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This page uses content from Wikipedia. The original article was at Point_in_polygon. The list of authors can be seen in the page history. As with Rosetta Code, the text of Wikipedia is available under the GNU FDL. (See links for details on variance)

Given a point and a polygon, check if the point is inside or outside the polygon using the ray-casting algorithm.

A pseudocode can be simply:

 count ← 0
 foreach side in polygon:
   if ray_intersects_segment(P,side) then
     count ← count + 1
 if is_odd(count) then
   return inside
 else
   return outside

Where the function ray_intersects_segment return true if the horizontal ray starting from the point P intersects the side (segment), false otherwise.

An intuitive explanation of why it works is that every time we cross a border, we change "country" (inside-outside, or outside-inside), but the last "country" we land on is surely outside (since the inside of the polygon is finite, while the ray continues towards infinity). So, if we crossed an odd number of borders we were surely inside, otherwise we were outside; we can follow the ray backward to see it better: starting from outside, only an odd number of crossing can give an inside: outside-inside, outside-inside-outside-inside, and so on (the - represents the crossing of a border).

So the main part of the algorithm is how we determine if a ray intersects a segment. The following text explain one of the possible ways.

Looking at the image on the right, we can easily be convinced of the fact that rays starting from points in the hatched area (like P₁ and P₂) surely do not intersect the segment AB. We also can easily see that rays starting from points in the greenish area surely intersect the segment AB (like point P₃).

So the problematic points are those inside the white area (the box delimited by the points A and B), like P₄.

Let us take into account a segment AB (the point A having y coordinate always smaller than B's y coordinate, i.e. point A is always below point B) and a point P. Let us use the cumbersome notation PAX to denote the angle between segment AP and AX, where X is always a point on the horizontal line passing by A with x coordinate bigger than the maximum between the x coordinate of A and the x coordinate of B. As explained graphically by the figures on the right, if PAX is greater than the angle BAX, then the ray starting from P intersects the segment AB. (In the images, the ray starting from P_A does not intersect the segment, while the ray starting from P_B in the second picture, intersects the segment).

Points on the boundary or "on" a vertex are someway special and through this approach we do not obtain coherent results. They could be treated apart, but it is not necessary to do so.

An algorithm for the previous speech could be (if P is a point, Px is its x coordinate):

 ray_intersects_segment:
    P : the point from which the ray starts
    A : the end-point of the segment with the smallest y coordinate
        (A must be "below" B)
    B : the end-point of the segment with the greatest y coordinate
        (B must be "above" A)
 if Py = Ay or Py = By then
   Py ← Py + ε
 end if
 if Py < Ay or Py > By then 
   return false
 else if Px > max(Ax, Bx) then 
   return false
 else
   if Px < min(Ax, Bx) then
     return true
   else
     if Ax ≠ Bx then
       m_red ← (By - Ay)/(Bx - Ax)
     else
       m_red ← ∞
     end if
     if Ax ≠ Px then
       m_blue ← (Py - Ay)/(Px - Ax)
     else
       m_blue ← ∞
     end if
     if m_blue ≥ m_red then
       return true
     else
       return false
     end if
   end if
 end if

(To avoid the "ray on vertex" problem, the point is moved upward of a small quantity ε)

Ada

polygons.ads: <lang Ada>package Polygons is

  type Point is record
     X, Y : Float;
  end record;
  type Point_List is array (Positive range <>) of Point;
  subtype Segment is Point_List (1 .. 2);
  type Polygon is array (Positive range <>) of Segment;

  function Create_Polygon (List : Point_List) return Polygon;

  function Is_Inside (Who : Point; Where : Polygon) return Boolean;

end Polygons;</lang>

polygons.adb: <lang Ada>package body Polygons is

  EPSILON : constant := 0.00001;

  function Ray_Intersects_Segment
    (Who   : Point;
     Where : Segment)
     return  Boolean
  is
     The_Point        : Point   := Who;
     Above            : Point;
     Below            : Point;
     M_Red            : Float;
     Red_Is_Infinity  : Boolean := False;
     M_Blue           : Float;
     Blue_Is_Infinity : Boolean := False;
  begin
     if Where (1).Y < Where (2).Y then
        Above := Where (2);
        Below := Where (1);
     else
        Above := Where (1);
        Below := Where (2);
     end if;
     if The_Point.Y = Above.Y or The_Point.Y = Below.Y then
        The_Point.Y := The_Point.Y + EPSILON;
     end if;
     if The_Point.Y < Below.Y or The_Point.Y > Above.Y then
        return False;
     elsif The_Point.X > Above.X and The_Point.X > Below.X then
        return False;
     elsif The_Point.X < Above.X and The_Point.X < Below.X then
        return True;
     else
        if Above.X /= Below.X then
           M_Red := (Above.Y - Below.Y) / (Above.X - Below.X);
        else
           Red_Is_Infinity := True;
        end if;
        if Below.X /= The_Point.X then
           M_Blue := (The_Point.Y - Below.Y) / (The_Point.X - Below.X);
        else
           Blue_Is_Infinity := True;
        end if;
        if Blue_Is_Infinity then
           return True;
        elsif Red_Is_Infinity then
           return False;
        elsif M_Blue >= M_Red then
           return True;
        else
           return False;
        end if;
     end if;
  end Ray_Intersects_Segment;

  function Create_Polygon (List : Point_List) return Polygon is
     Result : Polygon (List'Range);
     Side   : Segment;
  begin
     for I in List'Range loop
        Side (1) := List (I);
        if I = List'Last then
           Side (2) := List (List'First);
        else
           Side (2) := List (I + 1);
        end if;
        Result (I) := Side;
     end loop;
     return Result;
  end Create_Polygon;

  function Is_Inside (Who : Point; Where : Polygon) return Boolean is
     Count : Natural := 0;
  begin
     for Side in Where'Range loop
        if Ray_Intersects_Segment (Who, Where (Side)) then
           Count := Count + 1;
        end if;
     end loop;
     if Count mod 2 = 0 then
        return False;
     else
        return True;
     end if;
  end Is_Inside;

end Polygons;</lang>

Example use:

main.adb: <lang Ada>with Ada.Text_IO; with Polygons; procedure Main is

  package Float_IO is new Ada.Text_IO.Float_IO (Float);
  Test_Points : Polygons.Point_List :=
    ((  5.0,  5.0),
     (  5.0,  8.0),
     (-10.0,  5.0),
     (  0.0,  5.0),
     ( 10.0,  5.0),
     (  8.0,  5.0),
     ( 10.0, 10.0));
  Square      : Polygons.Polygon    :=
    ((( 0.0,  0.0), (10.0,  0.0)),
     ((10.0,  0.0), (10.0, 10.0)),
     ((10.0, 10.0), ( 0.0, 10.0)),
     (( 0.0, 10.0), ( 0.0,  0.0)));
  Square_Hole : Polygons.Polygon    :=
    ((( 0.0,  0.0), (10.0,  0.0)),
     ((10.0,  0.0), (10.0, 10.0)),
     ((10.0, 10.0), ( 0.0, 10.0)),
     (( 0.0, 10.0), ( 0.0,  0.0)),
     (( 2.5,  2.5), ( 7.5,  2.5)),
     (( 7.5,  2.5), ( 7.5,  7.5)),
     (( 7.5,  7.5), ( 2.5,  7.5)),
     (( 2.5,  7.5), ( 2.5,  2.5)));
  Strange     : Polygons.Polygon    :=
    ((( 0.0,  0.0), ( 2.5,  2.5)),
     (( 2.5,  2.5), ( 0.0, 10.0)),
     (( 0.0, 10.0), ( 2.5,  7.5)),
     (( 2.5,  7.5), ( 7.5,  7.5)),
     (( 7.5,  7.5), (10.0, 10.0)),
     ((10.0, 10.0), (10.0,  0.0)),
     ((10.0,  0.0), ( 2.5,  2.5)));
  Exagon      : Polygons.Polygon    :=
    ((( 3.0,  0.0), ( 7.0,  0.0)),
     (( 7.0,  0.0), (10.0,  5.0)),
     ((10.0,  5.0), ( 7.0, 10.0)),
     (( 7.0, 10.0), ( 3.0, 10.0)),
     (( 3.0, 10.0), ( 0.0,  5.0)),
     (( 0.0,  5.0), ( 3.0,  0.0)));

begin

  Ada.Text_IO.Put_Line ("Testing Square:");
  for Point in Test_Points'Range loop
     Ada.Text_IO.Put ("Point(");
     Float_IO.Put (Test_Points (Point).X, 0, 0, 0);
     Ada.Text_IO.Put (",");
     Float_IO.Put (Test_Points (Point).Y, 0, 0, 0);
     Ada.Text_IO.Put
       ("): " &
        Boolean'Image (Polygons.Is_Inside (Test_Points (Point), Square)));
     Ada.Text_IO.New_Line;
  end loop;
  Ada.Text_IO.New_Line;
  Ada.Text_IO.Put_Line ("Testing Square_Hole:");
  for Point in Test_Points'Range loop
     Ada.Text_IO.Put ("Point(");
     Float_IO.Put (Test_Points (Point).X, 0, 0, 0);
     Ada.Text_IO.Put (",");
     Float_IO.Put (Test_Points (Point).Y, 0, 0, 0);
     Ada.Text_IO.Put
       ("): " &
        Boolean'Image
           (Polygons.Is_Inside (Test_Points (Point), Square_Hole)));
     Ada.Text_IO.New_Line;
  end loop;
  Ada.Text_IO.New_Line;
  Ada.Text_IO.Put_Line ("Testing Strange:");
  for Point in Test_Points'Range loop
     Ada.Text_IO.Put ("Point(");
     Float_IO.Put (Test_Points (Point).X, 0, 0, 0);
     Ada.Text_IO.Put (",");
     Float_IO.Put (Test_Points (Point).Y, 0, 0, 0);
     Ada.Text_IO.Put
       ("): " &
        Boolean'Image (Polygons.Is_Inside (Test_Points (Point), Strange)));
     Ada.Text_IO.New_Line;
  end loop;
  Ada.Text_IO.New_Line;
  Ada.Text_IO.Put_Line ("Testing Exagon:");
  for Point in Test_Points'Range loop
     Ada.Text_IO.Put ("Point(");
     Float_IO.Put (Test_Points (Point).X, 0, 0, 0);
     Ada.Text_IO.Put (",");
     Float_IO.Put (Test_Points (Point).Y, 0, 0, 0);
     Ada.Text_IO.Put
       ("): " &
        Boolean'Image (Polygons.Is_Inside (Test_Points (Point), Exagon)));
     Ada.Text_IO.New_Line;
  end loop;

end Main;</lang>

Output:

Testing Square:
Point(5.0,5.0): TRUE
Point(5.0,8.0): TRUE
Point(-10.0,5.0): FALSE
Point(0.0,5.0): FALSE
Point(10.0,5.0): TRUE
Point(8.0,5.0): TRUE
Point(10.0,10.0): FALSE

Testing Square_Hole:
Point(5.0,5.0): FALSE
Point(5.0,8.0): TRUE
Point(-10.0,5.0): FALSE
Point(0.0,5.0): FALSE
Point(10.0,5.0): TRUE
Point(8.0,5.0): TRUE
Point(10.0,10.0): FALSE

Testing Strange:
Point(5.0,5.0): TRUE
Point(5.0,8.0): FALSE
Point(-10.0,5.0): FALSE
Point(0.0,5.0): FALSE
Point(10.0,5.0): TRUE
Point(8.0,5.0): TRUE
Point(10.0,10.0): FALSE

Testing Exagon:
Point(5.0,5.0): TRUE
Point(5.0,8.0): TRUE
Point(-10.0,5.0): FALSE
Point(0.0,5.0): FALSE
Point(10.0,5.0): TRUE
Point(8.0,5.0): TRUE
Point(10.0,10.0): FALSE

AutoHotkey

Works with: AutoHotkey L

<lang ahk>Points :=[{x: 5.0, y: 5.0} , {x: 5.0, y: 8.0} , {x:-10.0, y: 5.0} , {x: 0.0, y: 5.0} , {x: 10.0, y: 5.0} , {x: 8.0, y: 5.0} , {x: 10.0, y:10.0}] Square :=[{x: 0.0, y: 0.0}, {x:10.0, y: 0.0} , {x:10.0, y: 0.0}, {x:10.0, y:10.0} , {x:10.0, y:10.0}, {x: 0.0, y:10.0} , {x: 0.0, y:10.0}, {x: 0.0, y: 0.0}] Sq_Hole:=[{x: 0.0, y: 0.0}, {x:10.0, y: 0.0} , {x:10.0, y: 0.0}, {x:10.0, y:10.0} , {x:10.0, y:10.0}, {x: 0.0, y:10.0} , {x: 0.0, y:10.0}, {x: 0.0, y: 0.0} , {x: 2.5, y: 2.5}, {x: 7.5, y: 2.5} , {x: 7.5, y: 2.5}, {x: 7.5, y: 7.5} , {x: 7.5, y: 7.5}, {x: 2.5, y: 7.5} , {x: 2.5, y: 7.5}, {x: 2.5, y: 2.5}] Strange:=[{x: 0.0, y: 0.0}, {x: 2.5, y: 2.5} , {x: 2.5, y: 2.5}, {x: 0.0, y:10.0} , {x: 0.0, y:10.0}, {x: 2.5, y: 7.5} , {x: 2.5, y: 7.5}, {x: 7.5, y: 7.5} , {x: 7.5, y: 7.5}, {x:10.0, y:10.0} , {x:10.0, y:10.0}, {x:10.0, y: 0.0} , {x:10.0, y: 0.0}, {x: 2.5, y: 2.5}] Exagon :=[{x: 3.0, y: 0.0}, {x: 7.0, y: 0.0} , {x: 7.0, y: 0.0}, {x:10.0, y: 5.0} , {x:10.0, y: 5.0}, {x: 7.0, y:10.0} , {x: 7.0, y:10.0}, {x: 3.0, y:10.0} , {x: 3.0, y:10.0}, {x: 0.0, y: 5.0} , {x: 0.0, y: 5.0}, {x: 3.0, y: 0.0}] Polygons := {"Square":Square, "Sq_Hole":Sq_Hole, "Strange":Strange, "Exagon":Exagon} For j, Poly in Polygons For i, Point in Points If (point_in_polygon(Point,Poly)) s.= j " does contain point " i "`n" Else s.= j " doesn't contain point " i "`n" Msgbox %s%

point_in_polygon(Point,Poly) { n:=Poly.MaxIndex() count:=0 loop, %n% { if (ray_intersects_segment(Point,Poly[A_Index],Poly[mod(A_Index,n)+1])) { count++ } } if (mod(count,2)) { ; true = inside, false = outside return true ; P is in the polygon } else { return false ; P isn't in the polygon } }

ray_intersects_segment(P,A,B) { ;P = the point from which the ray starts ;A = the end-point of the segment with the smallest y coordinate ;B = the end-point of the segment with the greatest y coordinate if (A.y > B.y) { temp:=A A:=B B:=temp } if (P.y = A.y or P.y = B.y) { P.y += 0.000001 } if (P.y < A.y or P.y > B.y) { return false } else if (P.x > A.x && P.x > B.x) { return false } else { if (P.x < A.x && P.x < B.x) { return true } else { if (A.x != B.x) { m_red := (B.y - A.y)/(B.x - A.x) } else { m_red := "inf" } if (A.x != P.x) { m_blue := (P.y - A.y)/(P.x - A.x) } else { m_blue := "inf" } if (m_blue >= m_red) { return true } else { return false } } } }</lang>

