350j)))
Path(CubicBezier(start=(300+100j), control1=(100+100j), control2=(200+200j), end=(200+300j)),
     Line(start=(200+300j), end=(250+350j)))
True
M 300.0,100.0 C 100.0,100.0 200.0,200.0 200.0,300.0 L 250.0,350.0
True

The Path class is a mutable sequence, so it behaves much like a list. So segments can appended, inserted, set by index, deleted, enumerated, sliced out, etc.

# Let's append another to the end of it
path.append(CubicBezier(250+350j, 275+350j, 250+225j, 200+100j))
print(path)

# Let's replace the first segment with a Line object
path[0] = Line(200+100j, 200+300j)
print(path)

# You may have noticed that this path is connected and now is also closed (i.e. path.start == path.end)
print("path is continuous? ", path.iscontinuous())
print("path is closed? ", path.isclosed())

# The curve the path follows is not, however, smooth (differentiable)
from svgpathtools import kinks, smoothed_path
print("path contains non-differentiable points? ", len(kinks(path)) > 0)

# If we want, we can smooth these out (Experimental and only for line/cubic paths)
# Note:  smoothing will always works (except on 180 degree turns), but you may want
# to play with the maxjointsize and tightness parameters to get pleasing results
# Note also: smoothing will increase the number of segments in a path
spath = smoothed_path(path)
print("spath contains non-differentiable points? ", len(kinks(spath)) > 0)
print(spath)

# Let's take a quick look at the path and its smoothed relative
# The following commands will open two browser windows to display path and spaths
from svgpathtools import disvg
from time import sleep
disvg(path)
sleep(1)  # needed when not giving the SVGs unique names (or not using timestamp)
disvg(spath)
print("Notice that path contains {} segments and spath contains {} segments."
      "".format(len(path), len(spath)))

Path(CubicBezier(start=(300+100j), control1=(100+100j), control2=(200+200j), end=(200+300j)),
     Line(start=(200+300j), end=(250+350j)),
     CubicBezier(start=(250+350j), control1=(275+350j), control2=(250+225j), end=(200+100j)))
Path(Line(start=(200+100j), end=(200+300j)),
     Line(start=(200+300j), end=(250+350j)),
     CubicBezier(start=(250+350j), control1=(275+350j), control2=(250+225j), end=(200+100j)))
path is continuous?  True
path is closed?  True
path contains non-differentiable points?  True
spath contains non-differentiable points?  False
Path(Line(start=(200+101.5j), end=(200+298.5j)),
     CubicBezier(start=(200+298.5j), control1=(200+298.505j), control2=(201.057124638+301.057124638j), end=(201.060660172+301.060660172j)),
     Line(start=(201.060660172+301.060660172j), end=(248.939339828+348.939339828j)),
     CubicBezier(start=(248.939339828+348.939339828j), control1=(249.649982143+349.649982143j), control2=(248.995+350j), end=(250+350j)),
     CubicBezier(start=(250+350j), control1=(275+350j), control2=(250+225j), end=(200+100j)),
     CubicBezier(start=(200+100j), control1=(199.62675237+99.0668809257j), control2=(200+100.495j), end=(200+101.5j)))
Notice that path contains 3 segments and spath contains 6 segments.

Reading SVGSs

The svg2paths() function converts an svgfile to a list of Path objects and a separate list of dictionaries containing the attributes of each said path.

Note: Line, Polyline, Polygon, and Path SVG elements can all be converted to Path objects using this function.

