Source code for sympy.geometry.util
"""Utility functions for geometrical entities.
Contains
========
intersection
convex_hull
are_similar
"""
from __future__ import print_function, division
from sympy import Symbol, Function, solve
from sympy.core.compatibility import string_types, is_sequence
def idiff(eq, y, x, n=1):
"""Return ``dy/dx`` assuming that ``eq == 0``.
Parameters
==========
y : the dependent variable or a list of dependent variables (with y first)
x : the variable that the derivative is being taken with respect to
n : the order of the derivative (default is 1)
Examples
========
>>> from sympy.abc import x, y, a
>>> from sympy.geometry.util import idiff
>>> circ = x**2 + y**2 - 4
>>> idiff(circ, y, x)
-x/y
>>> idiff(circ, y, x, 2).simplify()
-(x**2 + y**2)/y**3
Here, ``a`` is assumed to be independent of ``x``:
>>> idiff(x + a + y, y, x)
-1
Now the x-dependence of ``a`` is made explicit by listing ``a`` after
``y`` in a list.
>>> idiff(x + a + y, [y, a], x)
-Derivative(a, x) - 1
See Also
========
sympy.core.function.Derivative: represents unevaluated derivatives
sympy.core.function.diff: explicitly differentiates wrt symbols
"""
if is_sequence(y):
dep = set(y)
y = y[0]
elif isinstance(y, Symbol):
dep = set([y])
else:
raise ValueError("expecting x-dependent symbol(s) but got: %s" % y)
f = dict([(s, Function(
s.name)(x)) for s in eq.atoms(Symbol) if s != x and s in dep])
dydx = Function(y.name)(x).diff(x)
eq = eq.subs(f)
derivs = {}
for i in range(n):
yp = solve(eq.diff(x), dydx)[0].subs(derivs)
if i == n - 1:
return yp.subs([(v, k) for k, v in f.items()])
derivs[dydx] = yp
eq = dydx - yp
dydx = dydx.diff(x)
def _symbol(s, matching_symbol=None):
"""Return s if s is a Symbol, else return either a new Symbol (real=True)
with the same name s or the matching_symbol if s is a string and it matches
the name of the matching_symbol.
>>> from sympy import Symbol
>>> from sympy.geometry.util import _symbol
>>> x = Symbol('x')
>>> _symbol('y')
y
>>> _.is_real
True
>>> _symbol(x)
x
>>> _.is_real is None
True
>>> arb = Symbol('foo')
>>> _symbol('arb', arb) # arb's name is foo so foo will not be returned
arb
>>> _symbol('foo', arb) # now it will
foo
NB: the symbol here may not be the same as a symbol with the same
name defined elsewhere as a result of different assumptions.
See Also
========
sympy.core.symbol.Symbol
"""
if isinstance(s, string_types):
if matching_symbol and matching_symbol.name == s:
return matching_symbol
return Symbol(s, real=True)
elif isinstance(s, Symbol):
return s
else:
raise ValueError('symbol must be string for symbol name or Symbol')
[docs]def intersection(*entities):
"""The intersection of a collection of GeometryEntity instances.
Parameters
==========
entities : sequence of GeometryEntity
Returns
=======
intersection : list of GeometryEntity
Raises
======
NotImplementedError
When unable to calculate intersection.
Notes
=====
The intersection of any geometrical entity with itself should return
a list with one item: the entity in question.
An intersection requires two or more entities. If only a single
entity is given then the function will return an empty list.
It is possible for `intersection` to miss intersections that one
knows exists because the required quantities were not fully
simplified internally.
Reals should be converted to Rationals, e.g. Rational(str(real_num))
or else failures due to floating point issues may result.
See Also
========
sympy.geometry.entity.GeometryEntity.intersection
Examples
========
>>> from sympy.geometry import Point, Line, Circle, intersection
>>> p1, p2, p3 = Point(0, 0), Point(1, 1), Point(-1, 5)
>>> l1, l2 = Line(p1, p2), Line(p3, p2)
>>> c = Circle(p2, 1)
>>> intersection(l1, p2)
[Point(1, 1)]
>>> intersection(l1, l2)
[Point(1, 1)]
>>> intersection(c, p2)
[]
>>> intersection(c, Point(1, 0))
[Point(1, 0)]
>>> intersection(c, l2)
[Point(-sqrt(5)/5 + 1, 2*sqrt(5)/5 + 1),
Point(sqrt(5)/5 + 1, -2*sqrt(5)/5 + 1)]
"""
from .entity import GeometryEntity
from .point import Point
if len(entities) <= 1:
return []
for i, e in enumerate(entities):
if not isinstance(e, GeometryEntity):
try:
entities[i] = Point(e)
except NotImplementedError:
raise ValueError('%s is not a GeometryEntity and cannot be made into Point' % str(e))
res = entities[0].intersection(entities[1])
for entity in entities[2:]:
newres = []
for x in res:
newres.extend(x.intersection(entity))
res = newres
return res
[docs]def convex_hull(*args):
"""The convex hull surrounding the Points contained in the list of entities.
Parameters
==========
args : a collection of Points, Segments and/or Polygons
Returns
=======
convex_hull : Polygon
Notes
=====
This can only be performed on a set of non-symbolic points.
