Find if a point is within a triangle
You are encouraged to solve this task according to the task description, using any language you may know.
Find if a point is within a triangle.
- Task
- Assume points are on a plane defined by (x, y) real number coordinates.
- Given a point P(x, y) and a triangle formed by points A, B, and C, determine if P is within triangle ABC.
- You may use any algorithm.
- Bonus: explain why the algorithm you chose works.
- Related tasks
- Also see
11l
V EPS = 0.001
V EPS_SQUARE = EPS * EPS
F side(p1, p2, p)
R (p2.y - p1.y) * (p.x - p1.x) + (-p2.x + p1.x) * (p.y - p1.y)
F distanceSquarePointToSegment(p1, p2, p)
V p1P2SquareLength = sqlen(p2 - p1)
V dotProduct = dot(p - p1, p2 - p1) / p1P2SquareLength
I dotProduct < 0
R sqlen(p - p1)
I dotProduct <= 1
V pP1SquareLength = sqlen(p1 - p)
R pP1SquareLength - dotProduct * dotProduct * p1P2SquareLength
R sqlen(p - p2)
T Triangle((DVec2 p1, DVec2 p2, DVec2 p3))
F String()
R ‘Triangle[’(.p1)‘, ’(.p2)‘, ’(.p3)‘]’
F.const pointInTriangleBoundingBox(p)
V xMin = min(.p1.x, min(.p2.x, .p3.x)) - :EPS
V xMax = max(.p1.x, max(.p2.x, .p3.x)) + :EPS
V yMin = min(.p1.y, min(.p2.y, .p3.y)) - :EPS
V yMax = max(.p1.y, max(.p2.y, .p3.y)) + :EPS
R !(p.x < xMin | xMax < p.x | p.y < yMin | yMax < p.y)
F.const nativePointInTriangle(p)
V checkSide1 = side(.p1, .p2, p) >= 0
V checkSide2 = side(.p2, .p3, p) >= 0
V checkSide3 = side(.p3, .p1, p) >= 0
R checkSide1 & checkSide2 & checkSide3
F.const accuratePointInTriangle(p)
I !.pointInTriangleBoundingBox(p)
R 0B
I .nativePointInTriangle(p)
R 1B
I distanceSquarePointToSegment(.p1, .p2, p) <= :EPS_SQUARE
R 1B
I distanceSquarePointToSegment(.p2, .p3, p) <= :EPS_SQUARE
R 1B
R distanceSquarePointToSegment(.p3, .p1, p) <= :EPS_SQUARE
F test(t, p)
print(t)
print(‘Point ’p‘ is within triangle ? ’(I t.accuratePointInTriangle(p) {‘true’} E ‘false’))
V p1 = (1.5, 2.4)
V p2 = (5.1, -3.1)
V p3 = (-3.8, 1.2)
V tri = Triangle(p1, p2, p3)
test(tri, (0.0, 0.0))
test(tri, (0.0, 1.0))
test(tri, (3.0, 1.0))
print()
p1 = (1.0 / 10, 1.0 / 9)
p2 = (100.0 / 8, 100.0 / 3)
p3 = (100.0 / 4, 100.0 / 9)
tri = Triangle(p1, p2, p3)
V pt = (p1.x + 3.0 / 7 * (p2.x - p1.x), p1.y + 3.0 / 7 * (p2.y - p1.y))
test(tri, pt)
print()
p3 = (-100.0 / 8, 100.0 / 6)
tri = Triangle(p1, p2, p3)
test(tri, pt)- Output:
Triangle[(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0, 0) is within triangle ? true Triangle[(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0, 1) is within triangle ? true Triangle[(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (3, 1) is within triangle ? false Triangle[(0.1, 0.111111111), (12.5, 33.333333333), (25, 11.111111111)] Point (5.414285714, 14.349206349) is within triangle ? true Triangle[(0.1, 0.111111111), (12.5, 33.333333333), (-12.5, 16.666666667)] Point (5.414285714, 14.349206349) is within triangle ? true
Ada
This uses a determinant method to calculate the area of triangles, and tests whether or not a point is in a triangle by adding up the areas of the triangles formed by each side of the triangle with the point in question, and seeing if the sum matches the whole.
It uses a generic two-dimensional geometry, that could be affine, euclidean, or a lot stranger than that. You only need to specify the type of one dimension, and the library should handle the rest. Edge cases probably exist where they shouldn't, as the area formula might add some imprecision.
-- triangle.ads
generic
type Dimension is private;
Zero, Two: Dimension;
with function "*"(Left, Right: in Dimension) return Dimension is <>;
with function "/"(Left, Right: in Dimension) return Dimension is <>;
with function "+"(Left, Right: in Dimension) return Dimension is <>;
with function "-"(Left, Right: in Dimension) return Dimension is <>;
with function ">"(Left, Right: in Dimension) return Boolean is <>;
with function "="(Left, Right: in Dimension) return Boolean is <>;
with function Image(D: in Dimension) return String is <>;
package Triangle is
type Point is record
X: Dimension;
Y: Dimension;
end record;
type Triangle_T is record
A,B,C: Point;
end record;
function Area(T: in Triangle_T) return Dimension;
function IsPointInTriangle(P: Point; T: Triangle_T) return Boolean;
function Image(P: Point) return String is
("(X="&Image(P.X)&", Y="&Image(P.Y)&")")
with Inline;
function Image(T: Triangle_T) return String is
("(A="&Image(T.A)&", B="&Image(T.B)&", C="&Image(T.C)&")")
with Inline;
end;
-- triangle.adb
package body Triangle is
function Area(T: in Triangle_T) return Dimension
is
tmp: Dimension;
begin
tmp:=((T.B.X*T.C.Y-T.C.X*T.B.Y)-(T.A.X*T.C.Y-T.C.X*T.A.Y)+(T.A.X*T.B.Y-T.B.X*T.A.Y))/Two;
if tmp>Zero then
return tmp;
else
return Zero-tmp;
end if;
end Area;
function IsPointInTriangle(P: Point; T: Triangle_T) return Boolean
is
begin
return Area(T)=Area((T.A,T.B,P))+Area((T.A,P,T.C))+Area((P,T.B,T.C));
end IsPointInTriangle;
end;
-- test_triangle.adb
with Ada.Text_IO;
use Ada.Text_IO;
with Triangle;
procedure test_triangle
is
package affine_tri is new Triangle(Dimension=>Integer, Zero=>0,Two=>2, Image=>Integer'Image);
use affine_tri;
tri1: Triangle_T:=((1,0),(2,0),(0,2));
tri2: Triangle_T:=((-1,0),(-1,-1),(2,2));
origin: Point:=(0,0);
begin
Put_Line("IsPointInTriangle("&Image(origin)&", "&Image(tri1)&") yields "&IsPointInTriangle(origin,tri1)'Image);
Put_Line("IsPointInTriangle("&Image(origin)&", "&Image(tri2)&") yields "&IsPointInTriangle(origin,tri2)'Image);
end test_triangle;
- Output:
IsPointInTriangle((X= 0, Y= 0), (A=(X= 1, Y= 0), B=(X= 2, Y= 0), C=(X= 0, Y= 2))) yields FALSE IsPointInTriangle((X= 0, Y= 0), (A=(X=-1, Y= 0), B=(X=-1, Y=-1), C=(X= 2, Y= 2))) yields TRUE
ALGOL 68
Started out as
With additional material
BEGIN # determine whether a point is within a triangle or not #
# tolerance for the accurate test #
REAL eps = 0.001;
REAL eps squared = eps * eps;
# mode to hold a point #
MODE POINT = STRUCT( REAL x, y );
# returns a readable representation of p #
OP TOSTRING = ( POINT p )STRING: "[" + fixed( x OF p, -8, 4 ) + "," + fixed( y OF p, -8, 4 ) + "]";
# returns 1 if p is to the right of the line ( a, b ), -1 if it is to the left and 0 if it is on it #
PROC side of line = ( POINT p, a, b )INT:
SIGN ( ( ( x OF b - x OF a ) * ( y OF p - y OF a ) )
- ( ( y OF b - y OF a ) * ( x OF p - x OF a ) )
);
# returns the minimum of a and b #
PROC min = ( REAL a, b )REAL: IF a < b THEN a ELSE b FI;
# returns the maximum of a and b #
PROC max = ( REAL a, b )REAL: IF a > b THEN a ELSE b FI;
# returns TRUE if p is within the bounding box of the triangle a, b, c, FALSE otherwise #
PROC point inside bounding box of triangle = ( POINT p, a, b, c )BOOL:
BEGIN
REAL x min = min( x OF a, min( x OF b, x OF c ) );
REAL y min = min( y OF a, min( y OF b, y OF c ) );
REAL x max = max( x OF a, max( x OF b, x OF c ) );
REAL y max = max( y OF a, max( y OF b, y OF c ) );
x min <= x OF p AND x OF p <= x max AND y min <= y OF p AND y OF p <= y max
END # point inside bounding box of triangle # ;
# returns the squared distance between p and the line a, b #
PROC distance square point to segment = ( POINT p, a, b )REAL:
IF REAL a b square length = ( ( x OF b - x OF a ) ^ 2 ) + ( ( y OF b - y OF a ) ^ 2 );
REAL dot product = ( ( ( x OF p - x OF a ) ^ 2 ) + ( ( y OF p - y OF a ) ^ 2 ) ) / a b square length;
dot product < 0
THEN ( ( x OF p - x OF a ) ^ 2 ) + ( ( y OF p - y OF a ) ^ 2 )
ELIF dot product <= 1
THEN ( ( x OF a - x OF p ) ^ 2 ) + ( ( y OF a - y OF p ) ^ 2 )
- ( dot product * dot product * a b square length )
ELSE ( ( x OF p - x OF b ) ^ 2 ) + ( ( y OF p - y OF b ) ^ 2 )
FI # distance square point to segment # ;
# returns TRUE if p is within the triangle defined by a, b and c, FALSE otherwise #
PROC point inside triangle = ( POINT p, a, b, c )BOOL:
IF NOT point inside bounding box of triangle( p, a, b, c )
THEN FALSE
ELIF INT side of ab = side of line( p, a, b );
INT side of bc = side of line( p, b, c );
side of ab /= side of bc
THEN FALSE
ELIF side of ab = side of line( p, c, a )
THEN TRUE
ELIF distance square point to segment( p, a, b ) <= eps squared
THEN TRUE
ELIF distance square point to segment( p, b, c ) <= eps squared
THEN TRUE
ELSE distance square point to segment( p, c, a ) <= eps squared
FI # point inside triangle # ;
# test the point inside triangle procedure #
PROC test point = ( POINT p, a, b, c )VOID:
print( ( TOSTRING p, " in ( ", TOSTRING a, ", ", TOSTRING b, ", ", TOSTRING c, ") -> "
, IF point inside triangle( p, a, b, c ) THEN "true" ELSE "false" FI
, newline
)
);
# test cases as in Commpn Lisp #
test point( ( 0, 0 ), ( 1.5, 2.4 ), ( 5.1, -3.1 ), ( -3.8, 1.2 ) );
test point( ( 0, 1 ), ( 1.5, 2.4 ), ( 5.1, -3.1 ), ( -3.8, 1.2 ) );
test point( ( 3, 1 ), ( 1.5, 2.4 ), ( 5.1, -3.1 ), ( -3.8, 1.2 ) );
test point( ( 5.414286, 14.349206 ), ( 0.1, 0.111111 ), ( 12.5, 33.333333 ), ( 25.0, 11.111111 ) );
test point( ( 5.414286, 14.349206 ), ( 0.1, 0.111111 ), ( 12.5, 33.333333 ), ( -12.5, 16.666667 ) );
# additional Wren test cases #
test point( ( 5.4142857142857, 14.349206349206 )
, ( 0.1, 0.11111111111111 ), ( 12.5, 33.333333333333 ), ( 25, 11.111111111111 )
);
test point( ( 5.4142857142857, 14.349206349206 )
, ( 0.1, 0.11111111111111 ), ( 12.5, 33.333333333333 ), ( -12.5, 16.666666666667 )
)
END- Output:
[ 0.0000, 0.0000] in ( [ 1.5000, 2.4000], [ 5.1000, -3.1000], [ -3.8000, 1.2000]) -> true [ 0.0000, 1.0000] in ( [ 1.5000, 2.4000], [ 5.1000, -3.1000], [ -3.8000, 1.2000]) -> true [ 3.0000, 1.0000] in ( [ 1.5000, 2.4000], [ 5.1000, -3.1000], [ -3.8000, 1.2000]) -> false [ 5.4143, 14.3492] in ( [ 0.1000, 0.1111], [ 12.5000, 33.3333], [ 25.0000, 11.1111]) -> true [ 5.4143, 14.3492] in ( [ 0.1000, 0.1111], [ 12.5000, 33.3333], [-12.5000, 16.6667]) -> false [ 5.4143, 14.3492] in ( [ 0.1000, 0.1111], [ 12.5000, 33.3333], [ 25.0000, 11.1111]) -> true [ 5.4143, 14.3492] in ( [ 0.1000, 0.1111], [ 12.5000, 33.3333], [-12.5000, 16.6667]) -> false
ATS
This is the same algorithm as the Common Lisp, although not a translation of the Common Lisp. I merely discovered the similarity while searching for Rosetta Code examples that had obtained similar outputs.
The algorithm is widely used for testing whether a point is inside a convex hull of any size. For each side of the hull, one computes the geometric product of some vectors and observes the sign of a component in the result. The test takes advantage of the sine (or cosine) being positive in two adjacent quadrants and negative in the other two. Two quadrants will represent the inside of the hull and two the outside, relative to any given side of the hull. More details are described in the comments of the program.
(* The principle employed here is to treat the triangle as a convex
hull oriented counterclockwise. Then a point is inside the hull if
it is to the left of every side of the hull.
This will prove easy to determine. Because the sine is positive in
quadrants 1 and 2 and negative in quadrants 3 and 4, the ‘sideness’
of a point can be determined by the sign of an outer product of
vectors. Or you can use any such trigonometric method, including
those that employ an inner product.
Suppose one side of the triangle goes from point p0 to point p1,
and that the point we are testing for ‘leftness’ is p2. Then we
compute the geometric outer product
(p1 - p0)∧(p2 - p0)
(or equivalently the cross product of Gibbs vector analysis) and
test the sign of its grade-2 component (or the sign of the
right-hand-rule Gibbs pseudovector along the z-axis). If the sign
is positive, then p2 is left of the side, because the sine of the
angle between the vectors is positive.
The algorithm considers the vertices and sides of the triangle as
as NOT inside the triangle.
Our algorithm is the same as that of the Common Lisp. We merely
have dressed it up in prêt-à-porter fashion and costume jewelry. *)
#include "share/atspre_staload.hats"
#define COUNTERCLOCKWISE 1
#define COLLINEAR 0
#define CLOCKWISE ~1
(* We will use some simple Euclidean geometric algebra. *)
typedef vector =
(* This type will represent either a point relative to the origin or
the difference between two points. The e1 component is the x-axis
and the e2 component is the y-axis. *)
@{e1 = double, e2 = double}
typedef rotor =
(* This type is analogous to a pseudovectors, complex numbers, and
so forth. It will be used to represent the outer product. *)
@{scalar = double, e1_e2 = double}
typedef triangle = @(vector, vector, vector)
fn
vector_sub (a : vector, b : vector) : vector =
@{e1 = a.e1 - b.e1, e2 = a.e2 - b.e2}
overload - with vector_sub
fn
outer_product (a : vector, b : vector) : rotor =
@{scalar = 0.0, (* The scalar term vanishes. *)
e1_e2 = a.e1 * b.e2 - a.e2 * b.e1}
fn
is_left_of (pt : vector,
side : @(vector, vector)) =
let
val r = outer_product (side.1 - side.0, pt - side.0)
in
r.e1_e2 > 0.0
end
fn
orient_triangle {orientation : int | abs (orientation) == 1}
(t : triangle,
orientation : int orientation) : triangle =
(* Return an equivalent triangle that is definitely either
counterclockwise or clockwise, unless the original was
collinear. If the original was collinear, return it unchanged. *)
let
val @(p1, p2, p3) = t
(* If the triangle is counterclockwise, the grade-2 component of
the following outer product will be positive. *)
val r = outer_product (p2 - p1, p3 - p1)
in
if r.e1_e2 = 0.0 then
t
else
let
val sign =
(if r.e1_e2 > 0.0 then COUNTERCLOCKWISE else CLOCKWISE)
: [sign : int | abs sign == 1] int sign
in
if orientation = sign then t else @(p1, p3, p2)
end
end
fn
is_inside_triangle (pt : vector,
t : triangle) : bool =
let
val @(p1, p2, p3) = orient_triangle (t, COUNTERCLOCKWISE)
in
is_left_of (pt, @(p1, p2))
&& is_left_of (pt, @(p2, p3))
&& is_left_of (pt, @(p3, p1))
end
fn
fprint_vector (outf : FILEref,
v : vector) : void =
fprint! (outf, "(", v.e1, ",", v.e2, ")")
fn
fprint_triangle (outf : FILEref,
t : triangle) : void =
begin
fprint_vector (outf, t.0);
fprint! (outf, "--");
fprint_vector (outf, t.1);
fprint! (outf, "--");
fprint_vector (outf, t.2);
fprint! (outf, "--cycle")
end
fn
try_it (outf : FILEref,
pt : vector,
t : triangle) : void =
begin
fprint_vector (outf, pt);
fprint! (outf, " is inside ");
fprint_triangle (outf, t);
fprintln! (outf);
fprintln! (outf, is_inside_triangle (pt, t))
end
implement
main () =
let
val outf = stdout_ref
val t1 = @(@{e1 = 1.5, e2 = 2.4},
@{e1 = 5.1, e2 = ~3.1},
@{e1 = ~3.8, e2 = 1.2})
val p1 = @{e1 = 0.0, e2 = 0.0}
val p2 = @{e1 = 0.0, e2 = 1.0}
val p3 = @{e1 = 3.0, e2 = 1.0}
val p4 = @{e1 = 1.5, e2 = 2.4}
val p5 = @{e1 = 5.414286, e2 = 14.349206}
val t2 = @(@{e1 = 0.100000, e2 = 0.111111},
@{e1 = 12.500000, e2 = 33.333333},
@{e1 = 25.000000, e2 = 11.111111})
val t3 = @(@{e1 = 0.100000, e2 = 0.111111},
@{e1 = 12.500000, e2 = 33.333333},
@{e1 = ~12.500000, e2 = 16.666667})
in
try_it (outf, p1, t1);
try_it (outf, p2, t1);
try_it (outf, p3, t1);
try_it (outf, p4, t1);
fprintln! (outf);
try_it (outf, p5, t2);
fprintln! (outf);
fprintln! (outf, "Some programs are returning TRUE for ",
"the following. The Common Lisp uses");
fprintln! (outf, "the same",
" algorithm we do (presented differently),",
" and returns FALSE.");
fprintln! (outf);
try_it (outf, p5, t3);
0
end- Output:
$ patscc -g -O3 -march=native -pipe -std=gnu2x point_inside_triangle.dats && ./a.out (0.000000,0.000000) is inside (1.500000,2.400000)--(5.100000,-3.100000)--(-3.800000,1.200000)--cycle true (0.000000,1.000000) is inside (1.500000,2.400000)--(5.100000,-3.100000)--(-3.800000,1.200000)--cycle true (3.000000,1.000000) is inside (1.500000,2.400000)--(5.100000,-3.100000)--(-3.800000,1.200000)--cycle false (1.500000,2.400000) is inside (1.500000,2.400000)--(5.100000,-3.100000)--(-3.800000,1.200000)--cycle false (5.414286,14.349206) is inside (0.100000,0.111111)--(12.500000,33.333333)--(25.000000,11.111111)--cycle true Some programs are returning TRUE for the following. The Common Lisp uses the same algorithm we do (presented differently), and returns FALSE. (5.414286,14.349206) is inside (0.100000,0.111111)--(12.500000,33.333333)--(-12.500000,16.666667)--cycle false
AutoHotkey
T := [[1.5, 2.4], [5.1, -3.1], [-3.8, 1.2]]
for i, p in [[0, 0], [0, 1], [3, 1], [5.4142857, 14.349206]]
result .= "[" p.1 ", " p.2 "] is within triangle?`t" (TriHasP(T, p) ? "ture" : "false") "`n"
MsgBox % result
return
TriHasP(T, P){
Ax := TriArea(T.1.1, T.1.2, T.2.1, T.2.2, T.3.1, T.3.2)
A1 := TriArea(P.1 , P.2 , T.2.1, T.2.2, T.3.1, T.3.2)
A2 := TriArea(T.1.1, T.1.2, P.1 , P.2 , T.3.1, T.3.2)
A3 := TriArea(T.1.1, T.1.2, T.2.1, T.2.2, P.1 , P.2)
return (Ax = A1 + A2 + A3)
}
TriArea(x1, y1, x2, y2, x3, y3){