# Read SVG into a list of path objects and list of dictionaries of attributes
from svgpathtools import svg2paths, wsvg
paths, attributes = svg2paths('test.svg')

# Update: You can now also extract the svg-attributes by setting
# return_svg_attributes=True, or with the convenience function svg2paths2
from svgpathtools import svg2paths2
paths, attributes, svg_attributes = svg2paths2('test.svg')

# Let's print out the first path object and the color it was in the SVG
# We'll see it is composed of two CubicBezier objects and, in the SVG file it
# came from, it was red
redpath = paths[0]
redpath_attribs = attributes[0]
print(redpath)
print(redpath_attribs['stroke'])

Path(CubicBezier(start=(10.5+80j), control1=(40+10j), control2=(65+10j), end=(95+80j)),
     CubicBezier(start=(95+80j), control1=(125+150j), control2=(150+150j), end=(180+80j)))
red

Writing SVGSs (and some geometric functions and methods)

The wsvg() function creates an SVG file from a list of path. This function can do many things (see docstring in paths2svg.py for more information) and is meant to be quick and easy to use. Note: Use the convenience function disvg() (or set 'openinbrowser=True') to automatically attempt to open the created svg file in your default SVG viewer.

# Let's make a new SVG that's identical to the first
wsvg(paths, attributes=attributes, svg_attributes=svg_attributes, filename='output1.svg')

output1.svg

There will be many more examples of writing and displaying path data below.

The .point() method and transitioning between path and path segment parameterizations

SVG Path elements and their segments have official parameterizations. These parameterizations can be accessed using the Path.point(), Line.point(), QuadraticBezier.point(), CubicBezier.point(), and Arc.point() methods. All these parameterizations are defined over the domain 0 <= t <= 1.

Note: In this document and in inline documentation and doctrings, I use a capital T when referring to the parameterization of a Path object and a lower case t when referring speaking about path segment objects (i.e. Line, QaudraticBezier, CubicBezier, and Arc objects).

Given a T value, the Path.T2t() method can be used to find the corresponding segment index, k, and segment parameter, t, such that path.point(T)=path[k].point(t).

There is also a Path.t2T() method to solve the inverse problem.

# Example:

# Let's check that the first segment of redpath starts
# at the same point as redpath
firstseg = redpath[0]
print(redpath.point(0) == firstseg.point(0) == redpath.start == firstseg.start)

# Let's check that the last segment of redpath ends on the same point as redpath
lastseg = redpath[-1]
print(redpath.point(1) == lastseg.point(1) == redpath.end == lastseg.end)

# This next boolean should return False as redpath is composed multiple segments
print(redpath.point(0.5) == firstseg.point(0.5))

# If we want to figure out which segment of redpoint the
# point redpath.point(0.5) lands on, we can use the path.T2t() method
k, t = redpath.T2t(0.5)
print(redpath[k].point(t) == redpath.point(0.5))

True
True
False
True

Tangent vectors and Bezier curves as numpy polynomial objects

Another great way to work with the parameterizations for Line, QuadraticBezier, and CubicBezier objects is to convert them to numpy.poly1d objects. This is done easily using the Line.poly(), QuadraticBezier.poly() and CubicBezier.poly() methods.

There's also a polynomial2bezier() function in the pathtools.py submodule to convert polynomials back to Bezier curves.

Note: cubic Bezier curves are parameterized as

\mathcal{B}(t) = P_0(1-t)^3 + 3P_1(1-t)^2t + 3P_2(1-t)t^2 + P_3t^3

where P_0, P_1, P_2, and P_3 are the control points start, control1, control2, and end, respectively, that svgpathtools uses to define a CubicBezier object. The CubicBezier.poly() method expands this polynomial to its standard form

\mathcal{B}(t) = c_0t^3 + c_1t^2 +c_2t+c3

where

\begin{bmatrix}c_0\\c_1\\c_2\\c_3\end{bmatrix} =
\begin{bmatrix}
-1 & 3 & -3 & 1\\
3 & -6 & -3 & 0\\
-3 & 3 & 0 & 0\\
1 & 0 & 0 & 0\\
\end{bmatrix}
\begin{bmatrix}P_0\\P_1\\P_2\\P_3\end{bmatrix}

QuadraticBezier.poly() and Line.poly() are defined similarly.

