What is most efficient way to find the intersectio

Question

I have a polygon consists of lots of points. I want to find the intersection of the polygon and a circle. Providing the circle center of [x0,y0] and radius of r0, I have wrote a rough function to simply solve the quadratic equation of the circle and a line. But what about the efficiency of find the intersection of every line segment of the polygon one by one? Is there more efficient way?

I know sympy already provide the feature to get the intersections of different geometry. But also what about the efficiency of external library like sympy compared to calculate it by my own function, if I want to deal with lots of polygons?

def LineIntersectCircle(p,lsp,lep):
# p is the circle parameter, lsp and lep is the two end of the line
  x0,y0,r0 = p
  x1,y1 = lsp
  x2,y2 = esp
  if x1 == x2:
    if abs(r0) >= abs(x1 - x0):
        p1 = x1, y0 - sqrt(r0**2 - (x1-x0)**2)
        p2 = x1, y0 + sqrt(r0**2 - (x1-x0)**2)
        inp = [p1,p2]
        # select the points lie on the line segment
        inp = [p for p in inp if p[1]>=min(y1,y2) and p[1]<=max(y1,y2)]
    else:
        inp = []
  else:
    k = (y1 - y2)/(x1 - x2)
    b0 = y1 - k*x1
    a = k**2 + 1
    b = 2*k*(b0 - y0) - 2*x0
    c = (b0 - y0)**2 + x0**2 - r0**2
    delta = b**2 - 4*a*c
    if delta >= 0:
        p1x = (-b - sqrt(delta))/(2*a)
        p2x = (-b + sqrt(delta))/(2*a)
        p1y = k*x1 + b0
        p2y = k*x2 + b0
        inp = [[p1x,p1y],[p2x,p2y]]
        # select the points lie on the line segment
        inp = [p for p in inp if p[0]>=min(x1,x2) and p[0]<=max(x1,x2)]
    else:
        inp = []
  return inp

Answer 1

A low cost spacial partition might be to divide the plane into 9 pieces

Here is a crappy diagram. Imagine, if you will, that the lines are just touching the circle.

  | |
__|_|__
__|O|__
  | |
  | |

8 of the areas we are interested in are surrounding the circle. The square in the centre isn't much use for a cheap test, but you can place a square of r/sqrt(2) inside the circle, so it's corners just touch the circle.

Lets label the areas

A |B| C
__|_|__
D_|O|_E
  | |
F |G| H

And the square of r/sqrt(2) in the centre we'll call J

We will call the set of points in the centre square shown in the diagram that aren't in J, Z

Label each vertex of the polygon with it's letter code.

Now we can quickly see

AA => Outside
AB => Outside
AC => Outside
...
AJ => Intersects
BJ => Intersects
...
JJ => Inside

This can turned into a lookup table

So depending on your dataset, you may have saved yourself a load of work. Anything with an endpoint in Z will need to be tested however.

Answer 2

Here's a solution that computes the intersection of a circle with either a line or a line segment defined by two (x, y) points:

def circle_line_segment_intersection(circle_center, circle_radius, pt1, pt2, full_line=True, tangent_tol=1e-9):
    """ Find the points at which a circle intersects a line-segment.  This can happen at 0, 1, or 2 points.

    :param circle_center: The (x, y) location of the circle center
    :param circle_radius: The radius of the circle
    :param pt1: The (x, y) location of the first point of the segment
    :param pt2: The (x, y) location of the second point of the segment
    :param full_line: True to find intersections along full line - not just in the segment.  False will just return intersections within the segment.
    :param tangent_tol: Numerical tolerance at which we decide the intersections are close enough to consider it a tangent
    :return Sequence[Tuple[float, float]]: A list of length 0, 1, or 2, where each element is a point at which the circle intercepts a line segment.

    Note: We follow: http://mathworld.wolfram.com/Circle-LineIntersection.html
    """

    (p1x, p1y), (p2x, p2y), (cx, cy) = pt1, pt2, circle_center
    (x1, y1), (x2, y2) = (p1x - cx, p1y - cy), (p2x - cx, p2y - cy)
    dx, dy = (x2 - x1), (y2 - y1)
    dr = (dx ** 2 + dy ** 2)**.5
    big_d = x1 * y2 - x2 * y1
    discriminant = circle_radius ** 2 * dr ** 2 - big_d ** 2

    if discriminant < 0:  # No intersection between circle and line
        return []
    else:  # There may be 0, 1, or 2 intersections with the segment
        intersections = [
            (cx + (big_d * dy + sign * (-1 if dy < 0 else 1) * dx * discriminant**.5) / dr ** 2,
             cy + (-big_d * dx + sign * abs(dy) * discriminant**.5) / dr ** 2)
            for sign in ((1, -1) if dy < 0 else (-1, 1))]  # This makes sure the order along the segment is correct
        if not full_line:  # If only considering the segment, filter out intersections that do not fall within the segment
            fraction_along_segment = [(xi - p1x) / dx if abs(dx) > abs(dy) else (yi - p1y) / dy for xi, yi in intersections]
            intersections = [pt for pt, frac in zip(intersections, fraction_along_segment) if 0 <= frac <= 1]
        if len(intersections) == 2 and abs(discriminant) <= tangent_tol:  # If line is tangent to circle, return just one point (as both intersections have same location)
            return [intersections[0]]
        else:
            return intersections

Answer 3

I guess maybe your question is about how to in theory do this in the fastest manner. But if you want to do this quickly, you should really use something which is written in C/C++.

I am quite used to Shapely, so I will provide an example of how to do this with this library. There are many geometry libraries for python. I will list them at the end of this answer.

from shapely.geometry import LineString
from shapely.geometry import Point

p = Point(5,5)
c = p.buffer(3).boundary
l = LineString([(0,0), (10, 10)])
i = c.intersection(l)

print i.geoms[0].coords[0]
(2.8786796564403576, 2.8786796564403576)

print i.geoms[1].coords[0]
(7.121320343559642, 7.121320343559642)

What is a little bit counter intuitive in Shapely is that circles are the boundries of points with buffer areas. This is why I do p.buffer(3).boundry

Also the intersection i is a list of geometric shapes, both of them points in this case, this is why I have to get both of them from i.geoms[]

There is another Stackoverflow question which goes into details about these libraries for those interested.

• SymPy
• CGAL Python bindings
• PyEuclid
• PythonOCC
• Geometry-simple

EDIT because comments:

Shapely is based on GEOS (trac.osgeo.org/geos) which is built in C++ and considerably faster than anything you write natively in python. SymPy seems to be based on mpmath (mpmath.org) which also seems to be python, but seems to have lots of quite complex math integrated into it. Implementing that yourself may require a lot of work, and will probably not be as fast as GEOS C++ implementations.

Answer 4

I think that the formula you use to find the coordinates of the two intersections cannot be optimized further. The only improvement (which is numerically important) is to distinguish the two cases: |x_2-x_1| >= |y_2-y_1| and |x_2-x1| < |y_2-y1| so that the quantity k is always between -1 and 1 (in your computation you can get very high numerical errors if |x_2-x_1| is very small). You can swap x-s and y-s to reduce one case to the other.

You could also implement a preliminary check: if both endpoints are internal to the circle there are no intersection. By computing the squared distance from the points to the center of the circle this becomes a simple formula which does not use the square root function. The other check: "whether the line is outside the circle" is already implemented in your code and corresponds to delta < 0. If you have a lot of small segments these two check should give a shortcut answer (no intersection) in most cases.