return Abs((x1 * (y2-y3) + x2 * (y3-y1) + x3 * (y1-y2)) / 2)
}
- Output:
[0, 0] is within triangle? ture [0, 1] is within triangle? ture [3, 1] is within triangle? false [5.4142857, 14.349206] is within triangle? false
C
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
const double EPS = 0.001;
const double EPS_SQUARE = 0.000001;
double side(double x1, double y1, double x2, double y2, double x, double y) {
return (y2 - y1) * (x - x1) + (-x2 + x1) * (y - y1);
}
bool naivePointInTriangle(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
double checkSide1 = side(x1, y1, x2, y2, x, y) >= 0;
double checkSide2 = side(x2, y2, x3, y3, x, y) >= 0;
double checkSide3 = side(x3, y3, x1, y1, x, y) >= 0;
return checkSide1 && checkSide2 && checkSide3;
}
bool pointInTriangleBoundingBox(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
double xMin = min(x1, min(x2, x3)) - EPS;
double xMax = max(x1, max(x2, x3)) + EPS;
double yMin = min(y1, min(y2, y3)) - EPS;
double yMax = max(y1, max(y2, y3)) + EPS;
return !(x < xMin || xMax < x || y < yMin || yMax < y);
}
double distanceSquarePointToSegment(double x1, double y1, double x2, double y2, double x, double y) {
double p1_p2_squareLength = (x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1);
double dotProduct = ((x - x1) * (x2 - x1) + (y - y1) * (y2 - y1)) / p1_p2_squareLength;
if (dotProduct < 0) {
return (x - x1) * (x - x1) + (y - y1) * (y - y1);
} else if (dotProduct <= 1) {
double p_p1_squareLength = (x1 - x) * (x1 - x) + (y1 - y) * (y1 - y);
return p_p1_squareLength - dotProduct * dotProduct * p1_p2_squareLength;
} else {
return (x - x2) * (x - x2) + (y - y2) * (y - y2);
}
}
bool accuratePointInTriangle(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
if (!pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y)) {
return false;
}
if (naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)) {
return true;
}
if (distanceSquarePointToSegment(x1, y1, x2, y2, x, y) <= EPS_SQUARE) {
return true;
}
if (distanceSquarePointToSegment(x2, y2, x3, y3, x, y) <= EPS_SQUARE) {
return true;
}
if (distanceSquarePointToSegment(x3, y3, x1, y1, x, y) <= EPS_SQUARE) {
return true;
}
return false;
}
void printPoint(double x, double y) {
printf("(%f, %f)", x, y);
}
void printTriangle(double x1, double y1, double x2, double y2, double x3, double y3) {
printf("Triangle is [");
printPoint(x1, y1);
printf(", ");
printPoint(x2, y2);
printf(", ");
printPoint(x3, y3);
printf("] \n");
}
void test(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
printTriangle(x1, y1, x2, y2, x3, y3);
printf("Point ");
printPoint(x, y);
printf(" is within triangle? ");
if (accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)) {
printf("true\n");
} else {
printf("false\n");
}
}
int main() {
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 0);
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 1);
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 3, 1);
printf("\n");
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, 25, 11.11111111111111, 5.414285714285714, 14.349206349206348);
printf("\n");
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, -12.5, 16.666666666666668, 5.414285714285714, 14.349206349206348);
printf("\n");
return 0;
}
- Output:
Triangle is [(1.500000, 2.400000), (5.100000, -3.100000), (-3.800000, 1.200000)] Point (0.000000, 0.000000) is within triangle? true Triangle is [(1.500000, 2.400000), (5.100000, -3.100000), (-3.800000, 1.200000)] Point (0.000000, 1.000000) is within triangle? true Triangle is [(1.500000, 2.400000), (5.100000, -3.100000), (-3.800000, 1.200000)] Point (3.000000, 1.000000) is within triangle? false Triangle is [(0.100000, 0.111111), (12.500000, 33.333333), (25.000000, 11.111111)] Point (5.414286, 14.349206) is within triangle? true Triangle is [(0.100000, 0.111111), (12.500000, 33.333333), (-12.500000, 16.666667)] Point (5.414286, 14.349206) is within triangle? true
C++
#include <iostream>
const double EPS = 0.001;
const double EPS_SQUARE = EPS * EPS;
double side(double x1, double y1, double x2, double y2, double x, double y) {
return (y2 - y1) * (x - x1) + (-x2 + x1) * (y - y1);
}
bool naivePointInTriangle(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
double checkSide1 = side(x1, y1, x2, y2, x, y) >= 0;
double checkSide2 = side(x2, y2, x3, y3, x, y) >= 0;
double checkSide3 = side(x3, y3, x1, y1, x, y) >= 0;
return checkSide1 && checkSide2 && checkSide3;
}
bool pointInTriangleBoundingBox(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
double xMin = std::min(x1, std::min(x2, x3)) - EPS;
double xMax = std::max(x1, std::max(x2, x3)) + EPS;
double yMin = std::min(y1, std::min(y2, y3)) - EPS;
double yMax = std::max(y1, std::max(y2, y3)) + EPS;
return !(x < xMin || xMax < x || y < yMin || yMax < y);
}
double distanceSquarePointToSegment(double x1, double y1, double x2, double y2, double x, double y) {
double p1_p2_squareLength = (x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1);
double dotProduct = ((x - x1) * (x2 - x1) + (y - y1) * (y2 - y1)) / p1_p2_squareLength;
if (dotProduct < 0) {
return (x - x1) * (x - x1) + (y - y1) * (y - y1);
} else if (dotProduct <= 1) {
double p_p1_squareLength = (x1 - x) * (x1 - x) + (y1 - y) * (y1 - y);
return p_p1_squareLength - dotProduct * dotProduct * p1_p2_squareLength;
} else {
return (x - x2) * (x - x2) + (y - y2) * (y - y2);
}
}
bool accuratePointInTriangle(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
if (!pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y)) {
return false;
}
if (naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)) {
return true;
}
if (distanceSquarePointToSegment(x1, y1, x2, y2, x, y) <= EPS_SQUARE) {
return true;
}
if (distanceSquarePointToSegment(x2, y2, x3, y3, x, y) <= EPS_SQUARE) {
return true;
}
if (distanceSquarePointToSegment(x3, y3, x1, y1, x, y) <= EPS_SQUARE) {
return true;
}
return false;
}
void printPoint(double x, double y) {
std::cout << '(' << x << ", " << y << ')';
}
void printTriangle(double x1, double y1, double x2, double y2, double x3, double y3) {
std::cout << "Triangle is [";
printPoint(x1, y1);
std::cout << ", ";
printPoint(x2, y2);
std::cout << ", ";
printPoint(x3, y3);
std::cout << "]\n";
}
void test(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
printTriangle(x1, y1, x2, y2, x3, y3);
std::cout << "Point ";
printPoint(x, y);
std::cout << " is within triangle? ";
if (accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)) {
std::cout << "true\n";
} else {
std::cout << "false\n";
}
}
int main() {
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 0);
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 1);
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 3, 1);
std::cout << '\n';
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, 25, 11.11111111111111, 5.414285714285714, 14.349206349206348);
std::cout << '\n';
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, -12.5, 16.666666666666668, 5.414285714285714, 14.349206349206348);
std::cout << '\n';
return 0;
}
- Output:
Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0, 0) is within triangle? true Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0, 1) is within triangle? true Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (3, 1) is within triangle? false Triangle is [(0.1, 0.111111), (12.5, 33.3333), (25, 11.1111)] Point (5.41429, 14.3492) is within triangle? true Triangle is [(0.1, 0.111111), (12.5, 33.3333), (-12.5, 16.6667)] Point (5.41429, 14.3492) is within triangle? true
Common Lisp
; There are different algorithms to solve this problem, such as adding areas, adding angles, etc... but these
; solutions are sensitive to rounding errors intrinsic to float operations. We want to avoid these issues, therefore we
; use the following algorithm which only uses multiplication and subtraction: we consider one side of the triangle
; and see on which side of it is the point P located. We can give +1 if it is on the right hand side, -1 for the
; left side, or 0 if it is on the line. If the point is located on the same side relative to all three sides of the triangle
; then the point is inside of it. This has an added advantage that it can be scaled up to other more complicated figures
; (even concave ones, with some minor modifications).