# Example:
b = CubicBezier(300+100j, 100+100j, 200+200j, 200+300j)
p = b.poly()

# p(t) == b.point(t)
print(p(0.235) == b.point(0.235))

# What is p(t)?  It's just the cubic b written in standard form.
bpretty = "{}*(1-t)^3 + 3*{}*(1-t)^2*t + 3*{}*(1-t)*t^2 + {}*t^3".format(*b.bpoints())
print("The CubicBezier, b.point(x) = \n\n" +
      bpretty + "\n\n" +
      "can be rewritten in standard form as \n\n" +
      str(p).replace('x','t'))

True
The CubicBezier, b.point(x) =

(300+100j)*(1-t)^3 + 3*(100+100j)*(1-t)^2*t + 3*(200+200j)*(1-t)*t^2 + (200+300j)*t^3

can be rewritten in standard form as

                3                2
(-400 + -100j) t + (900 + 300j) t - 600 t + (300 + 100j)

To illustrate the awesomeness of being able to convert our Bezier curve objects to numpy.poly1d objects and back, lets compute the unit tangent vector of the above CubicBezier object, b, at t=0.5 in four different ways.

Tangent vectors (and more on polynomials)

t = 0.5
### Method 1: the easy way
u1 = b.unit_tangent(t)

### Method 2: another easy way
# Note: This way will fail if it encounters a removable singularity.
u2 = b.derivative(t)/abs(b.derivative(t))

### Method 2: a third easy way
# Note: This way will also fail if it encounters a removable singularity.
dp = p.deriv()
u3 = dp(t)/abs(dp(t))

### Method 4: the removable-singularity-proof numpy.poly1d way
# Note: This is roughly how Method 1 works
from svgpathtools import real, imag, rational_limit
dx, dy = real(dp), imag(dp)  # dp == dx + 1j*dy
p_mag2 = dx**2 + dy**2  # p_mag2(t) = |p(t)|**2
# Note: abs(dp) isn't a polynomial, but abs(dp)**2 is, and,
#  the limit_{t->t0}[f(t) / abs(f(t))] ==
# sqrt(limit_{t->t0}[f(t)**2 / abs(f(t))**2])
from cmath import sqrt
u4 = sqrt(rational_limit(dp**2, p_mag2, t))

print("unit tangent check:", u1 == u2 == u3 == u4)

# Let's do a visual check
mag = b.length()/4  # so it's not hard to see the tangent line
tangent_line = Line(b.point(t), b.point(t) + mag*u1)
disvg([b, tangent_line], 'bg', nodes=[b.point(t)])

unit tangent check: True

Translations (shifts), reversing orientation, and normal vectors

# Speaking of tangents, let's add a normal vector to the picture
n = b.normal(t)
normal_line = Line(b.point(t), b.point(t) + mag*n)
disvg([b, tangent_line, normal_line], 'bgp', nodes=[b.point(t)])

# and let's reverse the orientation of b!
# the tangent and normal lines should be sent to their opposites
br = b.reversed()

# Let's also shift b_r over a bit to the right so we can view it next to b
# The simplest way to do this is br = br.translated(3*mag),  but let's use
# the .bpoints() instead, which returns a Bezier's control points
br.start, br.control1, br.control2, br.end = [3*mag + bpt for bpt in br.bpoints()]  #

tangent_line_r = Line(br.point(t), br.point(t) + mag*br.unit_tangent(t))
normal_line_r = Line(br.point(t), br.point(t) + mag*br.normal(t))
wsvg([b, tangent_line, normal_line, br, tangent_line_r, normal_line_r],
     'bgpkgp', nodes=[b.point(t), br.point(t)], filename='vectorframes.svg',
     text=["b's tangent", "br's tangent"], text_path=[tangent_line, tangent_line_r])

vectorframes.svg

Rotations and Translations

# Let's take a Line and an Arc and make some pictures
top_half = Arc(start=-1, radius=1+2j, rotation=0, large_arc=1, sweep=1, end=1)
midline = Line(-1.5, 1.5)

# First let's make our ellipse whole
bottom_half = top_half.rotated(180)
decorated_ellipse = Path(top_half, bottom_half)

# Now let's add the decorations
for k in range(12):
    decorated_ellipse.append(midline.rotated(30*k))

# Let's move it over so we can see the original Line and Arc object next
# to the final product
decorated_ellipse = decorated_ellipse.translated(4+0j)
wsvg([top_half, midline, decorated_ellipse], filename='decorated_ellipse.svg')

decorated_ellipse.svg

arc length and inverse arc length

Here we'll create an SVG that shows off the parametric and geometric midpoints of the paths from test.svg. We'll need to compute use the Path.length(), Line.length(), QuadraticBezier.length(), CubicBezier.length(), and Arc.length() methods, as well as the related inverse arc length methods .ilength() function to do this.