Output:

---------------------------
Ray-casting_algorithm.ahkl
---------------------------
Exagon does contain point 1
Exagon does contain point 2
Exagon doesn't contain point 3
Exagon doesn't contain point 4
Exagon does contain point 5
Exagon does contain point 6
Exagon doesn't contain point 7
Sq_Hole doesn't contain point 1
Sq_Hole does contain point 2
Sq_Hole doesn't contain point 3
Sq_Hole doesn't contain point 4
Sq_Hole does contain point 5
Sq_Hole does contain point 6
Sq_Hole doesn't contain point 7
Square does contain point 1
Square does contain point 2
Square doesn't contain point 3
Square doesn't contain point 4
Square does contain point 5
Square does contain point 6
Square doesn't contain point 7
Strange does contain point 1
Strange doesn't contain point 2
Strange doesn't contain point 3
Strange doesn't contain point 4
Strange does contain point 5
Strange does contain point 6
Strange doesn't contain point 7

---------------------------
OK   
---------------------------

C

<lang c>#include <stdio.h>

include <stdlib.h>
include <math.h>

typedef struct { double x, y; } vec; typedef struct { int n; vec* v; } polygon_t, *polygon;

define BIN_V(op, xx, yy) vec v##op(vec a,vec b){vec c;c.x=xx;c.y=yy;return c;}
define BIN_S(op, r) double v##op(vec a, vec b){ return r; }

BIN_V(sub, a.x - b.x, a.y - b.y); BIN_V(add, a.x + b.x, a.y + b.y); BIN_S(dot, a.x * b.x + a.y * b.y); BIN_S(cross, a.x * b.y - a.y * b.x);

/* return a + s * b */ vec vmadd(vec a, double s, vec b) { vec c; c.x = a.x + s * b.x; c.y = a.y + s * b.y; return c; }

/* check if x0->x1 edge crosses y0->y1 edge. dx = x1 - x0, dy = y1 - y0, then

  solve  x0 + a * dx == y0 + b * dy with a, b in real
  cross both sides with dx, then: (remember, cross product is a scalar)

x0 X dx = y0 X dx + b * (dy X dx)

  similarly,

x0 X dy + a * (dx X dy) == y0 X dy

  there is an intersection iff 0 <= a <= 1 and 0 <= b <= 1

  returns: 1 for intersect, -1 for not, 0 for hard to say (if the intersect
  point is too close to y0 or y1)

/

int intersect(vec x0, vec x1, vec y0, vec y1, double tol, vec *sect) { vec dx = vsub(x1, x0), dy = vsub(y1, y0); double d = vcross(dy, dx), a; if (!d) return 0; /* edges are parallel */

a = (vcross(x0, dx) - vcross(y0, dx)) / d; if (sect) *sect = vmadd(y0, a, dy);

if (a < -tol || a > 1 + tol) return -1; if (a < tol || a > 1 - tol) return 0;

a = (vcross(x0, dy) - vcross(y0, dy)) / d; if (a < 0 || a > 1) return -1;

return 1; }

/* distance between x and nearest point on y0->y1 segment. if the point

  lies outside the segment, returns infinity */

double dist(vec x, vec y0, vec y1, double tol) { vec dy = vsub(y1, y0); vec x1, s; int r;

x1.x = x.x + dy.y; x1.y = x.y - dy.x; r = intersect(x, x1, y0, y1, tol, &s); if (r == -1) return HUGE_VAL; s = vsub(s, x); return sqrt(vdot(s, s)); }

define for_v(i, z, p) for(i = 0, z = p->v; i < p->n; i++, z++)

/* returns 1 for inside, -1 for outside, 0 for on edge */ int inside(vec v, polygon p, double tol) { /* should assert p->n > 1 */ int i, k, crosses, intersectResult; vec *pv; double min_x, max_x, min_y, max_y;

for (i = 0; i < p->n; i++) { k = (i + 1) % p->n; min_x = dist(v, p->v[i], p->v[k], tol); if (min_x < tol) return 0; }

min_x = max_x = p->v[0].x; min_y = max_y = p->v[1].y;

/* calculate extent of polygon */ for_v(i, pv, p) { if (pv->x > max_x) max_x = pv->x; if (pv->x < min_x) min_x = pv->x; if (pv->y > max_y) max_y = pv->y; if (pv->y < min_y) min_y = pv->y; } if (v.x < min_x || v.x > max_x || v.y < min_y || v.y > max_y) return -1;

max_x -= min_x; max_x *= 2; max_y -= min_y; max_y *= 2; max_x += max_y;

vec e; while (1) { crosses = 0; /* pick a rand point far enough to be outside polygon */ e.x = v.x + (1 + rand() / (RAND_MAX + 1.)) * max_x; e.y = v.y + (1 + rand() / (RAND_MAX + 1.)) * max_x;

for (i = 0; i < p->n; i++) { k = (i + 1) % p->n; intersectResult = intersect(v, e, p->v[i], p->v[k], tol, 0);

/* picked a bad point, ray got too close to vertex. re-pick */ if (!intersectResult) break;

if (intersectResult == 1) crosses++; } if (i == p->n) break; } return (crosses & 1) ? 1 : -1; }

int main() { vec vsq[] = { {0,0}, {10,0}, {10,10}, {0,10}, {2.5,2.5}, {7.5,0.1}, {7.5,7.5}, {2.5,7.5}};

polygon_t sq = { 4, vsq }, /* outer square */ sq_hole = { 8, vsq }; /* outer and inner square, ie hole */

vec c = { 10, 5 }; /* on edge */ vec d = { 5, 5 };

printf("%d\n", inside(c, &sq, 1e-10)); printf("%d\n", inside(c, &sq_hole, 1e-10));

printf("%d\n", inside(d, &sq, 1e-10)); /* in */ printf("%d\n", inside(d, &sq_hole, 1e-10)); /* out (in the hole) */

return 0; }</lang>

CoffeeScript

Takes a polygon as a list of points joining segments, and creates segments between them. <lang coffeescript> Point = (@x,@y) ->

 pointInPoly = (point,poly) ->
   segments = for pointA, index in poly
                pointB = poly[(index + 1) % poly.length]
                [pointA,pointB]
   intesected = (segment for segment in segments when rayIntesectsSegment(point,segment))
   intesected.length % 2 != 0

 rayIntesectsSegment = (p,segment) ->
   [p1,p2] = segment
   [a,b] = if p1.y < p2.y
             [p1,p2]
           else
             [p2,p1]
   if p.y == b.y || p.y == a.y
     p.y += Number.MIN_VALUE

   if p.y > b.y || p.y < a.y
     false
   else if p.x > a.x && p.x > b.x
     false
   else if p.x < a.x && p.x < b.x
     true
   else
     mAB = (b.y - a.y) / (b.x - a.x)
     mAP = (p.y - a.y) / (p.x - a.x)
     mAP > mAB</lang>

Common Lisp

Points are represented as cons cells whose car is an x value and whose cdr is a y value. A line segment is a cons cell of two points. A polygon is a list of line segments. <lang lisp>(defun point-in-polygon (point polygon)

 (do ((in-p nil)) ((endp polygon) in-p)
   (when (ray-intersects-segment point (pop polygon))
     (setf in-p (not in-p)))))

(defun ray-intersects-segment (point segment &optional (epsilon .001))

 (destructuring-bind (px . py) point
   (destructuring-bind ((ax . ay) . (bx . by)) segment
     (when (< ay by)
       (rotatef ay by)
       (rotatef ax bx))
     (when (or (= py ay) (= py by))
       (incf py epsilon))
     (cond
      ;; point is above, below, or to the right of the rectangle
      ;; determined by segment; ray does not intesect the segment.
      ((or (> px (max ax bx)) (> py (max ay by)) (< py (min ay by)))
       nil)
      ;; point is to left of the rectangle; ray intersects segment
      ((< px (min ax bx))
       t)
      ;; point is within the rectangle...
      (t (let ((m-red (if (= ax bx) nil
                        (/ (- by ay) (- bx ax))))
               (m-blue (if (= px ax) nil
                         (/ (- py ay) (- px ax)))))
           (cond
            ((null m-blue) t)
            ((null m-red) nil)
            (t (>= m-blue m-red)))))))))</lang>

Testing code <lang lisp>(defparameter *points*

 #((0 . 0) (10 . 0) (10 . 10) (0 . 10)  
   (2.5 . 2.5) (7.5 . 2.5) (7.5 . 7.5) (2.5 . 7.5) 
   (0 . 5) (10 . 5) (3 . 0) (7 . 0)
   (7 . 10) (3 . 10)))

(defun create-polygon (indices &optional (points *points*))

 (loop for (a b) on indices by 'cddr
       collecting (cons (aref points (1- a))
                        (aref points (1- b)))))

(defun square ()

 (create-polygon '(1 2 2 3 3 4 4 1)))

(defun square-hole ()

 (create-polygon '(1 2 2 3 3 4 4 1 5 6 6 7 7 8 8 5)))

(defun strange ()

 (create-polygon '(1 5 5 4 4 8 8 7 7 3 3 2 2 5)))

(defun exagon ()

 (create-polygon '(11 12 12 10 10 13 13 14 14 9 9 11)))

(defparameter *test-points*

 #((5 . 5) (5 . 8) (-10 . 5) (0 . 5)
   (10 . 5) (8 . 5) (10 . 10)))

(defun test-pip ()

 (dolist (shape '(square square-hole strange exagon))
   (print shape)
   (loop with polygon = (funcall shape)
         for test-point across *test-points*
         do (format t "~&~w ~:[outside~;inside ~]."
                    test-point
                    (point-in-polygon test-point polygon)))))</lang>

D

<lang d>import std.stdio, std.math, std.algorithm;

immutable struct Point { double x, y; } immutable struct Edge { Point a, b; } immutable struct Figure {

   string name;
   Edge[] edges;

}

bool contains(in Figure poly, in Point p) pure nothrow @safe @nogc {

   static bool raySegI(in Point p, in Edge edge)
   pure nothrow @safe @nogc {
       enum double epsilon = 0.00001;
       with (edge) {
           if (a.y > b.y)
               //swap(a, b); // if edge is mutable
               return raySegI(p, Edge(b, a));
           if (p.y == a.y || p.y == b.y)
               //p.y += epsilon; // if p is mutable
               return raySegI(Point(p.x, p.y + epsilon), edge);
           if (p.y > b.y || p.y < a.y || p.x > max(a.x, b.x))
               return false;
           if (p.x < min(a.x, b.x))
               return true;
           immutable blue = (abs(a.x - p.x) > double.min_normal) ?
                            ((p.y - a.y) / (p.x - a.x)) :
                            double.max;
           immutable red = (abs(a.x - b.x) > double.min_normal) ?
                           ((b.y - a.y) / (b.x - a.x)) :
                           double.max;
           return blue >= red;
       }
   }

   return poly.edges.count!(e => raySegI(p, e)) % 2;

}

void main() {

   immutable Figure[] polys = [
 {"Square", [
   {{ 0.0,  0.0}, {10.0,  0.0}},  {{10.0,  0.0}, {10.0, 10.0}},
   {{10.0, 10.0}, { 0.0, 10.0}},  {{ 0.0, 10.0}, { 0.0,  0.0}}]},
 {"Square hole", [
   {{ 0.0,  0.0}, {10.0,  0.0}},  {{10.0,  0.0}, {10.0, 10.0}},
   {{10.0, 10.0}, { 0.0, 10.0}},  {{ 0.0, 10.0}, { 0.0,  0.0}},
   {{ 2.5,  2.5}, { 7.5,  2.5}},  {{ 7.5,  2.5}, { 7.5,  7.5}},
   {{ 7.5,  7.5}, { 2.5,  7.5}},  {{ 2.5,  7.5}, { 2.5,  2.5}}]},
 {"Strange", [
   {{ 0.0,  0.0}, { 2.5,  2.5}},  {{ 2.5,  2.5}, { 0.0, 10.0}},
   {{ 0.0, 10.0}, { 2.5,  7.5}},  {{ 2.5,  7.5}, { 7.5,  7.5}},
   {{ 7.5,  7.5}, {10.0, 10.0}},  {{10.0, 10.0}, {10.0,  0.0}},
   {{10.0,  0},   { 2.5,  2.5}}]},
 {"Exagon", [
   {{ 3.0,  0.0}, { 7.0,  0.0}},  {{ 7.0,  0.0}, {10.0,  5.0}},
   {{10.0,  5.0}, { 7.0, 10.0}},  {{ 7.0, 10.0}, { 3.0, 10.0}},
   {{ 3.0, 10.0}, { 0.0,  5.0}},  {{ 0.0,  5.0}, { 3.0,  0.0}}]}