(defun point-inside-triangle (P A B C)
"Is the point P inside the triangle formed by ABC?"
(= (side-of-line P A B)
(side-of-line P B C)
(side-of-line P C A) ))
; This is the version to include those points which are on one of the sides
(defun point-inside-or-on-triangle (P A B C)
"Is the point P inside the triangle formed by ABC or on one of the sides?"
(apply #'= (remove 0 (list (side-of-line P A B) (side-of-line P B C) (side-of-line P C A)))) )
(defun side-of-line (P A B)
"Return +1 if it is on the right side, -1 for the left side, or 0 if it is on the line"
; We use the sign of the determinant of vectors (AB,AM), where M(X,Y) is the query point:
; position = sign((Bx - Ax) * (Y - Ay) - (By - Ay) * (X - Ax))
(signum (- (* (- (car B) (car A))
(- (cdr P) (cdr A)) )
(* (- (cdr B) (cdr A))
(- (car P) (car A)) ))))
- Output:
(point-inside-triangle '(0 . 0) '(1.5 . 2.4) '(5.1 . -3.1) '(-3.8 . 1.2)) T (point-inside-triangle '(0 . 1) '(1.5 . 2.4) '(5.1 . -3.1) '(-3.8 . 1.2)) T (point-inside-triangle '(3 . 1) '(1.5 . 2.4) '(5.1 . -3.1) '(-3.8 . 1.2)) NIL (point-inside-triangle '(5.414286 . 14.349206) '(0.1 . 0.111111) '(12.5 . 33.333333) '(25.0 . 11.111111)) T (point-inside-triangle '(5.414286 . 14.349206) '(0.1 . 0.111111) '(12.5 . 33.333333) '(-12.5 . 16.666667)) NIL
D
import std.algorithm;
import std.stdio;
immutable EPS = 0.001;
immutable EPS_SQUARE = EPS * EPS;
double side(double x1, double y1, double x2, double y2, double x, double y) {
return (y2 - y1) * (x - x1) + (-x2 + x1) * (y - y1);
}
bool naivePointInTriangle(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
double checkSide1 = side(x1, y1, x2, y2, x, y) >= 0;
double checkSide2 = side(x2, y2, x3, y3, x, y) >= 0;
double checkSide3 = side(x3, y3, x1, y1, x, y) >= 0;
return checkSide1 && checkSide2 && checkSide3;
}
bool pointInTriangleBoundingBox(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
double xMin = min(x1, x2, x3) - EPS;
double xMax = max(x1, x2, x3) + EPS;
double yMin = min(y1, y2, y3) - EPS;
double yMax = max(y1, y2, y3) + EPS;
return !(x < xMin || xMax < x || y < yMin || yMax < y);
}
double distanceSquarePointToSegment(double x1, double y1, double x2, double y2, double x, double y) {
double p1_p2_squareLength = (x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1);
double dotProduct = ((x - x1) * (x2 - x1) + (y - y1) * (y2 - y1)) / p1_p2_squareLength;
if (dotProduct < 0) {
return (x - x1) * (x - x1) + (y - y1) * (y - y1);
} else if (dotProduct <= 1) {
double p_p1_squareLength = (x1 - x) * (x1 - x) + (y1 - y) * (y1 - y);
return p_p1_squareLength - dotProduct * dotProduct * p1_p2_squareLength;
} else {
return (x - x2) * (x - x2) + (y - y2) * (y - y2);
}
}
bool accuratePointInTriangle(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
if (!pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y)) {
return false;
}
if (naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)) {
return true;
}
if (distanceSquarePointToSegment(x1, y1, x2, y2, x, y) <= EPS_SQUARE) {
return true;
}
if (distanceSquarePointToSegment(x2, y2, x3, y3, x, y) <= EPS_SQUARE) {
return true;
}
if (distanceSquarePointToSegment(x3, y3, x1, y1, x, y) <= EPS_SQUARE) {
return true;
}
return false;
}
void printPoint(double x, double y) {
write('(', x, ", ", y, ')');
}
void printTriangle(double x1, double y1, double x2, double y2, double x3, double y3) {
write("Triangle is [");
printPoint(x1, y1);
write(", ");
printPoint(x2, y2);
write(", ");
printPoint(x3, y3);
writeln(']');
}
void test(double x1, double y1, double x2, double y2, double x3, double y3, double x, double y) {
printTriangle(x1, y1, x2, y2, x3, y3);
write("Point ");
printPoint(x, y);
write(" is within triangle? ");
writeln(accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y));
}
void main() {
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 0);
writeln;
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 1);
writeln;
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 3, 1);
writeln;
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, 25, 11.11111111111111, 5.414285714285714, 14.349206349206348);
writeln;
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, -12.5, 16.666666666666668, 5.414285714285714, 14.349206349206348);
writeln;
}
- Output:
Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0, 0) is within triangle? true Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0, 1) is within triangle? true Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (3, 1) is within triangle? false Triangle is [(0.1, 0.111111), (12.5, 33.3333), (25, 11.1111)] Point (5.41429, 14.3492) is within triangle? true Triangle is [(0.1, 0.111111), (12.5, 33.3333), (-12.5, 16.6667)] Point (5.41429, 14.3492) is within triangle? true
Delphi
This routine works by taking each line in the triangle and determining which side of the line the point is on. This is done using the "determinant" of the three points. If a point is on the same side of all sides in the triangle, the point is inside the triangle. Conversely, if a point isn't on the same side, it is out of side the triangle. Since there are only three points in a triangle, this applies no matter the order in which the points are presented, as long as the points are traversed in order. Points that fall on a line are treated as though they have the same "inside" sense when combined with other lines.
{Vector structs and operations - these would normally be in}
{a library, but are produced here so everything is explicit}
type T2DVector=packed record
X,Y: double;
end;
type T2DLine = packed record
P1,P2: T2DVector;
end;
type T2DTriangle = record
P1,P2,P3: T2DVector;
end;
function MakeVector2D(const X,Y: double): T2DVector;
begin
Result.X:=X;
Result.Y:=Y;
end;
function Make2DLine(const P1,P2: T2DVector): T2DLine; overload;
begin
Result.P1:=P1;
Result.P2:=P2;
end;
function MakeTriangle2D(P1,P2,P3: T2DVector): T2DTriangle;
begin
Result.P1:=P1; Result.P2:=P2; Result.P3:=P3;
end;
{Point-Line position constants}
const RightPos = -1;
const LeftPos = +1;
const ColinearPos = 0;
function LinePointPosition(Line: T2DLine; Point: T2DVector): integer;
{Test the position of point relative to the line}
{Returns +1 = right side, -1 = left side, 0 = on the line}
var Side: double;
begin
{ Use the determinate to find which side of the line a point is on }
Side := (Line.P2.X - Line.P1.X) * (Point.Y - Line.P1.Y) - (Point.X - Line.P1.X) * (Line.P2.Y - Line.P1.Y);
{Return +1 = right side, -1 = left side, 0 = on the line}
if Side > 0 then Result := LeftPos
else if Side < 0 then Result := RightPos
else Result := ColinearPos;
end;
function PointInTriangle2D(P: T2DVector; Tri: T2DTriangle): boolean; overload;
{Check if specified point is inside the specified Triangle}
var Side1,Side2,Side3: integer;
var L: T2DLine;
begin
{Get the side the point falls on for the first two sides of triangle}
Side1 := LinePointPosition(Make2DLine(Tri.P1,Tri.P2),P);
Side2 := LinePointPosition(Make2DLine(Tri.P2,Tri.P3),P);
{If they are on different sides, the point must be outside}
if (Side1 * Side2) = -1 then Result := False
else
begin
{The point is inside the first two sides, so check the third side}
Side3 := LinePointPosition(Make2DLine(Tri.P3,Tri.P1),P);
{Use the three}
if (Side1 = Side3) or (Side3 = 0) then Result := True
else if Side1 = 0 then Result := (Side2 * Side3) >= 0
else if Side2 = 0 then Result := (Side1 * Side3) >= 0
else Result := False;
end;
end;
{-------------- Test routines -------------------------------------------------}
procedure DrawTriangle(Canvas: TCanvas; T: T2DTriangle);
{Draw triangles on any canvas}
begin
Canvas.Pen.Color:=clBlack;
Canvas.Pen.Mode:=pmCopy;
Canvas.Pen.Style:=psSolid;
Canvas.Pen.Width:=2;
Canvas.MoveTo(Trunc(T.P1.X),Trunc(T.P1.Y));
Canvas.LineTo(Trunc(T.P2.X),Trunc(T.P2.Y));
Canvas.LineTo(Trunc(T.P3.X),Trunc(T.P3.Y));
Canvas.LineTo(Trunc(T.P1.X),Trunc(T.P1.Y));
end;
procedure DrawPoint(Canvas: TCanvas; X,Y: integer; InTri: boolean);
{Draw a test point on a canvas and mark if "In" or "Out"}
begin
Canvas.Pen.Color:=clRed;
Canvas.Pen.Width:=8;
Canvas.MoveTo(X-1,Y);
Canvas.LineTo(X+1,Y);
Canvas.MoveTo(X,Y-1);
Canvas.LineTo(X,Y+1);
Canvas.Font.Size:=12;
Canvas.Font.Style:=[fsBold];
if InTri then Canvas.TextOut(X+5,Y,'In')
else Canvas.TextOut(X+5,Y,'Out');
end;
procedure TestPointInTriangle(Image: TImage);
{Draw triangle and display test points}