# First we'll load the path data from the file test.svg
paths, attributes = svg2paths('test.svg')

# Let's mark the parametric midpoint of each segment
# I say "parametric" midpoint because Bezier curves aren't
# parameterized by arclength
# If they're also the geometric midpoint, let's mark them
# purple and otherwise we'll mark the geometric midpoint green
min_depth = 5
error = 1e-4
dots = []
ncols = []
nradii = []
for path in paths:
    for seg in path:
        parametric_mid = seg.point(0.5)
        seg_length = seg.length()
        if seg.length(0.5)/seg.length() == 1/2:
            dots += [parametric_mid]
            ncols += ['purple']
            nradii += [5]
        else:
            t_mid = seg.ilength(seg_length/2)
            geo_mid = seg.point(t_mid)
            dots += [parametric_mid, geo_mid]
            ncols += ['red', 'green']
            nradii += [5] * 2

# In 'output2.svg' the paths will retain their original attributes
wsvg(paths, nodes=dots, node_colors=ncols, node_radii=nradii,
     attributes=attributes, filename='output2.svg')

output2.svg

Intersections between Bezier curves

# Let's find all intersections between redpath and the other
redpath = paths[0]
redpath_attribs = attributes[0]
intersections = []
for path in paths[1:]:
    for (T1, seg1, t1), (T2, seg2, t2) in redpath.intersect(path):
        intersections.append(redpath.point(T1))

disvg(paths, filename='output_intersections.svg', attributes=attributes,
      nodes = intersections, node_radii = [5]*len(intersections))

output_intersections.svg

An Advanced Application: Offsetting Paths

Here we'll find the offset curve for a few paths.

from svgpathtools import parse_path, Line, Path, wsvg
def offset_curve(path, offset_distance, steps=1000):
    """Takes in a Path object, `path`, and a distance,
    `offset_distance`, and outputs an piecewise-linear approximation
    of the 'parallel' offset curve."""
    nls = []
    for seg in path:
        ct = 1
        for k in range(steps):
            t = k / steps
            offset_vector = offset_distance * seg.normal(t)
            nl = Line(seg.point(t), seg.point(t) + offset_vector)
            nls.append(nl)
    connect_the_dots = [Line(nls[k].end, nls[k+1].end) for k in range(len(nls)-1)]
    if path.isclosed():
        connect_the_dots.append(Line(nls[-1].end, nls[0].end))
    offset_path = Path(*connect_the_dots)
    return offset_path

# Examples:
path1 = parse_path("m 288,600 c -52,-28 -42,-61 0,-97 ")
path2 = parse_path("M 151,395 C 407,485 726.17662,160 634,339").translated(300)
path3 = parse_path("m 117,695 c 237,-7 -103,-146 457,0").translated(500+400j)
paths = [path1, path2, path3]

offset_distances = [10*k for k in range(1,51)]
offset_paths = []
for path in paths:
    for distances in offset_distances:
        offset_paths.append(offset_curve(path, distances))

# Note: This will take a few moments
wsvg(paths + offset_paths, 'g'*len(paths) + 'r'*len(offset_paths), filename='offset_curves.svg')

offset_curves.svg

Compatibility Notes for users of svg.path (v2.0)

• renamed Arc.arc attribute as Arc.large_arc
• Path.d() : For behavior similar2 to svg.path (v2.0), set both useSandT and use_closed_attrib to be True.

2 The behavior would be identical, but the string formatting used in this method has been changed to use default format (instead of the General format, {:G}), for inceased precision. ↩

Licence

This module is under a MIT License.