];

   immutable Point[] testPoints = [{ 5, 5}, {5, 8}, {-10,  5}, {0, 5},
                                   {10, 5}, {8, 5}, { 10, 10}];

   foreach (immutable poly; polys) {
       writefln(`Is point inside figure "%s"?`, poly.name);
       foreach (immutable p; testPoints)
           writefln("  (%3s, %2s): %s", p.x, p.y, contains(poly, p));
       writeln;
   }

}</lang>

Output:

Is point inside figure "Square"?
  (  5,  5): true
  (  5,  8): true
  (-10,  5): false
  (  0,  5): false
  ( 10,  5): true
  (  8,  5): true
  ( 10, 10): false

Is point inside figure "Square hole"?
  (  5,  5): false
  (  5,  8): true
  (-10,  5): false
  (  0,  5): false
  ( 10,  5): true
  (  8,  5): true
  ( 10, 10): false

Is point inside figure "Strange"?
  (  5,  5): true
  (  5,  8): false
  (-10,  5): false
  (  0,  5): false
  ( 10,  5): true
  (  8,  5): true
  ( 10, 10): false

Is point inside figure "Exagon"?
  (  5,  5): true
  (  5,  8): true
  (-10,  5): false
  (  0,  5): false
  ( 10,  5): true
  (  8,  5): true
  ( 10, 10): false

Factor

To test whether a ray intersects a line, we test that the starting point is between the endpoints in y value, and that it is to the left of the point on the segment with the same y value. Note that this implementation does not support polygons with horizontal edges. <lang factor>USING: kernel prettyprint sequences arrays math math.vectors ; IN: raycasting

between ( a b x -- ? ) [ last ] tri@ [ < ] curry bi@ xor ;

lincomb ( a b x -- w )

 3dup [ last ] tri@
 [ - ] curry bi@
 [ drop ] 2dip
 neg 2dup + [ / ] curry bi@
 [ [ v*n ] curry ] bi@ bi*  v+ ;

leftof ( a b x -- ? ) dup [ lincomb ] dip [ first ] bi@ > ;

ray ( a b x -- ? ) [ between ] [ leftof ] 3bi and ;

raycast ( poly x -- ? )

 [ dup first suffix [ rest-slice ] [ but-last-slice ] bi ] dip
 [ ray ] curry 2map
 f [ xor ] reduce ;</lang>

Usage: <lang factor>( scratchpad ) CONSTANT: square { { -2 -1 } { 1 -2 } { 2 1 } { -1 2 } } ( scratchpad ) square { 0 0 } raycast . t ( scratchpad ) square { 5 5 } raycast . f ( scratchpad ) square { 2 0 } raycast . f</lang>

Fortran

Works with: Fortran version 95 and later

The following code uses the Points_Module defined here.

This module defines "polygons". <lang fortran>module Polygons

 use Points_Module
 implicit none

 type polygon
    type(point), dimension(:), allocatable :: points
    integer, dimension(:), allocatable :: vertices
 end type polygon

contains

 function create_polygon(pts, vt)
   type(polygon) :: create_polygon
   type(point), dimension(:), intent(in) :: pts
   integer, dimension(:), intent(in) :: vt

   integer :: np, nv

   np = size(pts,1)
   nv = size(vt,1)

   allocate(create_polygon%points(np), create_polygon%vertices(nv))
   create_polygon%points = pts
   create_polygon%vertices = vt

 end function create_polygon

 subroutine free_polygon(pol)
   type(polygon), intent(inout) :: pol

   deallocate(pol%points, pol%vertices)

 end subroutine free_polygon

end module Polygons</lang>

The ray casting algorithm module: <lang fortran>module Ray_Casting_Algo

 use Polygons
 implicit none

 real, parameter, private :: eps = 0.00001
 private :: ray_intersects_seg

contains

 function ray_intersects_seg(p0, a0, b0) result(intersect)
   type(point), intent(in) :: p0, a0, b0
   logical :: intersect

   type(point) :: a, b, p
   real :: m_red, m_blue

   p = p0
   ! let variable "a" be the point with smallest y coordinate
   if ( a0%y > b0%y ) then
      b = a0
      a = b0
   else
      a = a0
      b = b0
   end if

   if ( (p%y == a%y) .or. (p%y == b%y) ) p%y = p%y + eps

   intersect = .false.

   if ( (p%y > b%y) .or. (p%y < a%y) ) return
   if ( p%x > max(a%x, b%x) ) return

   if ( p%x < min(a%x, b%x) ) then
      intersect = .true.
   else
      if ( abs(a%x - b%x) > tiny(a%x) ) then
         m_red = (b%y - a%y) / (b%x - a%x)
      else
         m_red = huge(m_red)
      end if
      if ( abs(a%x - p%x) > tiny(a%x) ) then
         m_blue = (p%y - a%y) / (p%x - a%x)
      else
         m_blue = huge(m_blue)
      end if
      if ( m_blue >= m_red ) then
         intersect = .true.
      else
         intersect = .false.
      end if
   end if

 end function ray_intersects_seg

 function point_is_inside(p, pol) result(inside)
   logical :: inside
   type(point), intent(in) :: p
   type(polygon), intent(in) :: pol
   
   integer :: i, cnt, pa, pb

   cnt = 0
   do i = lbound(pol%vertices,1), ubound(pol%vertices,1), 2
      pa = pol%vertices(i)
      pb = pol%vertices(i+1)
      if ( ray_intersects_seg(p, pol%points(pa), pol%points(pb)) ) cnt = cnt + 1
   end do
   
   inside = .true.
   if ( mod(cnt, 2) == 0 ) then
      inside = .false.
   end if

 end function point_is_inside

end module Ray_Casting_Algo</lang>

Testing <lang fortran>program Pointpoly

 use Points_Module
 use Ray_Casting_Algo
 implicit none

 character(len=16), dimension(4) :: names
 type(polygon), dimension(4) :: polys
 type(point), dimension(14) :: pts
 type(point), dimension(7) :: p

 integer :: i, j

 pts = (/ point(0,0), point(10,0), point(10,10), point(0,10), &
          point(2.5,2.5), point(7.5,2.5), point(7.5,7.5), point(2.5,7.5), &
          point(0,5), point(10,5), &
          point(3,0), point(7,0), point(7,10), point(3,10) /)

 polys(1) = create_polygon(pts, (/ 1,2, 2,3, 3,4, 4,1 /) )
 polys(2) = create_polygon(pts, (/ 1,2, 2,3, 3,4, 4,1, 5,6, 6,7, 7,8, 8,5 /) )
 polys(3) = create_polygon(pts, (/ 1,5, 5,4, 4,8, 8,7, 7,3, 3,2, 2,5 /) )
 polys(4) = create_polygon(pts, (/ 11,12, 12,10, 10,13, 13,14, 14,9, 9,11 /) )

 names = (/ "square", "square hole", "strange", "exagon" /)

 p = (/ point(5,5), point(5, 8), point(-10, 5), point(0,5), point(10,5), &
        point(8,5), point(10,10) /)

 do j = 1, size(p)
    do i = 1, size(polys)
       write(*, "('point (',F8.2,',',F8.2,') is inside ',A,'? ', L)") &
            p(j)%x, p(j)%y, names(i), point_is_inside(p(j), polys(i))
    end do
    print *, ""
 end do

 do i = 1, size(polys)
    call free_polygon(polys(i))
 end do

end program Pointpoly</lang>

FreeBASIC

Inpolygon by Winding number method <lang FreeBASIC> Type Point

   As Single x,y

End Type

Function inpolygon(p1() As Point,p2 As Point) As Integer

   #macro isleft2(L,p)
   -Sgn(  (L(1).x-L(2).x)*(p.y-L(2).y) - (p.x-L(2).x)*(L(1).y-L(2).y))
   #endmacro
   Dim As Integer index,nextindex
   Dim k As Integer=Ubound(p1)+1
   Dim send (1 To 2) As Point
   Dim wn As Integer=0
   For n As Integer=1 To Ubound(p1)
       index=n Mod k:nextindex=(n+1) Mod k
       If nextindex=0 Then nextindex=1
       send(1).x=p1(index).x:send(2).x=p1(nextindex).x
       send(1).y=p1(index).y:send(2).y=p1(nextindex).y
       If p1(index).y<=p2.y Then
           If p1(nextindex).y>p2.y Then 
               If isleft2(send,p2)>=0 Then '=
                   wn=wn+1
               End If
           End If
       Else
           If p1(nextindex).y<=p2.y Then
               If isleft2(send,p2)<=0 Then'=
                   wn=wn-1
               End If
           End If
       End If
   Next n
   Return wn

End Function

Dim As Point square(1 To 4) ={(0,0),(10,0),(10,10),(0,10)}

Dim As Point hole(1 To 4) ={(2.5,2.5),(7.5,2.5),(7.5,7.5),(2.5,7.5)}

Dim As Point strange(1 To 8) ={(0,0),(2.5,2.5),(0,10),(2.5,7.5),_

                             (7.5,7.5),(10,10),(10,0),(2.5,2.5)}

Dim As Point exagon(1 To 6) ={(3,0),(7,0),(10,5),(7,10),(3,10),(0,5)} 'printouts For z As Integer=1 To 4

   Select Case z
   Case 1: Print "squared"
   Print "(5,5)  " ;Tab(12);
   If inpolygon(square(),Type<Point>(5,5)) Then Print "in" Else Print "out"
   Print "(5,8)  " ;Tab(12);
   If inpolygon(square(),Type<Point>(5,8)) Then Print "in" Else Print "out"
   Print "(-10,5)  " ;Tab(12);
   If inpolygon(square(),Type<Point>(-10,5)) Then Print "in" Else Print "out"
   Print "(0,5)  " ;Tab(12);
   If inpolygon(square(),Type<Point>(0,5)) Then Print "in" Else Print "out"
   Print "(10,5)  " ;Tab(12);
   If inpolygon(square(),Type<Point>(10,5)) Then Print "in" Else Print "out"
   Print "(8,5)  " ;Tab(12);
   If inpolygon(square(),Type<Point>(8,5)) Then Print "in" Else Print "out"
   Print "(10,10)  " ;Tab(12);
   If inpolygon(square(),Type<Point>(10,10)) Then Print "in" Else Print "out"
   Print

Case 2:Print "squared hole" Print "(5,5) " ;Tab(12); If Not inpolygon(hole(),Type<Point>(5,5)) And inpolygon(square(),Type<Point>(5,5)) Then Print "in" Else Print "out" Print "(5,8) " ;Tab(12); If Not inpolygon(hole(),Type<Point>(5,8)) And inpolygon(square(),Type<Point>(5,8))Then Print "in" Else Print "out" Print "(-10,5) " ;Tab(12); If Not inpolygon(hole(),Type<Point>(-10,5))And inpolygon(square(),Type<Point>(-10,5)) Then Print "in" Else Print "out" Print "(0,5) " ;Tab(12); If Not inpolygon(hole(),Type<Point>(0,5))And inpolygon(square(),Type<Point>(0,5)) Then Print "in" Else Print "out" Print "(10,5) " ;Tab(12); If Not inpolygon(hole(),Type<Point>(10,5))And inpolygon(square(),Type<Point>(10,5)) Then Print "in" Else Print "out" Print "(8,5) " ;Tab(12); If Not inpolygon(hole(),Type<Point>(8,5))And inpolygon(square(),Type<Point>(8,5)) Then Print "in" Else Print "out" Print "(10,10) " ;Tab(12); If Not inpolygon(hole(),Type<Point>(10,10))And inpolygon(square(),Type<Point>(10,10)) Then Print "in" Else Print "out" Print Case 3:Print "strange" Print "(5,5) " ;Tab(12); If inpolygon(strange(),Type<Point>(5,5)) Then Print "in" Else Print "out" Print "(5,8) " ;Tab(12); If inpolygon(strange(),Type<Point>(5,8)) Then Print "in" Else Print "out" Print "(-10,5) " ;Tab(12); If inpolygon(strange(),Type<Point>(-10,5)) Then Print "in" Else Print "out" Print "(0,5) " ;Tab(12); If inpolygon(strange(),Type<Point>(0,5)) Then Print "in" Else Print "out" Print "(10,5) " ;Tab(12); If inpolygon(strange(),Type<Point>(10,5)) Then Print "in" Else Print "out" Print "(8,5) " ;Tab(12); If inpolygon(strange(),Type<Point>(8,5)) Then Print "in" Else Print "out" Print "(10,10) " ;Tab(12); If inpolygon(strange(),Type<Point>(10,10)) Then Print "in" Else Print "out" Print Case 4:Print "exagon" Print "(5,5) " ;Tab(12); If inpolygon(exagon(),Type<Point>(5,5)) Then Print "in" Else Print "out" Print "(5,8) " ;Tab(12); If inpolygon(exagon(),Type<Point>(5,8)) Then Print "in" Else Print "out" Print "(-10,5) " ;Tab(12); If inpolygon(exagon(),Type<Point>(-10,5)) Then Print "in" Else Print "out" Print "(0,5) " ;Tab(12); If inpolygon(exagon(),Type<Point>(0,5)) Then Print "in" Else Print "out" Print "(10,5) " ;Tab(12); If inpolygon(exagon(),Type<Point>(10,5)) Then Print "in" Else Print "out" Print "(8,5) " ;Tab(12); If inpolygon(exagon(),Type<Point>(8,5)) Then Print "in" Else Print "out" Print "(10,10) " ;Tab(12); If inpolygon(exagon(),Type<Point>(10,10)) Then Print "in" Else Print "out" Print End Select Next z sleep </lang> Output:

squared
(5,5)      in
(5,8)      in
(-10,5)    out
(0,5)      out
(10,5)     in
(8,5)      in
(10,10)    out

squared hole
(5,5)      out
(5,8)      in
(-10,5)    out
(0,5)      out
(10,5)     in
(8,5)      in
(10,10)    out

strange
(5,5)      in
(5,8)      out
(-10,5)    out
(0,5)      out
(10,5)     in
(8,5)      in
(10,10)    out

exagon
(5,5)      in
(5,8)      in
(-10,5)    out
(0,5)      out
(10,5)     in
(8,5)      in
(10,10)    out