var Tri: T2DTriangle;
var P: TPoint;
begin
{Create and draw Triangle}
Tri:=MakeTriangle2D(MakeVector2D(50,50),MakeVector2D(300,80),MakeVector2D(150,250));
DrawTriangle(Image.Canvas,Tri);
{Draw six test points}
P:=Point(62,193);
DrawPoint(Image.Canvas,P.X,P.Y, PointInTriangle2D(MakeVector2D(P.X,P.Y),Tri));
P:=Point(100,100);
DrawPoint(Image.Canvas,P.X,P.Y, PointInTriangle2D(MakeVector2D(P.X,P.Y),Tri));
P:=Point(200,100);
DrawPoint(Image.Canvas,P.X,P.Y, PointInTriangle2D(MakeVector2D(P.X,P.Y),Tri));
P:=Point(150,30);
DrawPoint(Image.Canvas,P.X,P.Y, PointInTriangle2D(MakeVector2D(P.X,P.Y),Tri));
P:=Point(250,200);
DrawPoint(Image.Canvas,P.X,P.Y, PointInTriangle2D(MakeVector2D(P.X,P.Y),Tri));
P:=Point(150,200);
DrawPoint(Image.Canvas,P.X,P.Y, PointInTriangle2D(MakeVector2D(P.X,P.Y),Tri));
end;
- Output:
Dart
import 'dart:math';
const double EPS = 0.001;
const double EPS_SQUARE = EPS * EPS;
double side(double x1, double y1, double x2, double y2, double x, double y) {
return (y2 - y1) * (x - x1) + (-x2 + x1) * (y - y1);
}
bool naivePointInTriangle(double x1, double y1, double x2, double y2, double x3,
double y3, double x, double y) {
double checkSide1 = side(x1, y1, x2, y2, x, y); // >= 0;
double checkSide2 = side(x2, y2, x3, y3, x, y); // >= 0;
double checkSide3 = side(x3, y3, x1, y1, x, y); // >= 0;
if (checkSide1 >= 0 && checkSide2 >= 0 && checkSide3 >= 0) {
return true;
} else {
return false;
}
}
bool pointInTriangleBoundingBox(double x1, double y1, double x2, double y2,
double x3, double y3, double x, double y) {
double xMin = min(x1, min(x2, x3)) - EPS;
double xMax = max(x1, max(x2, x3)) + EPS;
double yMin = min(y1, min(y2, y3)) - EPS;
double yMax = max(y1, max(y2, y3)) + EPS;
return !(x < xMin || xMax < x || y < yMin || yMax < y);
}
double distanceSquarePointToSegment(
double x1, double y1, double x2, double y2, double x, double y) {
double p1_p2_squareLength = (x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1);
double dotProduct =
((x - x1) * (x2 - x1) + (y - y1) * (y2 - y1)) / p1_p2_squareLength;
if (dotProduct < 0) {
return (x - x1) * (x - x1) + (y - y1) * (y - y1);
} else if (dotProduct <= 1) {
double p_p1_squareLength = (x1 - x) * (x1 - x) + (y1 - y) * (y1 - y);
return p_p1_squareLength - dotProduct * dotProduct * p1_p2_squareLength;
} else {
return (x - x2) * (x - x2) + (y - y2) * (y - y2);
}
}
bool accuratePointInTriangle(double x1, double y1, double x2, double y2,
double x3, double y3, double x, double y) {
if (!pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y)) {
return false;
}
if (naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)) {
return true;
}
if (distanceSquarePointToSegment(x1, y1, x2, y2, x, y) <= EPS_SQUARE) {
return true;
}
if (distanceSquarePointToSegment(x2, y2, x3, y3, x, y) <= EPS_SQUARE) {
return true;
}
if (distanceSquarePointToSegment(x3, y3, x1, y1, x, y) <= EPS_SQUARE) {
return true;
}
return false;
}
void printTriangle(
double x1, double y1, double x2, double y2, double x3, double y3) {
print("Triangle is [($x1, $y1), ($x2, $y2), ($x3, $y3)]");
}
void test(double x1, double y1, double x2, double y2, double x3, double y3,
double x, double y) {
printTriangle(x1, y1, x2, y2, x3, y3);
print("Point ($x, $y) is within triangle? ");
if (accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)) {
print("true");
} else {
print("false");
}
}
void main() {
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 0);
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 1);
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 3, 1);
print('');
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, 25, 11.11111111111111,
5.414285714285714, 14.349206349206348);
print('');
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, -12.5,
16.666666666666668, 5.414285714285714, 14.349206349206348);
print('');
}
EasyLang
func sgn px py ax ay bx by .
return sign ((px - bx) * (ay - by) - (ax - bx) * (py - by))
.
func isin px py ax ay bx by cx cy .
z1 = sgn px py ax ay bx by
z2 = sgn px py bx by cx cy
z3 = sgn px py cx cy ax ay
return if abs (z1 + z2 + z3) = 3
.
ax = 10 ; ay = 20
bx = 90 ; by = 30
cx = 50 ; cy = 80
#
move 5 90
textsize 4
text "Move mouse into the triangle"
color 444
polygon [ ax ay bx by cx cy ]
#
on mouse_move
if isin mouse_x mouse_y ax ay bx by cx cy <> in
in = 1 - in
if in = 1
color 822
else
color 444
.
polygon [ ax ay bx by cx cy ]
.
.
Evaldraw
This solution makes use of the (x,y,t,&r,&g,&b) plotting function. It evaluates an function in the cartesian plane. Given x,y inputs, the function is expected to set r,g,b color channels. The program tests all points in the viewport. You may pan and zoom. The current mouse position shows the computed RGB at that point. The isPointInsideTriangle-function here works in similar way to other solutions here;
for the 3 points of a triangle we compute 3 line equations that will be evaluated to get the signed distance from the line to a point. We can use this to return early from isPointInsideTriangle. Only if all three lines give a result with the point on the same side (same sign) then the point can be classified as inside the triangle. We can use this property of sidedness and sign to make the method work for both clockwise and anti-clockwise specification of the triangle vertices. If the triangle is clockwise, then the area function returns a positive value. If the triangle is anti clockwise, then the area function returns a negative value, and we can multiply the sgn checks by -1 so a point can still be considered inside. A point with distance 0 is also considered inside.
struct vec2{x,y;};
struct line_t{a,b,c;};
struct triangle_calc_t{
vec2 origin;
line_t lines[3];
vec2 min,max;
double area2;
winding_dir; // +1 if clockwise (positive angle) -1 if negative.
};
//static vec2 vertices[3] = {0,-2, -2,2, 4,0};
static vec2 vertices[3] = {-3,7, -6,-5, 2,2};
enum{TRI_OUT, TRI_ZERO, TRI_EDGE, TRI_INSIDE}
static triangle_calc_t triangle;
(x,y,t,&r,&g,&b)
{
if (numframes==0) {
precalc_tri( triangle, vertices);
}
d0 = d1 = d2 = 0; // Distances of point to lines
vec2 point = {x, y};
side = isPointInsideTriangle(point,triangle,d0,d1,d2);
if (side == TRI_INSIDE) {
if (triangle.winding_dir == -1) {
swap(d1,d2);
swap(d1,d0);
}
r_area = 1.0 / (triangle.winding_dir * triangle.area2);
r = 255 * r_area * d2;
g = 255 * r_area * d0;
b = 255 * r_area * d1; return 1;
}
r=0; g=0; b=0; return 0; // Set color to 0 if outside.
}
precalc_tri(triangle_calc_t t, vec2 verts[3]) {
t.area2 = triangleAreaTimes2(verts[0], verts[1], verts[2]);
if (t.area2 == 0) return;
t.winding_dir = sgn(t.area2);
t.origin = verts[0];
vec2 relative_vertices[3];
t.min.x = 1e32;
t.min.y = 1e32;
t.max.x = -1e32;
t.max.y = -1e32;
for(i=0; i<3; i++) {
t.min.x = min(t.min.x, verts[i].x);
t.min.y = min(t.min.y, verts[i].y);
t.max.x = max(t.max.x, verts[i].x);
t.max.y = max(t.max.y, verts[i].y);
relative_vertices[i].x = verts[i].x + t.origin.x;
relative_vertices[i].y = verts[i].y + t.origin.y;
}
makeLine(t.lines[0], relative_vertices[0], relative_vertices[1]);
makeLine(t.lines[1], relative_vertices[1], relative_vertices[2]);
makeLine(t.lines[2], relative_vertices[2], relative_vertices[0]);
}
triangleAreaTimes2(vec2 a, vec2 b, vec2 c) { // Same as the determinant, but dont div by 2
return c.x*(a.y-b.y)+a.x*(b.y-c.y)+b.x*(-a.y+c.y);
}
isPointInsideTriangle( vec2 point, triangle_calc_t t, &d0,&d1,&d2) {
if (t.area2 == 0) return TRI_ZERO;
if (point.x < t.min.x) return TRI_OUT;
if (point.y < t.min.y) return TRI_OUT;
if (point.x > t.max.x) return TRI_OUT;
if (point.y > t.max.y) return TRI_OUT;
vec2 p = {point.x + t.origin.x, point.y + t.origin.y };
d0 = t.winding_dir * lineDist( t.lines[0], p.x, p.y);
if (d0==0) { return TRI_EDGE; }else if ( sgn(d0) < 0 ) return TRI_OUT;
d1 = t.winding_dir * lineDist( t.lines[1], p.x, p.y);
if (d1==0) { return TRI_EDGE; } else if ( sgn(d1) < 0 ) return TRI_OUT;
d2 = t.winding_dir * lineDist( t.lines[2], p.x, p.y);
if (d2==0) { return TRI_EDGE; } else if ( sgn(d2) < 0 ) return TRI_OUT;
return TRI_INSIDE; // on inside
}
makeLine(line_t line, vec2 a, vec2 b) { // -dy,dx,axby-aybx
line.a = -(b.y - a.y);
line.b = (b.x - a.x);
line.c = a.x*b.y - a.y*b.x;
}
lineDist(line_t line, x,y) { return x*line.a + y*line.b + line.c; }
swap(&a,&b) {tmp = a; a=b; b=tmp; }
Factor
Uses the parametric equations method from [5].