References
==========
[1] http://en.wikipedia.org/wiki/Graham_scan
[2] Andrew's Monotone Chain Algorithm
(A.M. Andrew,
"Another Efficient Algorithm for Convex Hulls in Two Dimensions", 1979)
http://softsurfer.com/Archive/algorithm_0109/algorithm_0109.htm
See Also
========
sympy.geometry.point.Point, sympy.geometry.polygon.Polygon
Examples
========
>>> from sympy.geometry import Point, convex_hull
>>> points = [(1,1), (1,2), (3,1), (-5,2), (15,4)]
>>> convex_hull(*points)
Polygon(Point(-5, 2), Point(1, 1), Point(3, 1), Point(15, 4))
"""
from .entity import GeometryEntity
from .point import Point
from .line import Segment
from .polygon import Polygon
p = set()
for e in args:
if not isinstance(e, GeometryEntity):
try:
e = Point(e)
except NotImplementedError:
raise ValueError('%s is not a GeometryEntity and cannot be made into Point' % str(e))
if isinstance(e, Point):
p.add(e)
elif isinstance(e, Segment):
p.update(e.points)
elif isinstance(e, Polygon):
p.update(e.vertices)
else:
raise NotImplementedError(
'Convex hull for %s not implemented.' % type(e))
p = list(p)
if len(p) == 1:
return p[0]
elif len(p) == 2:
return Segment(p[0], p[1])
def _orientation(p, q, r):
'''Return positive if p-q-r are clockwise, neg if ccw, zero if
collinear.'''
return (q.y - p.y)*(r.x - p.x) - (q.x - p.x)*(r.y - p.y)
# scan to find upper and lower convex hulls of a set of 2d points.
U = []
L = []
p.sort(key=lambda x: x.args)
for p_i in p:
while len(U) > 1 and _orientation(U[-2], U[-1], p_i) <= 0:
U.pop()
while len(L) > 1 and _orientation(L[-2], L[-1], p_i) >= 0:
L.pop()
U.append(p_i)
L.append(p_i)
U.reverse()
convexHull = tuple(L + U[1:-1])
if len(convexHull) == 2:
return Segment(convexHull[0], convexHull[1])
return Polygon(*convexHull)
[docs]def are_similar(e1, e2):
"""Are two geometrical entities similar.
Can one geometrical entity be uniformly scaled to the other?
Parameters
==========
e1 : GeometryEntity
e2 : GeometryEntity
Returns
=======
are_similar : boolean
Raises
======
GeometryError
When `e1` and `e2` cannot be compared.
Notes
=====
If the two objects are equal then they are similar.
See Also
========
sympy.geometry.entity.GeometryEntity.is_similar
Examples
========
>>> from sympy import Point, Circle, Triangle, are_similar
>>> c1, c2 = Circle(Point(0, 0), 4), Circle(Point(1, 4), 3)
>>> t1 = Triangle(Point(0, 0), Point(1, 0), Point(0, 1))
>>> t2 = Triangle(Point(0, 0), Point(2, 0), Point(0, 2))
>>> t3 = Triangle(Point(0, 0), Point(3, 0), Point(0, 1))
>>> are_similar(t1, t2)
True
>>> are_similar(t1, t3)
False
"""
from .exceptions import GeometryError
if e1 == e2:
return True
try:
return e1.is_similar(e2)
except AttributeError:
try:
return e2.is_similar(e1)
except AttributeError:
n1 = e1.__class__.__name__
n2 = e2.__class__.__name__
raise GeometryError(
"Cannot test similarity between %s and %s" % (n1, n2))
[docs]def centroid(*args):
"""Find the centroid (center of mass) of the collection containing only Points,
Segments or Polygons. The centroid is the weighted average of the individual centroid
where the weights are the lengths (of segments) or areas (of polygons).
Overlapping regions will add to the weight of that region.
If there are no objects (or a mixture of objects) then None is returned.
See Also
========
sympy.geometry.point.Point, sympy.geometry.line.Segment,
sympy.geometry.polygon.Polygon
Examples
========
>>> from sympy import Point, Segment, Polygon
>>> from sympy.geometry.util import centroid
>>> p = Polygon((0, 0), (10, 0), (10, 10))
>>> q = p.translate(0, 20)
>>> p.centroid, q.centroid
(Point(20/3, 10/3), Point(20/3, 70/3))
>>> centroid(p, q)
Point(20/3, 40/3)
>>> p, q = Segment((0, 0), (2, 0)), Segment((0, 0), (2, 2))
>>> centroid(p, q)
Point(1, -sqrt(2) + 2)
>>> centroid(Point(0, 0), Point(2, 0))
Point(1, 0)
Stacking 3 polygons on top of each other effectively triples the
weight of that polygon:
>>> p = Polygon((0, 0), (1, 0), (1, 1), (0, 1))
>>> q = Polygon((1, 0), (3, 0), (3, 1), (1, 1))
>>> centroid(p, q)
Point(3/2, 1/2)
>>> centroid(p, p, p, q) # centroid x-coord shifts left
Point(11/10, 1/2)
Stacking the squares vertically above and below p has the same
effect:
>>> centroid(p, p.translate(0, 1), p.translate(0, -1), q)
Point(11/10, 1/2)
"""
from sympy.geometry import Polygon, Segment, Point
if args:
if all(isinstance(g, Point) for g in args):
c = Point(0, 0)
for g in args:
c += g
den = len(args)
elif all(isinstance(g, Segment) for g in args):
c = Point(0, 0)
L = 0
for g in args:
l = g.length
c += g.midpoint*l
L += l
den = L
elif all(isinstance(g, Polygon) for g in args):
c = Point(0, 0)
A = 0
for g in args:
a = g.area
c += g.centroid*a
A += a
den = A
c /= den
return c.func(*[i.simplify() for i in c.args])