Go

<lang go>package main

import (

   "math"
   "fmt"

)

type xy struct {

   x, y float64

}

type seg struct {

   p1, p2 xy

}

type poly struct {

   name  string
   sides []seg

}

func inside(pt xy, pg poly) (i bool) {

   for _, side := range pg.sides {
       if rayIntersectsSegment(pt, side) {
           i = !i
       }
   }
   return

}

func rayIntersectsSegment(p xy, s seg) bool {

   var a, b xy
   if s.p1.y < s.p2.y {
       a, b = s.p1, s.p2
   } else {
       a, b = s.p2, s.p1
   }
   for p.y == a.y || p.y == b.y {
       p.y = math.Nextafter(p.y, math.Inf(1))
   }
   if p.y < a.y || p.y > b.y {
       return false
   }
   if a.x > b.x {
       if p.x > a.x {
           return false
       }
       if p.x < b.x {
           return true
       }
   } else {
       if p.x > b.x {
           return false
       }
       if p.x < a.x {
           return true
       }
   }
   return (p.y-a.y)/(p.x-a.x) >= (b.y-a.y)/(b.x-a.x)

}

var (

   p1  = xy{0, 0}
   p2  = xy{10, 0}
   p3  = xy{10, 10}
   p4  = xy{0, 10}
   p5  = xy{2.5, 2.5}
   p6  = xy{7.5, 2.5}
   p7  = xy{7.5, 7.5}
   p8  = xy{2.5, 7.5}
   p9  = xy{0, 5}
   p10 = xy{10, 5}
   p11 = xy{3, 0}
   p12 = xy{7, 0}
   p13 = xy{7, 10}
   p14 = xy{3, 10}

)

var tpg = []poly{

   {"square", []seg{{p1, p2}, {p2, p3}, {p3, p4}, {p4, p1}}},
   {"square hole", []seg{{p1, p2}, {p2, p3}, {p3, p4}, {p4, p1},
       {p5, p6}, {p6, p7}, {p7, p8}, {p8, p5}}},
   {"strange", []seg{{p1, p5},
       {p5, p4}, {p4, p8}, {p8, p7}, {p7, p3}, {p3, p2}, {p2, p5}}},
   {"exagon", []seg{{p11, p12}, {p12, p10}, {p10, p13},
       {p13, p14}, {p14, p9}, {p9, p11}}},

}

var tpt = []xy{{5, 5}, {5, 8}, {-10, 5}, {0, 5}, {10, 5}, {8, 5}, {10, 10}}

func main() {

   for _, pg := range tpg {
       fmt.Printf("%s:\n", pg.name)
       for _, pt := range tpt {
           fmt.Println(pt, inside(pt, pg))
       }
   }

}</lang> Output:

square:
{5 5} true
{5 8} true
{-10 5} false
{0 5} false
{10 5} true
{8 5} true
{10 10} false
square hole:
{5 5} false
{5 8} true
{-10 5} false
{0 5} false
{10 5} true
{8 5} true
{10 10} false
strange:
{5 5} true
{5 8} false
{-10 5} false
{0 5} false
{10 5} true
{8 5} true
{10 10} false
exagon:
{5 5} true
{5 8} true
{-10 5} false
{0 5} false
{10 5} true
{8 5} true
{10 10} false

Closed polygon solution

The solution above follows the model of most other solutions on the page in defining a polygon as a list of segments. Input to the ray-casting algorithm however, as noted in the WP article, is specified to be a closed polygon. Input is more properly given then, as a list of N verticies defining N segments, where one segment extends from each vertex to the next, and one more extends from the last vertex to the first. Code below implements this. The test points used below demonstrate that this model gives good results for all shapes, whereas the model above gives curious results for these test points in the case of the "strange" shape. <lang go>package main

import (

   "math"
   "fmt"

)

type xy struct {

   x, y float64

}

type closedPoly struct {

   name string
   vert []xy

}

func inside(pt xy, pg closedPoly) bool {

   if len(pg.vert) < 3 {
       return false
   }
   in := rayIntersectsSegment(pt, pg.vert[len(pg.vert)-1], pg.vert[0])
   for i := 1; i < len(pg.vert); i++ {
       if rayIntersectsSegment(pt, pg.vert[i-1], pg.vert[i]) {
           in = !in
       }
   }
   return in

}

func rayIntersectsSegment(p, a, b xy) bool {

   if a.y > b.y {
       a, b = b, a
   }
   for p.y == a.y || p.y == b.y {
       p.y = math.Nextafter(p.y, math.Inf(1))
   }
   if p.y < a.y || p.y > b.y {
       return false
   }
   if a.x > b.x {
       if p.x > a.x {
           return false
       }
       if p.x < b.x {
           return true
       }
   } else {
       if p.x > b.x {
           return false
       }
       if p.x < a.x {
           return true
       }
   }
   return (p.y-a.y)/(p.x-a.x) >= (b.y-a.y)/(b.x-a.x)

}

var tpg = []closedPoly{

   {"square", []xy{{0, 0}, {10, 0}, {10, 10}, {0, 10}}},
   {"square hole", []xy{{0, 0}, {10, 0}, {10, 10}, {0, 10}, {0, 0},
       {2.5, 2.5}, {7.5, 2.5}, {7.5, 7.5}, {2.5, 7.5}, {2.5, 2.5}}},
   {"strange", []xy{{0, 0}, {2.5, 2.5}, {0, 10}, {2.5, 7.5}, {7.5, 7.5},
       {10, 10}, {10, 0}, {2.5, 2.5}}},
   {"exagon", []xy{{3, 0}, {7, 0}, {10, 5}, {7, 10}, {3, 10}, {0, 5}}},

}

var tpt = []xy{{1, 2}, {2, 1}}

func main() {

   for _, pg := range tpg {
       fmt.Printf("%s:\n", pg.name)
       for _, pt := range tpt {
           fmt.Println(pt, inside(pt, pg))
       }
   }

}</lang> Output:

square:
{1 2} true
{2 1} true
square hole:
{1 2} true
{2 1} true
strange:
{1 2} false
{2 1} false
exagon:
{1 2} false
{2 1} false

Haskell

<lang haskell>import Data.Ratio

type Point = (Rational, Rational) type Polygon = [Point] data Line = Sloped {lineSlope, lineYIntercept :: Rational} |

           Vert {lineXIntercept :: Rational}

polygonSides :: Polygon -> [(Point, Point)] polygonSides poly@(p1 : ps) = zip poly $ ps ++ [p1]

intersects :: Point -> Line -> Bool {- @intersects (px, py) l@ is true if the ray {(x, py) | x ≥ px} intersects l. -} intersects (px, _) (Vert xint) = px <= xint intersects (px, py) (Sloped m b) | m < 0 = py <= m * px + b

                                | otherwise = py >= m * px + b

onLine :: Point -> Line -> Bool {- Is the point on the line? -} onLine (px, _) (Vert xint) = px == xint onLine (px, py) (Sloped m b) = py == m * px + b

carrier :: (Point, Point) -> Line {- Finds the line containing the given line segment. -} carrier ((ax, ay), (bx, by)) | ax == bx = Vert ax

                            | otherwise = Sloped slope yint
 where slope = (ay - by) / (ax - bx)
       yint = ay - slope * ax

between :: Ord a => a -> a -> a -> Bool between x a b | a > b = b <= x && x <= a

             | otherwise = a <= x && x <= b

inPolygon :: Point -> Polygon -> Bool inPolygon p@(px, py) = f 0 . polygonSides

 where f n []                             = odd n
       f n (side : sides) | far           = f n       sides
                          | onSegment     = True
                          | rayIntersects = f (n + 1) sides
                          | otherwise     = f n       sides
         where far = not $ between py ay by
               onSegment | ay == by  = between px ax bx
                         | otherwise = p `onLine` line
               rayIntersects =
                   intersects p line &&
                   (py /= ay || by < py) &&
                   (py /= by || ay < py)
               ((ax, ay), (bx, by)) = side
               line = carrier side</lang>

J

<lang j>NB.*crossPnP v point in closed polygon, crossing number NB. bool=. points crossPnP polygon crossPnP=: 4 : 0"2

 'X Y'=. |:x
 'x0 y0 x1 y1'=. |:2 ,/\^:(2={:@$@]) y
 p1=. ((y0<:/Y)*. y1>/Y) +. (y0>/Y)*. y1<:/Y
 p2=. (x0-/X) < (x0-x1) * (y0-/Y) % (y0 - y1)
 2|+/ p1*.p2

)</lang>

Sample data: <lang j>SQUAREV=: 0 0 , 10 0 , 10 10 ,: 0 10 SQUAREV=: SQUAREV, 2.5 2.5 , 7.5 0.1 , 7.5 7.5 ,: 2.5 7.5

ESAV=: 3 0 , 7 0 , 10 5 , 7 10 , 3 10 ,: 0 5

ESA=: (0 1,1 2,2 3,3 4,4 5,:5 0) , .{ ESAV SQUARE=: (0 1,1 2,2 3,:3 0) , .{ SQUAREV SQUAREHOLE=: (0 1,1 2,2 3,3 0,4 5,5 6,6 7,:7 4) , .{ SQUAREV STRANGE=: (0 4,4 3,3 7,7 6,6 2,2 1,1 5,:5 0) , .{ SQUAREV

POINTS=: 5 5,5 8,2 2,0 0,10 10,2.5 2.5,0.01 5,2.2 7.4,0 5,10 5,:_4 10</lang>

Testing: <lang j> (<POINTS) crossPnP every ESA;SQUARE;SQUAREHOLE;STRANGE 1 1 1 0 0 1 1 1 0 1 0 1 1 1 0 0 1 1 1 0 1 0 0 1 1 0 0 1 1 1 0 1 0 1 0 0 0 0 0 0 1 0 1 0</lang>

Liberty BASIC

Translated from C code at: http://alienryderflex.com/polygon/

Displays interactively on-screen. <lang lb>NoMainWin Global sw, sh, verts

sw = 640 : sh = 480 WindowWidth = sw+8 : WindowHeight = sh+31 UpperLeftX = (DisplayWidth -sw)/2 UpperLeftY = (DisplayHeight-sh)/2 Open"Ray Casting Algorithm" For Graphics_nf_nsb As #g

g "Down; TrapClose [halt]"

h$ = "#g"

Dim xp(15),yp(15)

g "when leftButtonDown [halt];when mouseMove checkPoint"
g "when rightButtonDown [Repeat]"

[Repeat]

   #g "Cls;Fill 32 160 255; Color white;BackColor 32 160 255"
   #g "Place 5 460;\L-click to exit"
   #g "Place 485 460;\R-click for new polygon"

   'generate polygon from random points
   numPoints =  rand(4,15)
   verts = numPoints
   For i = 0 To numPoints-1
       xp(i) = rand(20,620)
       yp(i) = rand(40,420)
   Next
   Call drawPoly h$, verts, "white"
   #g "Flush"
   Wait

[halt] Close #g End

'Point In Polygon Function Function pnp(x, y, numSides)

   j= numSides-1: oddNodes = 0
   For i = 0 To numSides-1
       If ((yp(i)<y) And (yp(j)>=y)) Or ((yp(j)<y) And (yp(i)>=y)) Then
           f1 = y - yp(i):f2 = yp(j) - yp(i): f3 = xp(j) - xp(i)
           If (xp(i) + f1 / f2 * f3) < x Then oddNodes = 1 - oddNodes
       End If
       j = i
   Next
   pnp = oddNodes

End Function

'draw the polygon Sub drawPoly h$, verts, colour$

   #h$, "Color ";colour$
   j = verts-1
   For i = 0 To verts-1
       #h$ "Line ";xp(j);" ";yp(j);" ";xp(i);" ";yp(i)
       j = i
   Next

End Sub

'change message and color of polygon Sub checkPoint h$, x, y

   If pnp(x,y,verts) Then
       #h$ "Color 32 160 255;BackColor 32 160 255"
       #h$ "Place 5 0;BoxFilled 150 20;Color white"
       #h$ "Place 7 15;\Mouse In Polygon"
       Call drawPoly h$, verts, "red"
   Else
       #h$ "Color 32 160 255;BackColor 32 160 255"
       #h$ "Place 5 0;BoxFilled 150 20;Color white"
       #h$ "Place 7 15;\Mouse Not In Polygon"
       Call drawPoly h$, verts, "white"
   End If

End Sub

Function rand(loNum,hiNum)

   rand = Int(Rnd(0)*(hiNum-loNum+1)+loNum)