USING: accessors fry io kernel locals math math.order sequences ;
TUPLE: point x y ;
C: <point> point
: >point< ( point -- x y ) [ x>> ] [ y>> ] bi ;
TUPLE: triangle p1 p2 p3 ;
C: <triangle> triangle
: >triangle< ( triangle -- x1 y1 x2 y2 x3 y3 )
[ p1>> ] [ p2>> ] [ p3>> ] tri [ >point< ] tri@ ;
:: point-in-triangle? ( point triangle -- ? )
point >point< triangle >triangle< :> ( x y x1 y1 x2 y2 x3 y3 )
y2 y3 - x1 * x3 x2 - y1 * + x2 y3 * + y2 x3 * - :> d
y3 y1 - x * x1 x3 - y * + x1 y3 * - y1 x3 * + d / :> t1
y2 y1 - x * x1 x2 - y * + x1 y2 * - y1 x2 * + d neg / :> t2
t1 t2 + :> s
t1 t2 [ 0 1 between? ] bi@ and s 1 <= and ;
! Test if it works.
20 <iota> dup [ swap <point> ] cartesian-map ! Make a matrix of points
3 3 <point> 16 10 <point> 10 16 <point> <triangle> ! Make a triangle
'[ [ _ point-in-triangle? "#" "." ? write ] each nl ] each nl ! Show points inside the triangle with '#'
- Output:
.................... .................... .................... ...#................ ....#............... .....##............. .....####........... ......#####......... ......#######....... .......########..... .......##########... ........########.... ........#######..... .........#####...... .........####....... ..........##........ ..........#......... .................... .................... ....................
Fortran
PROGRAM POINT_WITHIN_TRIANGLE
IMPLICIT NONE
REAL (KIND = SELECTED_REAL_KIND (8)) px, py, ax, ay, bx, by, cx, cy
px = 0.0
py = 0.0
ax = 1.5
ay = 2.4
bx = 5.1
by = -3.1
cx = -3.8
cy = 1.2
IF (IS_P_IN_ABC (px, py, ax, ay, bx, by, cx, cy)) THEN
WRITE (*, *) 'Point (', px, ', ', py, ') is within triangle &
[(', ax, ', ', ay,'), (', bx, ', ', by, '), (', cx, ', ', cy, ')].'
ELSE
WRITE (*, *) 'Point (', px, ', ', py, ') is not within triangle &
[(', ax, ', ', ay,'), (', bx, ', ', by, '), (', cx, ', ', cy, ')].'
END IF
CONTAINS
!Provide xy values of points P, A, B, C, respectively.
LOGICAL FUNCTION IS_P_IN_ABC (px, py, ax, ay, bx, by, cx, cy)
REAL (KIND = SELECTED_REAL_KIND (8)), INTENT (IN) :: px, py, ax, ay, bx, by, cx, cy
REAL (KIND = SELECTED_REAL_KIND (8)) :: vabx, vaby, vacx, vacy, a, b
vabx = bx - ax
vaby = by - ay
vacx = cx - ax
vacy = cy - ay
a = ((px * vacy - py * vacx) - (ax * vacy - ay * vacx)) / &
(vabx * vacy - vaby * vacx)
b = -((px * vaby - py * vabx) - (ax * vaby - ay * vabx)) / &
(vabx * vacy - vaby * vacx)
IF ((a .GT. 0) .AND. (b .GT. 0) .AND. (a + b < 1)) THEN
IS_P_IN_ABC = .TRUE.
ELSE
IS_P_IN_ABC = .FALSE.
END IF
END FUNCTION IS_P_IN_ABC
END PROGRAM POINT_WITHIN_TRIANGLE
- Output:
Point ( 0.0000000000000000 , 0.0000000000000000 ) is within triangle [( 1.5000000000000000 , 2.4000000953674316 ), ( 5.0999999046325684 , -3.0999999046325684 ), ( -3.7999999523162842 , 1.2000000476837158 )].
FreeBASIC
type p2d
x as double 'define a two-dimensional point
y as double
end type
function in_tri( A as p2d, B as p2d, C as p2d, P as p2d ) as boolean
'uses barycentric coordinates to determine if point P is inside
'the triangle defined by points A, B, C
dim as double AreaD = (-B.y*C.x + A.y*(-B.x + C.x) + A.x*(B.y - C.y) + B.x*C.y)
dim as double s = (A.y*C.x - A.x*C.y + (C.y - A.y)*P.x + (A.x - C.x)*P.y)/AreaD
dim as double t = (A.x*B.y - A.y*B.x + (A.y - B.y)*P.x + (B.x - A.x)*P.y)/AreaD
if s<=0 then return false
if t<=0 then return false
if s+t>=1 then return false
return true
end function
dim as p2d A,B,C,P 'generate some arbitrary triangle
A.x = 4.14 : A.y = -1.12
B.x = 8.1 : B.y =-4.9
C.x = 1.5: C.y = -9.3
for y as double = -0.25 to -9.75 step -0.5 'display a 10x10 square
for x as double = 0.125 to 9.875 step 0.25
P.x = x : P.y = y
if in_tri(A,B,C,P) then print "@"; else print "."; 'with all the points inside the triangle indicated
next x
print
next y
- Output:
................................................................................ ................@....................... ................@@@..................... ...............@@@@@@................... ..............@@@@@@@@@................. ..............@@@@@@@@@@@............... .............@@@@@@@@@@@@@@@............ .............@@@@@@@@@@@@@@@@@.......... ............@@@@@@@@@@@@@@@@@@@@........ ...........@@@@@@@@@@@@@@@@@@@.......... ...........@@@@@@@@@@@@@@@@............. ..........@@@@@@@@@@@@@@................ .........@@@@@@@@@@@@................... .........@@@@@@@@@...................... ........@@@@@@@......................... .......@@@@@............................ .......@@............................... ........................................ ........................................
FutureBasic
_window = 1
begin enum 1
_textLabel
end enum
void local fn BuildWindow
window _window, @"Find if a point is within a triangle", (0, 0, 340, 360 )
WindowCenter(_window)
WindowSubclassContentView(_window)
ViewSetFlipped( _windowContentViewTag, YES )
ViewSetNeedsDisplay( _windowContentViewTag )
subclass textLabel _textLabel, @"", ( 20, 320, 300, 20 ), _window
end fn
void local fn DrawInView( tag as NSInteger )
BezierPathRef path = fn BezierPathInit
BezierPathMoveToPoint( path, fn CGPointMake( 30, 300 ) )
BezierPathLineToPoint( path, fn CGPointMake( 300, 300 ) )
BezierPathLineToPoint( path, fn CGPointMake( 150, 30 ) )
BezierPathClose( path )
BezierPathStrokeFill( path, 3.0, fn ColorBlack, fn ColorGreen )
AppSetProperty( @"path", path )
end fn
void local fn DoMouse( tag as NSInteger )
CGPoint pt = fn EventLocationInView( tag )
if ( fn BezierPathContainsPoint( fn AppProperty( @"path" ), pt ) )
ControlSetStringValue( _textLabel, fn StringWithFormat( @"Inside triangle: x = %.f y = %.f", pt.x, pt.y ) )
else
ControlSetStringValue( _textLabel, fn StringWithFormat( @"Outside triangle: x = %.f y = %.f", pt.x, pt.y ) )
end if
end fn
void local fn DoDialog( ev as long, tag as long )
select ( ev )
case _viewDrawRect : fn DrawInView(tag)
case _viewMouseDown : fn DoMouse( tag )
case _viewMouseMoved : fn DoMouse( tag )
end select
end fn
fn BuildWindow
on dialog fn DoDialog
HandleEvents- Output:
Go
package main
import (
"fmt"
"math"
)
const EPS = 0.001
const EPS_SQUARE = EPS * EPS
func side(x1, y1, x2, y2, x, y float64) float64 {
return (y2-y1)*(x-x1) + (-x2+x1)*(y-y1)
}
func naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y float64) bool {
checkSide1 := side(x1, y1, x2, y2, x, y) >= 0
checkSide2 := side(x2, y2, x3, y3, x, y) >= 0
checkSide3 := side(x3, y3, x1, y1, x, y) >= 0
return checkSide1 && checkSide2 && checkSide3
}
func pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y float64) bool {
xMin := math.Min(x1, math.Min(x2, x3)) - EPS
xMax := math.Max(x1, math.Max(x2, x3)) + EPS
yMin := math.Min(y1, math.Min(y2, y3)) - EPS
yMax := math.Max(y1, math.Max(y2, y3)) + EPS
return !(x < xMin || xMax < x || y < yMin || yMax < y)
}
func distanceSquarePointToSegment(x1, y1, x2, y2, x, y float64) float64 {
p1_p2_squareLength := (x2-x1)*(x2-x1) + (y2-y1)*(y2-y1)
dotProduct := ((x-x1)*(x2-x1) + (y-y1)*(y2-y1)) / p1_p2_squareLength
if dotProduct < 0 {
return (x-x1)*(x-x1) + (y-y1)*(y-y1)
} else if dotProduct <= 1 {
p_p1_squareLength := (x1-x)*(x1-x) + (y1-y)*(y1-y)
return p_p1_squareLength - dotProduct*dotProduct*p1_p2_squareLength
} else {
return (x-x2)*(x-x2) + (y-y2)*(y-y2)
}
}
func accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y float64) bool {
if !pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y) {
return false
}
if naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y) {
return true
}
if distanceSquarePointToSegment(x1, y1, x2, y2, x, y) <= EPS_SQUARE {
return true
}
if distanceSquarePointToSegment(x2, y2, x3, y3, x, y) <= EPS_SQUARE {
return true
}
if distanceSquarePointToSegment(x3, y3, x1, y1, x, y) <= EPS_SQUARE {
return true
}
return false
}
func main() {
pts := [][2]float64{{0, 0}, {0, 1}, {3, 1}}
tri := [][2]float64{{3.0 / 2, 12.0 / 5}, {51.0 / 10, -31.0 / 10}, {-19.0 / 5, 1.2}}
fmt.Println("Triangle is", tri)
x1, y1 := tri[0][0], tri[0][1]
x2, y2 := tri[1][0], tri[1][1]
x3, y3 := tri[2][0], tri[2][1]
for _, pt := range pts {
x, y := pt[0], pt[1]
within := accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)
fmt.Println("Point", pt, "is within triangle?", within)
}
fmt.Println()
tri = [][2]float64{{1.0 / 10, 1.0 / 9}, {100.0 / 8, 100.0 / 3}, {100.0 / 4, 100.0 / 9}}
fmt.Println("Triangle is", tri)
x1, y1 = tri[0][0], tri[0][1]
x2, y2 = tri[1][0], tri[1][1]
x3, y3 = tri[2][0], tri[2][1]
x := x1 + (3.0/7)*(x2-x1)
y := y1 + (3.0/7)*(y2-y1)
pt := [2]float64{x, y}
within := accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)
fmt.Println("Point", pt, "is within triangle ?", within)
fmt.Println()
tri = [][2]float64{{1.0 / 10, 1.0 / 9}, {100.0 / 8, 100.0 / 3}, {-100.0 / 8, 100.0 / 6}}
fmt.Println("Triangle is", tri)
x3 = tri[2][0]
y3 = tri[2][1]
within = accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)
fmt.Println("Point", pt, "is within triangle ?", within)
}
- Output:
Triangle is [[1.5 2.4] [5.1 -3.1] [-3.8 1.2]] Point [0 0] is within triangle? true Point [0 1] is within triangle? true Point [3 1] is within triangle? false Triangle is [[0.1 0.1111111111111111] [12.5 33.333333333333336] [25 11.11111111111111]] Point [5.414285714285714 14.349206349206348] is within triangle ? true Triangle is [[0.1 0.1111111111111111] [12.5 33.333333333333336] [-12.5 16.666666666666668]] Point [5.414285714285714 14.349206349206348] is within triangle ? true