End Function </lang>

OCaml

Translation of: C

<lang ocaml>type point = { x:float; y:float }

type polygon = {

 vertices: point array;
 edges: (int * int) list;

}

let p x y = { x=x; y=y }

let square_v = [|

 (p 0. 0.); (p 10. 0.); (p 10. 10.); (p 0. 10.);
 (p 2.5 2.5); (p 7.5 0.1); (p 7.5 7.5); (p 2.5 7.5)

|]

let esa_v = [|

 (p 3. 0.); (p 7. 0.); (p 10. 5.); (p 7. 10.); (p 3. 10.); (p 0. 5.)

|]

let esa = {

 vertices = esa_v;
 edges = [ (0,1); (1,2); (2,3); (3,4); (4,5); (5,0) ]

}

let square = {

 vertices = square_v;
 edges = [ (0,1); (1,2); (2,3); (3,0) ]

}

let squarehole = {

 vertices = square_v;
 edges = [ (0,1); (1,2); (2,3); (3,0); (4,5); (5,6); (6,7); (7,4) ]

}

let strange = {

 vertices = square_v;
 edges = [ (0,4); (4,3); (3,7); (7,6); (6,2); (2,1); (1,5); (5,0) ]

}

let min_y ~a ~b = if a.y > b.y then (b) else (a)

let coeff_ang ~pa ~pb = (pb.y -. pa.y) /. (pb.x -. pa.x)

let huge_val = infinity

let hseg_intersect_seg ~s ~a ~b =

 let _eps =
   if s.y = (max a.y b.y) ||
      s.y = (min a.y b.y) then 0.00001 else 0.0
 in
 if  (s.y +. _eps) > (max a.y b.y) ||
     (s.y +. _eps) < (min a.y b.y) ||
      s.x > (max a.x b.x) then (false)
 else if s.x <= (min a.x b.x) then (true)
 else
   let ca = if a.x <> b.x then (coeff_ang a b) else (huge_val) in
   let my = min_y ~a ~b in
   let cp = if (s.x -. my.x) <> 0.0 then (coeff_ang my s) else (huge_val) in
   (cp >= ca)

let point_is_inside ~poly ~pt =

 let cross = ref 0 in
 List.iter (fun (a,b) ->
   if hseg_intersect_seg pt
            poly.vertices.(a)
            poly.vertices.(b)
   then incr cross
 ) poly.edges;
 ( (!cross mod 2) <> 0)

let make_test p label s =

 Printf.printf "point (%.5f,%.5f) is " p.x p.y;
 print_string (if point_is_inside s p
               then "INSIDE "
               else "OUTSIDE ");
 print_endline label;

let () =

 let test_points = [
   (p 5. 5.); (p 5. 8.); (p 2. 2.); (p 0. 0.);
   (p 10. 10.); (p 2.5 2.5); (p 0.01 5.);
   (p 2.2 7.4); (p 0. 5.); (p 10. 5.); (p (-4.) 10.) ] in

 List.iter (fun p ->
   make_test p "square"     square;
   make_test p "squarehole" squarehole;
   make_test p "strange"    strange;
   make_test p "esa"        esa;
   print_newline()
 ) test_points;

</lang>

Perl

This example is incorrect. Please fix the code and remove this message.

Details: Says everything is outside with v5.14.

<lang perl>use strict; use List::Util qw(max min);

sub point_in_polygon {

   my ( $point, $polygon ) = @_;

   my $count = 0;
   foreach my $side ( @$polygon ) {

$count++ if ray_intersect_segment($point, $side);

   }
   return ($count % 2 == 0) ? 0 : 1;

}

my $eps = 0.0001; my $inf = 1e600;

sub ray_intersect_segment {

   my ($point, $segment) = @_;

   my ($A, $B) = @$segment;

   my @P = @$point; # copy it

   ($A, $B) = ($B, $A) if $A->[1] > $B->[1];

   $P[1] += $eps if ($P[1] == $A->[1]) || ($P[1] == $B->[1]);

   return 0 if ($P[1] < $A->[1]) || ( $P[1] > $B->[1]) || ($P[0] > max($A->[0],$B->[1]) );
   return 1 if $P[0] < min($A->[0], $B->[0]);

   my $m_red = ($A->[0] != $B->[0]) ? ( $B->[1] - $A->[1] )/($B->[0] - $A->[0]) : $inf;
   my $m_blue = ($A->[0] != $P[0]) ? ( $P[1] - $A->[1] )/($P[0] - $A->[0]) : $inf;

   return ($m_blue >= $m_red) ? 1 : 0;

}</lang>

Testing: <lang perl># the following are utilities to use the same Fortran data... sub point {

   [shift, shift];

} sub create_polygon {

   my ($pts, $sides) = @_;
   my @poly;
   for(my $i = 0; $i < $#$sides; $i += 2) {

push @poly, [ $pts->[$sides->[$i]-1], $pts->[$sides->[$i+1]-1] ];

   }
   @poly;

}

my @pts = ( point(0,0), point(10,0), point(10,10), point(0,10), point(2.5,2.5), point(7.5,2.5), point(7.5,7.5), point(2.5,7.5), point(0,5), point(10,5), point(3,0), point(7,0), point(7,10), point(3,10) );

my @squared = create_polygon(\@pts, [ 1,2, 2,3, 3,4, 4,1 ] ); my @squaredhole = create_polygon(\@pts, [ 1,2, 2,3, 3,4, 4,1, 5,6, 6,7, 7,8, 8,5 ] ); my @strange = create_polygon(\@pts, [ 1,5, 5,4, 4,8, 8,7, 7,3, 3,2, 2,5 ] ); my @exagon = create_polygon(\@pts, [ 11,12, 12,10, 10,13, 13,14, 14,9, 9,11 ]) ;

my @p = ( point(5,5), point(5, 8), point(-10, 5), point(0,5), point(10,5), & point(8,5), point(10,10) );

foreach my $pol ( qw(squared squaredhole strange exagon) ) {

   no strict 'refs';
   print "$pol\n";
   my @rp = @{$pol};
   foreach my $tp ( @p ) {

print "\t($tp->[0],$tp->[1]) " .

          ( point_in_polygon($tp, \@rp) ? "INSIDE" : "OUTSIDE" ) . "\n";
   }

}</lang>

Perl 6

<lang perl6>constant ε = 0.0001;

sub ray-hits-seg([\Px,\Py], [[\Ax,\Ay], [\Bx,\By]] --> Bool) {

   Py += ε if Py == Ay | By;

   if Py < Ay or Py > By or Px > (Ax max Bx) {

False;

   }
   elsif Px < (Ax min Bx) {

True;

   }
   else {

my \red = Ax == Bx ?? Inf !! (By - Ay) / (Bx - Ax); my \blue = Ax == Px ?? Inf !! (Py - Ay) / (Px - Ax); blue >= red;

}

sub point-in-poly(@point, @polygon --> Bool) {

   so 2 R% [+] gather for @polygon -> @side {

take ray-hits-seg @point, @side.sort(*.[1]);

}

my %poly =

   squared =>

[[[ 0.0, 0.0], [10.0, 0.0]], [[10.0, 0.0], [10.0, 10.0]], [[10.0, 10.0], [ 0.0, 10.0]], [[ 0.0, 10.0], [ 0.0, 0.0]]],

   squaredhole =>

[[[ 0.0, 0.0], [10.0, 0.0]], [[10.0, 0.0], [10.0, 10.0]], [[10.0, 10.0], [ 0.0, 10.0]], [[ 0.0, 10.0], [ 0.0, 0.0]], [[ 2.5, 2.5], [ 7.5, 2.5]], [[ 7.5, 2.5], [ 7.5, 7.5]], [[ 7.5, 7.5], [ 2.5, 7.5]], [[ 2.5, 7.5], [ 2.5, 2.5]]],

   strange =>

[[[ 0.0, 0.0], [ 2.5, 2.5]], [[ 2.5, 2.5], [ 0.0, 10.0]], [[ 0.0, 10.0], [ 2.5, 7.5]], [[ 2.5, 7.5], [ 7.5, 7.5]], [[ 7.5, 7.5], [10.0, 10.0]], [[10.0, 10.0], [10.0, 0.0]], [[10.0, 0.0], [ 2.5, 2.5]], [[ 2.5, 2.5], [ 0.0, 0.0]]], # conjecturally close polygon

   exagon =>

[[[ 3.0, 0.0], [ 7.0, 0.0]], [[ 7.0, 0.0], [10.0, 5.0]], [[10.0, 5.0], [ 7.0, 10.0]], [[ 7.0, 10.0], [ 3.0, 10.0]], [[ 3.0, 10.0], [ 0.0, 5.0]], [[ 0.0, 5.0], [ 3.0, 0.0]]];

my @test-points = [ 5.0, 5.0], [ 5.0, 8.0], [-10.0, 5.0], [ 0.0, 5.0], [ 10.0, 5.0], [ 8.0, 5.0], [ 10.0, 10.0];

for <squared squaredhole strange exagon> -> $polywanna {

   say "$polywanna";
   my @poly = %poly{$polywanna}[];
   for @test-points -> @point {

say "\t(@point.fmt('%.1f',','))\t{ point-in-poly(@point, @poly) ?? 'IN' !! 'OUT' }";

}</lang>

Output:

squared
	(5.0,5.0)	IN
	(5.0,8.0)	IN
	(-10.0,5.0)	OUT
	(0.0,5.0)	OUT
	(10.0,5.0)	IN
	(8.0,5.0)	IN
	(10.0,10.0)	OUT
squaredhole
	(5.0,5.0)	OUT
	(5.0,8.0)	IN
	(-10.0,5.0)	OUT
	(0.0,5.0)	OUT
	(10.0,5.0)	IN
	(8.0,5.0)	IN
	(10.0,10.0)	OUT
strange
	(5.0,5.0)	IN
	(5.0,8.0)	OUT
	(-10.0,5.0)	OUT
	(0.0,5.0)	OUT
	(10.0,5.0)	IN
	(8.0,5.0)	IN
	(10.0,10.0)	OUT
exagon
	(5.0,5.0)	IN
	(5.0,8.0)	IN
	(-10.0,5.0)	OUT
	(0.0,5.0)	OUT
	(10.0,5.0)	IN
	(8.0,5.0)	IN
	(10.0,10.0)	OUT

PicoLisp

<lang PicoLisp>(scl 4)

(de intersects (Px Py Ax Ay Bx By)

  (when (> Ay By)
     (xchg 'Ax 'Bx)
     (xchg 'Ay 'By) )
  (when (or (= Py Ay) (= Py By))
     (inc 'Py) )
  (and
     (>= Py Ay)
     (>= By Py)
     (>= (max Ax Bx) Px)
     (or
        (> (min Ax Bx) Px)
        (= Ax Px)
        (and
           (<> Ax Bx)
           (>=
              (*/ (- Py Ay) 1.0 (- Px Ax))            # Blue
              (*/ (- By Ay) 1.0 (- Bx Ax)) ) ) ) ) )  # Red

(de inside (Pt Poly)

  (let Res NIL
     (for Edge Poly
        (when (apply intersects Edge (car Pt) (cdr Pt))
           (onOff Res) ) )
     Res ) )</lang>

Test data:

(de Square
   ( 0.0  0.0  10.0  0.0)
   (10.0  0.0  10.0 10.0)
   (10.0 10.0   0.0 10.0)
   ( 0.0 10.0   0.0  0.0) )

(de SquareHole
   ( 0.0  0.0  10.0  0.0)
   (10.0  0.0  10.0 10.0)
   (10.0 10.0   0.0 10.0)
   ( 0.0 10.0   0.0  0.0)
   ( 2.5  2.5   7.5  2.5)
   ( 7.5  2.5   7.5  7.5)
   ( 7.5  7.5   2.5  7.5)
   ( 2.5  7.5   2.5  2.5) )

(de Strange
   ( 0.0  0.0   2.5  2.5)
   ( 2.5  2.5   0.0 10.0)
   ( 0.0 10.0   2.5  7.5)
   ( 2.5  7.5   7.5  7.5)
   ( 7.5  7.5  10.0 10.0)
   (10.0 10.0  10.0  0.0)
   (10.0  0.0   2.5  2.5) )

(de Exagon
   ( 3.0  0.0   7.0  0.0)
   ( 7.0  0.0  10.0  5.0)
   (10.0  5.0   7.0 10.0)
   ( 7.0 10.0   3.0 10.0)
   ( 3.0 10.0   0.0  5.0)
   ( 0.0  5.0   3.0  0.0) )

Output:

: (inside (5.0 . 5.0) Square)
-> T
: (inside (5.0 . 8.0) Square)
-> T
: (inside (-10.0 . 5.0) Square)
-> NIL
: (inside (0.0 . 5.0) Square)
-> NIL
: (inside (10.0 . 5.0) Square)
-> T
: (inside (8.0 . 5.0) Square)
-> T
: (inside (10.0 . 10.0) Square)
-> NIL

: (inside (5.0 . 5.0) SquareHole)
-> NIL
: (inside (5.0 . 8.0) SquareHole)
-> T
: (inside (-10.0 . 5.0) SquareHole)
-> NIL
: (inside (0 . 5.0) SquareHole)
-> NIL
: (inside (10.0 . 5.0) SquareHole)
-> T
: (inside (8.0 . 5.0) SquareHole)
-> T
: (inside (10.0 . 10.0) SquareHole)
-> NIL

: (inside (5.0 . 5.0) Strange)
-> T
: (inside (5.0 . 8.0) Strange)
-> NIL
: (inside (-10.0 . 5.0) Strange)
-> NIL
: (inside (0 . 5.0) Strange)
-> NIL
: (inside (10.0 . 5.0) Strange)
-> T
: (inside (8.0 . 5.0) Strange)
-> T
: (inside (10.0 . 10.0) Strange)
-> NIL

: (inside (5.0 . 5.0) Exagon)
-> T
: (inside (5.0 . 8.0) Exagon)
-> T
: (inside (-10.0 . 5.0) Exagon)
-> NIL
: (inside (0.0 . 5.0) Exagon)
-> NIL
: (inside (10.0 . 5.0) Exagon)
-> T
: (inside (8.0 . 5.0) Exagon)
-> T
: (inside (10.0 . 10.0) Exagon)
-> NIL

PureBasic

The code below is includes a GUI for drawing a polygon with the mouse that constantly tests whether the mouse is inside or outside the polygon. It displays a message and changes the windows color slightly to indicate if the pointer is inside or outside the polygon being drawn. The routine that does the checking is called inpoly() and it returns a value of one if the point is with the polygon and zero if it isn't. <lang PureBasic>Structure point_f

 x.f
 y.f

EndStructure Procedure inpoly(*p.point_f, List poly.point_f())

 Protected.point_f new, old, lp, rp
 Protected inside
 If ListSize(poly()) < 3: ProcedureReturn 0: EndIf 
 LastElement(poly()): old = poly()
 ForEach poly()
   ;find leftmost endpoint 'lp' and the rightmost endpoint 'rp' based on x value
   If poly()\x > old\x 
     lp = old
     rp = poly()
   Else
     lp = poly()
     rp = old
   EndIf 
   If lp\x < *p\x And *p\x <= rp\x And (*p\y - lp\y) * (rp\x - lp\x) < (rp\y - lp\y) * (*p\x - lp\x)
     inside = ~inside
   EndIf 
   old = poly()
 Next 
 ProcedureReturn inside & 1

EndProcedure

If InitSprite()

 If InitKeyboard() And InitMouse()
   OpenWindow(0, 0, 0, 800, 600, "Press [Esc] to close, [Left mouse button] Add Point, [Right mouse button] Clear All Points.", #PB_Window_ScreenCentered | #PB_Window_SystemMenu)
   OpenWindowedScreen(WindowID(0), 0, 0, 800, 600, 1, 0, 0)
   SetFrameRate(60)
 EndIf

Else

 MessageRequester("", "Unable to initsprite"): End

EndIf

NewList v.point_f() Define.point_f pvp, mp Define Col, EventID, mode.b, modetxt.s Repeat

 Delay(1)
 EventID = WindowEvent()
 ExamineKeyboard()
 ExamineMouse()
 ClearScreen(Col)
 
 mp\x = MouseX()
 mp\y = MouseY()
 If MouseButton(#PB_MouseButton_Left)
   AddElement(v())
   v()\x = mp\x
   v()\y = mp\y
   Delay(100)
 EndIf
 
 If MouseButton(#PB_MouseButton_Right)
   ClearList(v())
   Delay(100)
 EndIf
 
 StartDrawing(ScreenOutput())
   If LastElement(v())
     pvp = v()
     ForEach v()
       LineXY(pvp\x, pvp\y, v()\x, v()\y, RGB(0, $FF, 0)) ;Green
       Circle(pvp\x, pvp\y, 5, RGB($FF, 0, 0)) ;Red
       pvp = v()
     Next
   EndIf 
   Circle(MouseX(), MouseY(), 5, RGB($C0, $C0, $FF)) ;LightBlue

   If inpoly(mp, v())
     modetxt = "You are in the polygon."
     Col = RGB(0, 0, 0)
   Else
     modetxt = "You are not in the polygon."
     Col = RGB($50, $50, $50)
   EndIf
   DrawText((800 - TextWidth(modetxt)) / 2, 0, modetxt) 
 StopDrawing()
 
 FlipBuffers()

Until KeyboardReleased(#PB_Key_Escape) Or EventID = #PB_Event_CloseWindow</lang>

Python

<lang python>from collections import namedtuple from pprint import pprint as pp import sys

Pt = namedtuple('Pt', 'x, y') # Point Edge = namedtuple('Edge', 'a, b') # Polygon edge from a to b Poly = namedtuple('Poly', 'name, edges') # Polygon

_eps = 0.00001 _huge = sys.float_info.max _tiny = sys.float_info.min

def rayintersectseg(p, edge):

    takes a point p=Pt() and an edge of two endpoints a,b=Pt() of a line segment returns boolean
   
   a,b = edge
   if a.y > b.y:
       a,b = b,a
   if p.y == a.y or p.y == b.y:
       p = Pt(p.x, p.y + _eps)

   intersect = False

   if (p.y > b.y or p.y < a.y) or (
       p.x > max(a.x, b.x)):
       return False

   if p.x < min(a.x, b.x):
       intersect = True
   else:
       if abs(a.x - b.x) > _tiny:
           m_red = (b.y - a.y) / float(b.x - a.x)
       else:
           m_red = _huge
       if abs(a.x - p.x) > _tiny:
           m_blue = (p.y - a.y) / float(p.x - a.x)
       else:
           m_blue = _huge
       intersect = m_blue >= m_red
   return intersect

def _odd(x): return x%2 == 1

def ispointinside(p, poly):

   ln = len(poly)
   return _odd(sum(rayintersectseg(p, edge)
                   for edge in poly.edges ))

def polypp(poly):

   print "\n  Polygon(name='%s', edges=(" % poly.name
   print '   ', ',\n    '.join(str(e) for e in poly.edges) + '\n    ))'

if __name__ == '__main__':

   polys = [
     Poly(name='square', edges=(
       Edge(a=Pt(x=0, y=0), b=Pt(x=10, y=0)),
       Edge(a=Pt(x=10, y=0), b=Pt(x=10, y=10)),
       Edge(a=Pt(x=10, y=10), b=Pt(x=0, y=10)),
       Edge(a=Pt(x=0, y=10), b=Pt(x=0, y=0))
       )),
     Poly(name='square_hole', edges=(
       Edge(a=Pt(x=0, y=0), b=Pt(x=10, y=0)),
       Edge(a=Pt(x=10, y=0), b=Pt(x=10, y=10)),
       Edge(a=Pt(x=10, y=10), b=Pt(x=0, y=10)),
       Edge(a=Pt(x=0, y=10), b=Pt(x=0, y=0)),
       Edge(a=Pt(x=2.5, y=2.5), b=Pt(x=7.5, y=2.5)),
       Edge(a=Pt(x=7.5, y=2.5), b=Pt(x=7.5, y=7.5)),
       Edge(a=Pt(x=7.5, y=7.5), b=Pt(x=2.5, y=7.5)),
       Edge(a=Pt(x=2.5, y=7.5), b=Pt(x=2.5, y=2.5))
       )),
     Poly(name='strange', edges=(
       Edge(a=Pt(x=0, y=0), b=Pt(x=2.5, y=2.5)),
       Edge(a=Pt(x=2.5, y=2.5), b=Pt(x=0, y=10)),
       Edge(a=Pt(x=0, y=10), b=Pt(x=2.5, y=7.5)),
       Edge(a=Pt(x=2.5, y=7.5), b=Pt(x=7.5, y=7.5)),
       Edge(a=Pt(x=7.5, y=7.5), b=Pt(x=10, y=10)),
       Edge(a=Pt(x=10, y=10), b=Pt(x=10, y=0)),
       Edge(a=Pt(x=10, y=0), b=Pt(x=2.5, y=2.5))
       )),
     Poly(name='exagon', edges=(
       Edge(a=Pt(x=3, y=0), b=Pt(x=7, y=0)),
       Edge(a=Pt(x=7, y=0), b=Pt(x=10, y=5)),
       Edge(a=Pt(x=10, y=5), b=Pt(x=7, y=10)),
       Edge(a=Pt(x=7, y=10), b=Pt(x=3, y=10)),
       Edge(a=Pt(x=3, y=10), b=Pt(x=0, y=5)),
       Edge(a=Pt(x=0, y=5), b=Pt(x=3, y=0))
       )),
     ]
   testpoints = (Pt(x=5, y=5), Pt(x=5, y=8),
                 Pt(x=-10, y=5), Pt(x=0, y=5),
                 Pt(x=10, y=5), Pt(x=8, y=5),
                 Pt(x=10, y=10))
   
   print "\n TESTING WHETHER POINTS ARE WITHIN POLYGONS"
   for poly in polys:
       polypp(poly)
       print '   ', '\t'.join("%s: %s" % (p, ispointinside(p, poly))
                              for p in testpoints[:3])
       print '   ', '\t'.join("%s: %s" % (p, ispointinside(p, poly))
                              for p in testpoints[3:6])
       print '   ', '\t'.join("%s: %s" % (p, ispointinside(p, poly))
                              for p in testpoints[6:])</lang>

Sample output

 TESTING WHETHER POINTS ARE WITHIN POLYGONS

  Polygon(name='square', edges=(
    Edge(a=Pt(x=0, y=0), b=Pt(x=10, y=0)),
    Edge(a=Pt(x=10, y=0), b=Pt(x=10, y=10)),
    Edge(a=Pt(x=10, y=10), b=Pt(x=0, y=10)),
    Edge(a=Pt(x=0, y=10), b=Pt(x=0, y=0))
    ))
    Pt(x=5, y=5): True	Pt(x=5, y=8): True	Pt(x=-10, y=5): False
    Pt(x=0, y=5): False	Pt(x=10, y=5): True	Pt(x=8, y=5): True
    Pt(x=10, y=10): False

  Polygon(name='square_hole', edges=(
    Edge(a=Pt(x=0, y=0), b=Pt(x=10, y=0)),
    Edge(a=Pt(x=10, y=0), b=Pt(x=10, y=10)),
    Edge(a=Pt(x=10, y=10), b=Pt(x=0, y=10)),
    Edge(a=Pt(x=0, y=10), b=Pt(x=0, y=0)),
    Edge(a=Pt(x=2.5, y=2.5), b=Pt(x=7.5, y=2.5)),
    Edge(a=Pt(x=7.5, y=2.5), b=Pt(x=7.5, y=7.5)),
    Edge(a=Pt(x=7.5, y=7.5), b=Pt(x=2.5, y=7.5)),
    Edge(a=Pt(x=2.5, y=7.5), b=Pt(x=2.5, y=2.5))
    ))
    Pt(x=5, y=5): False	Pt(x=5, y=8): True	Pt(x=-10, y=5): False
    Pt(x=0, y=5): False	Pt(x=10, y=5): True	Pt(x=8, y=5): True
    Pt(x=10, y=10): False

  Polygon(name='strange', edges=(
    Edge(a=Pt(x=0, y=0), b=Pt(x=2.5, y=2.5)),
    Edge(a=Pt(x=2.5, y=2.5), b=Pt(x=0, y=10)),
    Edge(a=Pt(x=0, y=10), b=Pt(x=2.5, y=7.5)),
    Edge(a=Pt(x=2.5, y=7.5), b=Pt(x=7.5, y=7.5)),
    Edge(a=Pt(x=7.5, y=7.5), b=Pt(x=10, y=10)),
    Edge(a=Pt(x=10, y=10), b=Pt(x=10, y=0)),
    Edge(a=Pt(x=10, y=0), b=Pt(x=2.5, y=2.5))
    ))
    Pt(x=5, y=5): True	Pt(x=5, y=8): False	Pt(x=-10, y=5): False
    Pt(x=0, y=5): False	Pt(x=10, y=5): True	Pt(x=8, y=5): True
    Pt(x=10, y=10): False

  Polygon(name='exagon', edges=(
    Edge(a=Pt(x=3, y=0), b=Pt(x=7, y=0)),
    Edge(a=Pt(x=7, y=0), b=Pt(x=10, y=5)),
    Edge(a=Pt(x=10, y=5), b=Pt(x=7, y=10)),
    Edge(a=Pt(x=7, y=10), b=Pt(x=3, y=10)),
    Edge(a=Pt(x=3, y=10), b=Pt(x=0, y=5)),
    Edge(a=Pt(x=0, y=5), b=Pt(x=3, y=0))
    ))
    Pt(x=5, y=5): True	Pt(x=5, y=8): True	Pt(x=-10, y=5): False
    Pt(x=0, y=5): False	Pt(x=10, y=5): True	Pt(x=8, y=5): True
    Pt(x=10, y=10): False

Helper routine to convert Fortran Polygons and points to Python <lang python>def _convert_fortran_shapes():

   point = Pt
   pts = (point(0,0), point(10,0), point(10,10), point(0,10), 
          point(2.5,2.5), point(7.5,2.5), point(7.5,7.5), point(2.5,7.5), 
          point(0,5), point(10,5), 
          point(3,0), point(7,0), point(7,10), point(3,10))
   p = (point(5,5), point(5, 8), point(-10, 5), point(0,5), point(10,5),
        point(8,5), point(10,10) )

   def create_polygon(pts,vertexindex):
       return [tuple(Edge(pts[vertexindex[i]-1], pts[vertexindex[i+1]-1])
                      for i in range(0, len(vertexindex), 2) )]
   polys=[]
   polys += create_polygon(pts, ( 1,2, 2,3, 3,4, 4,1 ) )
   polys += create_polygon(pts, ( 1,2, 2,3, 3,4, 4,1, 5,6, 6,7, 7,8, 8,5 ) )
   polys += create_polygon(pts, ( 1,5, 5,4, 4,8, 8,7, 7,3, 3,2, 2,5 ) )
   polys += create_polygon(pts, ( 11,12, 12,10, 10,13, 13,14, 14,9, 9,11 ) )

   names = ( "square", "square_hole", "strange", "exagon" )
   polys = [Poly(name, edges)
            for name, edges in zip(names, polys)]
   print 'polys = ['
   for p in polys:
       print "  Poly(name='%s', edges=(" % p.name
       print '   ', ',\n    '.join(str(e) for e in p.edges) + '\n    )),'
   print '  ]'
_convert_fortran_shapes()</lang>