GW-BASIC
10 PIT1X! = 3 : PIT1Y! = 1.3 : REM arbitrary triangle for demonstration
20 PIT2X! = 17.222 : PIT2Y! = 10
30 PIT3X! = 5.5 : PIT3Y! = 18.212
40 FOR PITPY! = 0 TO 19 STEP 1
50 FOR PITPX! = 0 TO 20 STEP .5
60 GOSUB 1000
70 IF PITRES% = 0 THEN PRINT "."; ELSE PRINT "#";
80 NEXT PITPX!
90 PRINT
100 NEXT PITPY!
110 END
1000 REM Detect if point is in triangle. Takes 8 double-precision
1010 REM values: (PIT1X!, PIT1Y!), (PIT2X!, PIT2Y!), (PIT3X!, PIT3Y!)
1020 REM for the coordinates of the corners of the triangle
1030 REM and (PITPX!, PITPY!) for the coordinates of the test point
1040 REM Returns PITRES%: 1=in triangle, 0=not in it
1050 PITDAR! = -PIT2Y!*PIT3X! + PIT1Y!*(-PIT2X! + PIT3X!) + PIT1X!*(PIT2Y - PIT3Y!) + PIT2X!*PIT3Y!
1060 PITXXS = (PIT1Y!*PIT3X! - PIT1X!*PIT3Y! + (PIT3Y! - PIT1Y!)*PITPX! + (PIT1X! - PIT3X!)*PITPY!)/PITDAR!
1070 PITXXT = (PIT1X!*PIT2Y! - PIT1Y!*PIT2X! + (PIT1Y! - PIT2Y!)*PITPX! + (PIT2X! - PIT1X!)*PITPY!)/PITDAR!
1080 PITRES% = 0
1090 IF PITXXS!<=0 THEN RETURN
1100 IF PITXXT!<=0 THEN RETURN
1110 IF PITXXS!+PITXXT!>=1 THEN RETURN
1120 PITRES% = 1
1130 RETURN
- Output:
.................................................................................. .......##................................ .......#####............................. .......########.......................... ........###########...................... ........##############................... ........#################................ ........####################............. .........#######################......... .........##########################...... .........#######################......... ..........###################............ ..........################............... ..........##############................. ...........##########.................... ...........#######....................... ...........####.......................... ...........#............................. .........................................
Haskell
The point to be tested is transformed by affine transformation which turns given triangle to the simplex: Triangle (0,0) (0,s) (s,0), where s is half of the triangles' area. After that criteria of overlapping become trivial. Affinity allows to avoid division, so all functions work for points on the integer, or rational, or even modular meshes as well.
type Pt a = (a, a)
data Overlapping = Inside | Outside | Boundary
deriving (Show, Eq)
data Triangle a = Triangle (Pt a) (Pt a) (Pt a)
deriving Show
vertices (Triangle a b c) = [a, b, c]
-- Performs the affine transformation
-- which turns a triangle to Triangle (0,0) (0,s) (s,0)
-- where s is half of the triangles' area
toTriangle :: Num a => Triangle a -> Pt a -> (a, Pt a)
toTriangle t (x,y) = let
[(x0,y0), (x1,y1), (x2,y2)] = vertices t
s = x2*(y0-y1)+x0*(y1-y2)+x1*(-y0+y2)
in ( abs s
, ( signum s * (x2*(-y+y0)+x0*(y-y2)+x*(-y0+y2))
, signum s * (x1*(y-y0)+x*(y0-y1)+x0*(-y+y1))))
overlapping :: (Eq a, Ord a, Num a) =>
Triangle a -> Pt a -> Overlapping
overlapping t p = case toTriangle t p of
(s, (x, y))
| s == 0 && (x == 0 || y == 0) -> Boundary
| s == 0 -> Outside
| x > 0 && y > 0 && y < s - x -> Inside
| (x <= s && x >= 0) &&
(y <= s && y >= 0) &&
(x == 0 || y == 0 || y == s - x) -> Boundary
| otherwise -> Outside
Testing
tests = let
t1 = Triangle (2,0) (-1,2) (-2,-2)
bs = [(2,0), (-1,2), (-2,-2), (0,-1), (1/2,1), (-3/2,0)]
is = [(0,0), (0,1), (-1,0), (-1,1), (-1,-1)]
os = [(1,1), (-2,2), (100,100), (2.00000001, 0)]
t2 = Triangle (1,2) (1,2) (-1,3)
ps = [(1,2), (0,5/2), (0,2), (1,3)]
in mapM_ print [ overlapping t1 <$> bs
, overlapping t1 <$> is
, overlapping t1 <$> os
, overlapping t2 <$> ps]
test2 = unlines
[ [case overlapping t (i,j) of
Inside -> '∗'
Boundary -> '+'
Outside -> '·'
| i <- [-10..10] :: [Int] ]
| j <- [-5..5] :: [Int] ]
where t = Triangle (-8,-3) (8,1) (-1,4)
λ> tests [Boundary,Boundary,Boundary,Boundary,Boundary,Boundary] [Inside,Inside,Inside,Inside,Inside] [Outside,Outside,Outside,Outside] [Boundary,Boundary,Outside,Outside] λ> putStrLn test2 ····················· ····················· ··+·················· ···+∗∗+·············· ····+∗∗∗∗∗+·········· ·····+∗∗∗∗∗∗∗∗+······ ······+∗∗∗∗∗∗∗∗∗∗∗+·· ·······+∗∗∗∗∗∗∗+····· ········+∗∗∗+········ ·········+··········· ·····················
J
Implementation, using complex numbers to represent x,y coordinates:
area=: [:| 0.5-/ .*@,.+. NB. signed area of triangle
I3=: =i.3 NB. identity matrix
inside=: {{ (area y)=+/area"1|:(I3*x)+(1-I3)*y }}
This is based on the algorithm documented for the ada implementation: compute the area of triangles using the determinant method (we want the absolute area here), and check whether the triangles formed with the test point and the sides of the test triangle matches the area of the test triangle.