R

<lang R>point_in_polygon <- function(polygon, p) {

 count <- 0
 for(side in polygon) {
   if ( ray_intersect_segment(p, side) ) {
     count <- count + 1
   }
 }
 if ( count %% 2 == 1 )
   "INSIDE"
 else
   "OUTSIDE"

}

ray_intersect_segment <- function(p, side) {

 eps <- 0.0001
 a <- side$A
 b <- side$B
 if ( a$y > b$y ) {
   a <- side$B
   b <- side$A
 }
 if ( (p$y == a$y) || (p$y == b$y) ) {
   p$y <- p$y + eps
 }
 if ( (p$y < a$y) || (p$y > b$y) )
   return(FALSE)
 else if ( p$x > max(a$x, b$x) )
   return(FALSE)
 else {
   if ( p$x < min(a$x, b$x) )
     return(TRUE)
   else {
     if ( a$x != b$x )
       m_red <- (b$y - a$y) / (b$x - a$x)
     else
       m_red <- Inf
     if ( a$x != p$x )
       m_blue <- (p$y - a$y) / (p$x - a$x)
     else
       m_blue <- Inf
     return( m_blue >= m_red )
   }
 }

}</lang>

<lang R>######## utility functions #########

point <- function(x,y) list(x=x, y=y)

pts = list(p1, p2, ... )... coords
segs = list(c(1,2), c(2,1) ...) indices

createPolygon <- function(pts, segs) {

 pol <- list()
 for(pseg in segs) {
   pol <- c(pol, list(list(A=pts[[pseg[1]]], B=pts[[pseg[2]]])))
 }
 pol

}</lang>

<lang R>#### testing ####

pts <- list(point(0,0), point(10,0), point(10,10), point(0,10),

           point(2.5,2.5), point(7.5,2.5), point(7.5,7.5), point(2.5,7.5), 
           point(0,5), point(10,5), 
           point(3,0), point(7,0), point(7,10), point(3,10))

polygons <-

 list(
      square = createPolygon(pts, list(c(1,2), c(2,3), c(3,4), c(4,1))),
      squarehole = createPolygon(pts, list(c(1,2), c(2,3), c(3,4), c(4,1), c(5,6), c(6,7), c(7,8), c(8,5))),
      exagon = createPolygon(pts, list(c(11,12), c(12,10), c(10,13), c(13,14), c(14,9), c(9,11)))
     )

testpoints <-

 list(
      point(5,5), point(5, 8), point(-10, 5), point(0,5), point(10,5),
      point(8,5), point(9.9,9.9)
     )

for(p in testpoints) {

 for(polysi in 1:length(polygons)) {
   cat(sprintf("point (%lf, %lf) is %s polygon (%s)\n",
                 p$x, p$y, point_in_polygon(polygonspolysi, p), names(polygons[polysi])))
 }

}</lang>

Racket

Straightforward implementation of pseudocode <lang racket>

lang racket

(module pip racket

 (require racket/contract)
 
 (provide point)
 (provide seg)
 (provide (contract-out [point-in-polygon? (-> 
                                            point? 
                                            list? 
                                            boolean?)]))

 (struct point (x y) #:transparent)
 (struct seg (Ax Ay Bx By))
 (define ε 0.000001)
 (define (neq? x y) (not (eq? x y)))

 (define (ray-cross-seg? r s)
   (let* ([Ax (seg-Ax s)] [Ay (seg-Ay s)]
          [Bx (seg-Bx s)] [By (seg-By s)]
          [Px (point-x r)] [Pyo (point-y r)]
          [Py (+ Pyo (if (or (eq? Pyo Ay) 
                             (eq? Pyo By)) 
                         ε 0))])
     
     (cond [(or (< Py Ay) (> Py By)) #f]
           [(> Px (max Ax Bx)) #f]
           [(< Px (min Ax Bx)) #t]
           [else
            (let ([red (if (neq? Ax Px)
                           (/ (- By Ay) (- Bx Ax))
                           +inf.0)]
                  [blue (if (neq? Ax Px)
                            (/ (- Py Ax) (- Px Ax))
                            +inf.0)])
              (if (>= blue red) #t #f))])))

 (define (point-in-polygon? point polygon)
   (odd? 
    (for/fold ([c 0]) ([seg polygon])
      (+ c (if (ray-cross-seg? point seg) 1 0))))))

(require 'pip)

(define test-point-list

 (list
  (point 5.0    5.0) 
  (point 5.0    8.0) 
  (point -10.0  5.0) 
  (point  0.0   5.0) 
  (point 10.0   5.0) 
  (point  8.0   5.0) 
  (point 10.0  10.0)))

(define square

 (list (seg 0.0   0.0  10.0   0.0) 
       (seg 10.0  0.0  10.0  10.0) 
       (seg 10.0  10.0  0.0  10.0) 
       (seg 0.0   0.0  0.0   10.0)))

(define exagon

 (list (seg  3.0   0.0   7.0   0.0) 
       (seg  7.0   0.0  10.0   5.0) 
       (seg 10.0   5.0   7.0  10.0) 
       (seg  7.0  10.0   3.0  10.0) 
       (seg  0.0   5.0   3.0   10.0) 
       (seg  3.0   0.0 0.0   5.0)))

(define (test-figure fig name)

 (printf "\ntesting ~a: \n" name)
 (for ([p test-point-list])
   (printf "testing ~v: ~a\n"  p (point-in-polygon? p fig))))

(test-figure square "square") (test-figure exagon "exagon") </lang>

Output:

testing square: 
testing (point 5.0 5.0): #t
testing (point 5.0 8.0): #t
testing (point -10.0 5.0): #f
testing (point 0.0 5.0): #f
testing (point 10.0 5.0): #t
testing (point 8.0 5.0): #t
testing (point 10.0 10.0): #f

testing exagon: 
testing (point 5.0 5.0): #t
testing (point 5.0 8.0): #t
testing (point -10.0 5.0): #f
testing (point 0.0 5.0): #f
testing (point 10.0 5.0): #t
testing (point 8.0 5.0): #t
testing (point 10.0 10.0): #f

REXX

Over half of the REXX program is devoted to specifing/defining/assigning the points for
the test cases and the polygons.
Code was added to facilitate easier specification of polygon sides by
just specifying their vertices instead of specifying line segments.

Points on a vertex (or side) don't obtain coherent results, but casual observation
seems to indicate that this program considers those points as outside the polygon. <lang rexx>/*REXX program to see if a horizontal ray from pt P intersects a polygon*/ call points 5 5, 5 8, -10 5, 0 5, 10 5, 8 5, 10 10 call polygon 0 0, 10 0, 10 10, 0 10 ; call test 'square' call polygon 0 0, 10 0, 10 10, 0 10, 2.5 2.5, 7.5 2.5, 7.5 7.5, 2.5 7.5 ; call test 'square hole' call polygon 0 0, 2.5 2.5, 0 10, 2.5 7.5, 7.5 7.5, 10 10, 10 0 ; call test 'irregular' call polygon 3 0, 7 0, 10 5, 7 10, 3 10, 0 5 ; call test 'exagon' exit /*stick a fork in it, we're done.*/ /*──────────────────────────────────IN_OUT subroutine────────────────────*/ in_out: procedure expose point. poly. /*note: // is division remainder.*/ parse arg p; #=0; do side=1 to poly.0 by 2; #=#+ray_intersect(p,side)

                     end   /*side*/

return #//2 /*odd=inside, return 1; else 0.*/ /*──────────────────────────────────POINTS subroutine───────────────────*/ points: n=0; v='POINT.'; do j=1 for arg(); n=n+1

                              call  value v||n'.X', word(arg(j),1)
                              call  value v||n'.Y', word(arg(j),2)
                              end   /*j*/

call value v'0',n /*define number of points.*/ return /*──────────────────────────────────POLYGON subroutine──────────────────*/ polygon: n=0; v='POLY.'; parse arg Fx Fy

         do j=1  for arg();   _=arg(j);    n=n+1
         call value v||n'.X', word(_,1);   call value v||n'.Y', word(_,2)
         if n//2  then iterate
         n=n+1
         call value v||n'.X', word(_,1);   call value v||n'.Y', word(_,2)
         end   /*j*/

n=n+1 call value v||n".X", Fx; call value v||n".Y", Fy; call value v'0',n return /*POLY.0 is # of segments/sides.*/ /*──────────────────────────────────RAY_INTERSECT subroutine────────────*/ ray_intersect: procedure expose point. poly.; parse arg ?,s; sp=s+1 epsilon = '1e'||(digits()%2); infinity = '1e'||(digits()*2) Px=point.?.x; Py=point.?.y Ax=poly.s.x; Bx=poly.sp.x ; Ay=poly.s.y; By=poly.sp.y if Ay>By then parse value Ax Ay Bx By with Bx By Ax Ay if Py=Ay | Py=By then Py=Py+epsilon if Py<Ay | Py>By | Px>max(Ax,Bx) then return 0 if Px<min(Ax,Bx) then return 1 if Ax\=Bx then m_red = (By-Ay) / (Bx-Ax); else m_red = infinity if Ax\=Px then m_blue = (Py-Ay) / (Px-Ax); else return 1 return m_blue>=m_red /*──────────────────────────────────TEST procedure──────────────────────*/ test: say; do k=1 for point.0 /*traipse through each test point*/

           say '  ['arg(1)"]  point:"   right(point.k.x','point.k.y, 9),
               "  is  "     word('outside inside', in_out(k)+1)
           end   /*k*/

return</lang> output

  [square]  point:       5,5   is   inside
  [square]  point:       5,8   is   inside
  [square]  point:     -10,5   is   outside
  [square]  point:       0,5   is   outside
  [square]  point:      10,5   is   inside
  [square]  point:       8,5   is   inside
  [square]  point:     10,10   is   outside

  [square hole]  point:       5,5   is   outside
  [square hole]  point:       5,8   is   inside
  [square hole]  point:     -10,5   is   outside
  [square hole]  point:       0,5   is   outside
  [square hole]  point:      10,5   is   inside
  [square hole]  point:       8,5   is   inside
  [square hole]  point:     10,10   is   outside

  [irregular]  point:       5,5   is   inside
  [irregular]  point:       5,8   is   outside
  [irregular]  point:     -10,5   is   outside
  [irregular]  point:       0,5   is   outside
  [irregular]  point:      10,5   is   inside
  [irregular]  point:       8,5   is   inside
  [irregular]  point:     10,10   is   outside

  [exagon]  point:       5,5   is   outside
  [exagon]  point:       5,8   is   inside
  [exagon]  point:     -10,5   is   outside
  [exagon]  point:       0,5   is   outside
  [exagon]  point:      10,5   is   outside
  [exagon]  point:       8,5   is   outside
  [exagon]  point:     10,10   is   outside

Scala

From D snippet. <lang scala> case class Figure(name: String, edges: ((Double, Double), (Double, Double))*) {}

object Ray_casting extends App {

   import Math._
   import Double._

   val figures = Array(Figure("Square", ((0.0, 0.0), (10.0, 0.0)),
       ((10.0, 0.0), (10.0, 10.0)), ((10.0, 10.0), (0.0, 10.0)),
       ((0.0, 10.0), (0.0, 0.0))),
       Figure("Square hole", ((0.0, 0.0), (10.0, 0.0)), ((10.0, 0.0), (10.0, 10.0)),
           ((10.0, 10.0), (0.0, 10.0)), ((0.0, 10.0), (0.0, 0.0)),
           ((2.5, 2.5), (7.5, 2.5)), ((7.5, 2.5), (7.5, 7.5)),
           ((7.5, 7.5), (2.5, 7.5)), ((2.5, 7.5), (2.5, 2.5))),
       Figure("Strange", ((0.0, 0.0), (2.5, 2.5)), ((2.5, 2.5), (0.0, 10.0)),
           ((0.0, 10.0), (2.5, 7.5)), ((2.5, 7.5), (7.5, 7.5)),
           ((7.5, 7.5), (10.0, 10.0)), ((10.0, 10.0), (10.0, 0.0)),
           ((10.0, 0), (2.5, 2.5))),
       Figure("Exagon", ((3.0, 0.0), (7.0, 0.0)), ((7.0, 0.0), (10.0, 5.0)),
           ((10.0, 5.0), (7.0, 10.0)), ((7.0, 10.0), (3.0, 10.0)),
           ((3.0, 10.0), (0.0, 5.0)), ((0.0, 5.0), (3.0, 0.0))))

   val points = Array((5.0, 5.0), (5.0, 8.0), (-10.0, 5.0), (0.0, 5.0), (10.0, 5.0), (8.0, 5.0), (10.0, 10.0))

   figures foreach { f =>
       println("Is point inside figure " + f.name + '?')
       points foreach { p => println("  " + p + ": " + contains(f, p)) }
       println
   }

   private def raySegI(p: (Double, Double), e: ((Double, Double), (Double, Double))): Boolean = {
       val epsilon = 0.00001
       if (e._1._2 > e._2._2)
           return raySegI(p, (e._2, e._1))
       if (p._2 == e._1._2 || p._2 == e._2._2)
           return raySegI((p._1, p._2 + epsilon), e)
       if (p._2 > e._2._2 || p._2 < e._1._2 || p._1 > max(e._1._1, e._2._1))
           return false
       if (p._1 < min(e._1._1, e._2._1))
           return true
       val blue = if (abs(e._1._1 - p._1) > MinValue) (p._2 - e._1._2) / (p._1 - e._1._1) else MaxValue
       val red = if (abs(e._1._1 - e._2._1) > MinValue) (e._2._2 - e._1._2) / (e._2._1 - e._1._1) else MaxValue
       blue >= red
   }

   private def contains(f: Figure, p: (Double, Double)) = f.edges.count(raySegI(p, _)) % 2 != 0

}</lang>

Smalltalk

Works with: GNU Smalltalk

The class Segment holds the code to test if a ray starting from a point (and going towards "right") intersects the segment (method doesIntersectRayFrom); while the class Polygon hosts the code to test if a point is inside the polygon (method pointInside). <lang smalltalk>Object subclass: Segment [

   |pts|
   Segment class >> new: points [ |a|
     a := super new.
     ^ a init: points
   ]
   init: points [ pts := points copy. ^self ]
   endPoints [ ^pts ]
   "utility methods"
   first [ ^ pts at: 1]
   second [ ^ pts at: 2]
   leftmostEndPoint [ 
     ^ (self first x > self second x) ifTrue: [ self second ] ifFalse: [ self first ]
   ]
   rightmostEndPoint [
     ^ (self first x > self second x) ifTrue: [ self first ] ifFalse: [ self second ] 
   ]
   topmostEndPoint [
     ^ (self first y > self second y) ifTrue: [ self first ] ifFalse: [ self second ]
   ]
   bottommostEndPoint [
     ^ (self first y > self second y) ifTrue: [ self second ] ifFalse: [ self first ]
   ]

   slope [
     (pts at: 1) x ~= (pts at: 2) x
     ifTrue: [ ^ ((pts at: 1) y - (pts at: 2) y) / ((pts at: 1) x - (pts at: 2) x) ]
     ifFalse: [ ^ FloatD infinity ]
   ]

   doesIntersectRayFrom: point [ |p A B|
     (point y = (pts at: 1) y) | (point y = (pts at: 2) y)
     ifTrue: [ p := Point x: (point x) y: (point y) + 0.00001 ]
     ifFalse: [ p := point copy ].
     A := self bottommostEndPoint.
     B := self topmostEndPoint.
     (p y < A y) | (p y > B y) | (p x > (self rightmostEndPoint x))
       ifTrue: [ ^false ]
       ifFalse: [ (p x < (self leftmostEndPoint x))
                    ifTrue: [ ^true ]
                    ifFalse: [ |s| 
                       s := Segment new: { A . point }.

(s slope) >= (self slope) ifTrue: [ ^ true ]

].

Object subclass: Polygon [

   |polysegs|
   Polygon class >> new [ |a| a := super new. ^ a init. ]
   Polygon class >> fromSegments: segments [ |a|
     a := super new.
     ^ a initWithSegments: segments
   ]
   Polygon class >> fromPoints: pts and: indexes [ |a|
     a := self new.
     indexes do: [ :i |
       a addSegment: ( Segment new: { pts at: (i at: 1) . pts at: (i at: 2) } )
     ].
     ^ a
   ]
   initWithSegments: segments [
     polysegs := segments copy. ^self
   ]
   init [ polysegs := OrderedCollection new. ^ self ]
   addSegment: segment [ polysegs add: segment ]
   
   pointInside: point [ |cnt|
     cnt := 0.
     polysegs do: [ :s | (s doesIntersectRayFrom: point)
                         ifTrue: [ cnt := cnt + 1 ] ].
     ^ ( cnt \\ 2 = 0 ) not
   ]

].</lang>

Testing <lang smalltalk>|points names polys|

points := {

          0@0 . 10@0 . 10@10 . 0@10 .
          2.5@2.5 . 7.5@2.5 . 7.5@7.5 .
          2.5@7.5 . 0@5 . 10@5 .
          3@0 . 7@0 . 7@10 . 3@10
         }.

names := { 'square' . 'square hole' . 'strange' . 'exagon' }.

polys := OrderedCollection new.

polys add:

     ( 
       Polygon fromPoints: points 
               and: { {1 . 2}. {2 . 3}. {3 . 4}. {4 . 1} }
     ) ;
     add:
     (
       Polygon fromPoints: points 
               and: { {1 . 2}. {2 . 3}. {3 . 4}. {4 . 1}. {5 . 6}. {6 . 7}. {7 . 8}. {8 . 5} }
     ) ;
     add:
     (
       Polygon fromPoints: points 
               and: { {1 . 5}. {5 . 4}. {4 . 8}. {8 . 7}. {7 . 3}. {3 . 2}. {2 . 5} }
     ) ;
     add:
     (
       Polygon fromPoints: points 
               and: { {11 . 12}. {12 . 10}. {10 . 13}. {13 . 14}. {14 . 9}. {9 . 11} }
     ).

{ 5@5 . 5@8 . -10@5 . 0@5 . 10@5 . 8@5 . 10@10 } do: [ :p |

 1 to: 4 do: [ :i |
  ('point %1 inside %2? %3' %
    { p . names at: i. (polys at: i) pointInside: p }) displayNl
 ].
 ' ' displayNl.

]</lang>

Tcl

<lang Tcl>package require Tcl 8.5

proc point_in_polygon {point polygon} {

   set count 0
   foreach side [sides $polygon] {
       if {[ray_intersects_line $point $side]} {
           incr count
       }
   }
   expr {$count % 2} ;#-- 1 = odd = true, 0 = even = false

} proc sides polygon {

   lassign $polygon x0 y0
   foreach {x y} [lrange [lappend polygon $x0 $y0] 2 end] {
       lappend res [list $x0 $y0 $x $y]
       set x0 $x
       set y0 $y
   }
   return $res

} proc ray_intersects_line {point line} {

   lassign $point Px Py
   lassign $line Ax Ay Bx By
   # Reverse line direction if necessary
   if {$By < $Ay} {

lassign $line Bx By Ax Ay

   }
   # Add epsilon to 
   if {$Py == $Ay || $Py == $By} {

set Py [expr {$Py + abs($Py)/1e6}]

   }
   # Bounding box checks
   if {$Py < $Ay || $Py > $By || $Px > max($Ax,$Bx)} {

return 0

   } elseif {$Px < min($Ax,$Bx)} {

return 1

   }
   # Compare dot products to compare (cosines of) angles
   set mRed [expr {$Ax != $Bx ? ($By-$Ay)/($Bx-$Ax) : Inf}]
   set mBlu [expr {$Ax != $Px ? ($Py-$Ay)/($Px-$Ax) : Inf}]
   return [expr {$mBlu >= $mRed}]

}

foreach {point poly} {

   {0 0}	{-1 -1  -1 1  1 1  1 -1}
   {2 2}	{-1 -1  -1 1  1 1  1 -1}
   {0 0}	{-2 -2  -2 2  2 2  2 -2   2 -1  1 1  -1 1  -1 -1  1 -1  2 -1}
   {1.5 1.5}	{-2 -2  -2 2  2 2  2 -2   2 -1  1 1  -1 1  -1 -1  1 -1  2 -1}
   {5 5}	{0 0  2.5 2.5  0 10  2.5 7.5  7.5 7.5  10 10  10 0  7.5 0.1}
   {5 8}	{0 0  2.5 2.5  0 10  2.5 7.5  7.5 7.5  10 10  10 0  7.5 0.1}
   {2 2}	{0 0  2.5 2.5  0 10  2.5 7.5  7.5 7.5  10 10  10 0  7.5 0.1}
   {0 0}	{0 0  2.5 2.5  0 10  2.5 7.5  7.5 7.5  10 10  10 0  7.5 0.1}
   {10 10}	{0 0  2.5 2.5  0 10  2.5 7.5  7.5 7.5  10 10  10 0  7.5 0.1}
   {2.5 2.5}	{0 0  2.5 2.5  0 10  2.5 7.5  7.5 7.5  10 10  10 0  7.5 0.1}
   {-5 5}	{3 0  7 0  10 5  7 10  3 10  0 5}

} {

   puts "$point in $poly = [point_in_polygon $point $poly]"

}</lang>

Ursala

This function takes a point $(x,y)$ and a polygon $\langle (x_{1},y_{1})\dots (x_{n},y_{n})\rangle$ to a true value if the point is enclosed by the polygon and a false value if it's outside, using the algorithm described above. For points on the boundary the result is unspecified. <lang Ursala>#import flo

in =

@lrzyCipPX ~|afatPRZaq ~&EZ+fleq~~lrPrbr2G&& ~&B+fleq~~lrPrbl2G!| -&

  ~&Y+ ~~lrPrbl2G fleq,
  ^E(fleq@lrrPX,@rl fleq\0.)^/~&lr ^(~&r,times)^/minus@llPrll2X vid+ minus~~rbbI&-</lang>

This test program tries it on a couple of examples. <lang Ursala>#cast %bL

examples =

in* <

  ((0.5,0.6),<(0.,0.),(1.,2.),(1.,0.)>),
  ((0.5,0.6),<(0.,0.),(1.,1.),(1.,0.)>)></lang>

output:

<true,false>

zkl

Translation of: Perl 6

<lang zkl>const E = 0.0001;

fcn rayHitsSeg([(Px,Py)],[(Ax,Ay)],[(Bx,By)]){ // --> Bool

  if(Py==Ay or Py==By) Py+=E;
  if(Py<Ay or Py>By or Px>Ax.max(Bx)) False
  else if(Px<Ax.min(Bx))              True
  else try{ ( (Py - Ay)/(Px - Ax) )>=( (By - Ay)/(Bx - Ax) ) } //blue>=red
       catch(MathError){ Px==Ax } // divide by zero == ∞, only blue?

}

fcn pointInPoly(point, polygon){ // --> Bool, polygon is ( (a,b),(c,d).. )

  polygon.filter('wrap([(ab,cd)]){ a,b:=ab; c,d:=cd;
     if(b<=d) rayHitsSeg(point,ab,cd); // left point has smallest y coordinate
     else     rayHitsSeg(point,cd,ab);
  })
  .len().isOdd; // True if crossed an odd number of borders ie inside polygon

}</lang> <lang zkl>polys:=T( //(name,( ((a,b),(c,d)),((a,b),(c,d))... ), ... )==(nm,(ln,ln..) ..)

   T("squared",

T(T(T( 0.0, 0.0), T(10.0, 0.0)), T(T(10.0, 0.0), T(10.0, 10.0)), T(T(10.0, 10.0), T( 0.0, 10.0)), T(T( 0.0, 10.0), T( 0.0, 0.0)))),

   T("squaredhole",

T(T(T( 0.0, 0.0), T(10.0, 0.0)), T(T(10.0, 0.0), T(10.0, 10.0)), T(T(10.0, 10.0), T( 0.0, 10.0)), T(T( 0.0, 10.0), T( 0.0, 0.0)), T(T( 2.5, 2.5), T( 7.5, 2.5)), T(T( 7.5, 2.5), T( 7.5, 7.5)), T(T( 7.5, 7.5), T( 2.5, 7.5)), T(T( 2.5, 7.5), T( 2.5, 2.5)))),

   T("strange",

T(T(T( 0.0, 0.0), T( 2.5, 2.5)), T(T( 2.5, 2.5), T( 0.0, 10.0)), T(T( 0.0, 10.0), T( 2.5, 7.5)), T(T( 2.5, 7.5), T( 7.5, 7.5)), T(T( 7.5, 7.5), T(10.0, 10.0)), T(T(10.0, 10.0), T(10.0, 0.0)), T(T(10.0, 0.0), T( 2.5, 2.5)), T(T( 2.5, 2.5), T( 0.0, 0.0)))), # conjecturally close polygon

   T("exagon",

T(T(T( 3.0, 0.0), T( 7.0, 0.0)), T(T( 7.0, 0.0), T(10.0, 5.0)), T(T(10.0, 5.0), T( 7.0, 10.0)), T(T( 7.0, 10.0), T( 3.0, 10.0)), T(T( 3.0, 10.0), T( 0.0, 5.0)), T(T( 0.0, 5.0), T( 3.0, 0.0)))), );

testPoints:=T( T( 5.0, 5.0), T( 5.0, 8.0), T(-10.0, 5.0), T( 0.0, 5.0), T( 10.0, 5.0), T( 8.0, 5.0), T( 10.0, 10.0) );

foreach name,polywanna in (polys){

  name.println();
  foreach testPoint in (testPoints){
     println("\t(%6.1f,%6.1f)\t".fmt(testPoint.xplode()),
        pointInPoly(testPoint,polywanna) and "IN" or "OUT");
  }

}</lang>

Output:

squared
	(   5.0,   5.0)	IN
	(   5.0,   8.0)	IN
	( -10.0,   5.0)	OUT
	(   0.0,   5.0)	OUT
	(  10.0,   5.0)	IN
	(   8.0,   5.0)	IN
	(  10.0,  10.0)	OUT
squaredhole
	(   5.0,   5.0)	OUT
	(   5.0,   8.0)	IN
	( -10.0,   5.0)	OUT
	(   0.0,   5.0)	OUT
	(  10.0,   5.0)	IN
	(   8.0,   5.0)	IN
	(  10.0,  10.0)	OUT
strange
	(   5.0,   5.0)	IN
	(   5.0,   8.0)	OUT
	( -10.0,   5.0)	OUT
	(   0.0,   5.0)	OUT
	(  10.0,   5.0)	IN
	(   8.0,   5.0)	IN
	(  10.0,  10.0)	OUT
exagon
	(   5.0,   5.0)	IN
	(   5.0,   8.0)	IN
	( -10.0,   5.0)	OUT
	(   0.0,   5.0)	OUT
	(  10.0,   5.0)	IN
	(   8.0,   5.0)	IN
	(  10.0,  10.0)	OUT

@@ Line 2,457: / Line 2,457: @@
 return</lang>
 '''output'''
-<pre style="height:50ex;overflow:scroll">
+<pre style="height:50ex">
   [square]  point:       5,5   is   inside
   [square]  point:       5,8   is   inside