Examples:
0j0 inside 1.5j2.4 5.1j_3.1 _3.8j1.2
1
0j1 inside 1.5j2.4 5.1j_3.1 _3.8j1.2
1
3j1 inside 1.5j2.4 5.1j_3.1 _3.8j1.2
0
5.414285714285714j14.349206349206348 inside 0.1j1r9 12.5j100r3 25j100r9
1
5.414285714285714j14.349206349206348 inside 0.1j1r9 12.5j100r3 _12.5j100r6
1
Java
import java.util.Objects;
public class FindTriangle {
private static final double EPS = 0.001;
private static final double EPS_SQUARE = EPS * EPS;
public static class Point {
private final double x, y;
public Point(double x, double y) {
this.x = x;
this.y = y;
}
public double getX() {
return x;
}
public double getY() {
return y;
}
@Override
public String toString() {
return String.format("(%f, %f)", x, y);
}
}
public static class Triangle {
private final Point p1, p2, p3;
public Triangle(Point p1, Point p2, Point p3) {
this.p1 = Objects.requireNonNull(p1);
this.p2 = Objects.requireNonNull(p2);
this.p3 = Objects.requireNonNull(p3);
}
public Point getP1() {
return p1;
}
public Point getP2() {
return p2;
}
public Point getP3() {
return p3;
}
private boolean pointInTriangleBoundingBox(Point p) {
var xMin = Math.min(p1.getX(), Math.min(p2.getX(), p3.getX())) - EPS;
var xMax = Math.max(p1.getX(), Math.max(p2.getX(), p3.getX())) + EPS;
var yMin = Math.min(p1.getY(), Math.min(p2.getY(), p3.getY())) - EPS;
var yMax = Math.max(p1.getY(), Math.max(p2.getY(), p3.getY())) + EPS;
return !(p.getX() < xMin || xMax < p.getX() || p.getY() < yMin || yMax < p.getY());
}
private static double side(Point p1, Point p2, Point p) {
return (p2.getY() - p1.getY()) * (p.getX() - p1.getX()) + (-p2.getX() + p1.getX()) * (p.getY() - p1.getY());
}
private boolean nativePointInTriangle(Point p) {
boolean checkSide1 = side(p1, p2, p) >= 0;
boolean checkSide2 = side(p2, p3, p) >= 0;
boolean checkSide3 = side(p3, p1, p) >= 0;
return checkSide1 && checkSide2 && checkSide3;
}
private double distanceSquarePointToSegment(Point p1, Point p2, Point p) {
double p1_p2_squareLength = (p2.getX() - p1.getX()) * (p2.getX() - p1.getX()) + (p2.getY() - p1.getY()) * (p2.getY() - p1.getY());
double dotProduct = ((p.getX() - p1.getX()) * (p2.getX() - p1.getX()) + (p.getY() - p1.getY()) * (p2.getY() - p1.getY())) / p1_p2_squareLength;
if (dotProduct < 0) {
return (p.getX() - p1.getX()) * (p.getX() - p1.getX()) + (p.getY() - p1.getY()) * (p.getY() - p1.getY());
}
if (dotProduct <= 1) {
double p_p1_squareLength = (p1.getX() - p.getX()) * (p1.getX() - p.getX()) + (p1.getY() - p.getY()) * (p1.getY() - p.getY());
return p_p1_squareLength - dotProduct * dotProduct * p1_p2_squareLength;
}
return (p.getX() - p2.getX()) * (p.getX() - p2.getX()) + (p.getY() - p2.getY()) * (p.getY() - p2.getY());
}
private boolean accuratePointInTriangle(Point p) {
if (!pointInTriangleBoundingBox(p)) {
return false;
}
if (nativePointInTriangle(p)) {
return true;
}
if (distanceSquarePointToSegment(p1, p2, p) <= EPS_SQUARE) {
return true;
}
if (distanceSquarePointToSegment(p2, p3, p) <= EPS_SQUARE) {
return true;
}
return distanceSquarePointToSegment(p3, p1, p) <= EPS_SQUARE;
}
public boolean within(Point p) {
Objects.requireNonNull(p);
return accuratePointInTriangle(p);
}
@Override
public String toString() {
return String.format("Triangle[%s, %s, %s]", p1, p2, p3);
}
}
private static void test(Triangle t, Point p) {
System.out.println(t);
System.out.printf("Point %s is within triangle? %s\n", p, t.within(p));
}
public static void main(String[] args) {
var p1 = new Point(1.5, 2.4);
var p2 = new Point(5.1, -3.1);
var p3 = new Point(-3.8, 1.2);
var tri = new Triangle(p1, p2, p3);
test(tri, new Point(0, 0));
test(tri, new Point(0, 1));
test(tri, new Point(3, 1));
System.out.println();
p1 = new Point(1.0 / 10, 1.0 / 9);
p2 = new Point(100.0 / 8, 100.0 / 3);
p3 = new Point(100.0 / 4, 100.0 / 9);
tri = new Triangle(p1, p2, p3);
var pt = new Point(p1.getX() + (3.0 / 7) * (p2.getX() - p1.getX()), p1.getY() + (3.0 / 7) * (p2.getY() - p1.getY()));
test(tri, pt);
System.out.println();
p3 = new Point(-100.0 / 8, 100.0 / 6);
tri = new Triangle(p1, p2, p3);
test(tri, pt);
}
}
- Output:
Triangle[(1.500000, 2.400000), (5.100000, -3.100000), (-3.800000, 1.200000)] Point (0.000000, 0.000000) is within triangle? true Triangle[(1.500000, 2.400000), (5.100000, -3.100000), (-3.800000, 1.200000)] Point (0.000000, 1.000000) is within triangle? true Triangle[(1.500000, 2.400000), (5.100000, -3.100000), (-3.800000, 1.200000)] Point (3.000000, 1.000000) is within triangle? false Triangle[(0.100000, 0.111111), (12.500000, 33.333333), (25.000000, 11.111111)] Point (5.414286, 14.349206) is within triangle? true Triangle[(0.100000, 0.111111), (12.500000, 33.333333), (-12.500000, 16.666667)] Point (5.414286, 14.349206) is within triangle? true
JavaScript
const EPS = 0.001;
const EPS_SQUARE = EPS * EPS;
function side(x1, y1, x2, y2, x, y) {
return (y2 - y1) * (x - x1) + (-x2 + x1) * (y - y1);
}
function naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y) {
const checkSide1 = side(x1, y1, x2, y2, x, y) >= 0;
const checkSide2 = side(x2, y2, x3, y3, x, y) >= 0;
const checkSide3 = side(x3, y3, x1, y1, x, y) >= 0;
return checkSide1 && checkSide2 && checkSide3;
}
function pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y) {
const xMin = Math.min(x1, Math.min(x2, x3)) - EPS;
const xMax = Math.max(x1, Math.max(x2, x3)) + EPS;
const yMin = Math.min(y1, Math.min(y2, y3)) - EPS;
const yMax = Math.max(y1, Math.max(y2, y3)) + EPS;
return !(x < xMin || xMax < x || y < yMin || yMax < y);
}
function distanceSquarePointToSegment(x1, y1, x2, y2, x, y) {
const p1_p2_squareLength = (x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1);
const dotProduct =
((x - x1) * (x2 - x1) + (y - y1) * (y2 - y1)) / p1_p2_squareLength;
if (dotProduct < 0) {
return (x - x1) * (x - x1) + (y - y1) * (y - y1);
} else if (dotProduct <= 1) {
const p_p1_squareLength = (x1 - x) * (x1 - x) + (y1 - y) * (y1 - y);
return p_p1_squareLength - dotProduct * dotProduct * p1_p2_squareLength;
} else {
return (x - x2) * (x - x2) + (y - y2) * (y - y2);
}
}
function accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y) {
if (!pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y)) {
return false;
}
if (naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)) {
return true;
}
if (distanceSquarePointToSegment(x1, y1, x2, y2, x, y) <= EPS_SQUARE) {
return true;
}
if (distanceSquarePointToSegment(x2, y2, x3, y3, x, y) <= EPS_SQUARE) {
return true;
}
if (distanceSquarePointToSegment(x3, y3, x1, y1, x, y) <= EPS_SQUARE) {
return true;
}
return false;
}
function printPoint(x, y) {
return "(" + x + ", " + y + ")";
}
function printTriangle(x1, y1, x2, y2, x3, y3) {
return (
"Triangle is [" +
printPoint(x1, y1) +
", " +
printPoint(x2, y2) +
", " +
printPoint(x3, y3) +
"]"
);
}
function test(x1, y1, x2, y2, x3, y3, x, y) {
console.log(
printTriangle(x1, y1, x2, y2, x3, y3) +
"Point " +
printPoint(x, y) +
" is within triangle? " +
(accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y) ? "true" : "false")
);
}
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 0);
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 1);
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 3, 1);
console.log();
test(
0.1,
0.1111111111111111,
12.5,
33.333333333333336,
25,
11.11111111111111,
5.414285714285714,
14.349206349206348
);
console.log();
test(
0.1,
0.1111111111111111,
12.5,
33.333333333333336,
-12.5,
16.666666666666668,
5.414285714285714,
14.349206349206348
);
console.log();
jq
Works with gojq, the Go implementation of jq
Adapted from Wren
A point is represented by [x,y] and denoted by P1, P2, P3, or Q.
A triangle is represented by an array of points: [P1, P2, P3].
Preliminaries
def sum_of_squares(stream): reduce stream as $x (0; . + $x * $x);
def distanceSquared(P1; P2): sum_of_squares(P1[0]-P2[0], P1[1]-P2[1]);
# Emit {x1,y1, ...} for the input triangle
def xy:
{ x1: .[0][0],
y1: .[0][1],
x2: .[1][0],
y2: .[1][1],
x3: .[2][0],
y3: .[2][1] };
def EPS: 0.001;
def EPS_SQUARE: EPS * EPS;def side(P1; P2; Q):
[P1, P2, Q]
| xy
| (.y2 - .y1)*(.x3 - .x1) + (-.x2 + .x1)*(.y3 - .y1);
def naivePointInTriangle(P1; P2; P3; Q):
side(P1; P2; Q) >= 0
and side(P2; P3; Q) >= 0
and side(P3; P1; Q) >= 0;
def pointInTriangleBoundingBox(P1; P2; P3; Q):
[P1,P2,P3]
| (map(.[0]) | min - EPS) as $xMin
| (map(.[0]) | max + EPS) as $xMax
| (map(.[1]) | min - EPS) as $yMin
| (map(.[1]) | max + EPS) as $yMax
| (Q[0] < $xMin or $xMax < Q[0] or Q[1] < $yMin or $yMax < Q[1]) | not;
def distanceSquarePointToSegment(P1; P2; Q):
distanceSquared(P1; P2) as $p1_p2_squareLength
| [P1, P2, Q]
| xy
| (((.x3 - .x1)*(.x2 - .x1) + (.y3 - .y1)*(.y2 - .y1)) / $p1_p2_squareLength) as $dotProduct
| if $dotProduct < 0
then sum_of_squares(.x3 - .x1, .y3 - .y1)
elif $dotProduct <= 1
then sum_of_squares(.x1 - .x3, .y1 - .y3) as $p_p1_squareLength
| $p_p1_squareLength - $dotProduct * $dotProduct * $p1_p2_squareLength
else sum_of_squares(.x3 - .x2, .y3 - .y2)
end;
def accuratePointInTriangle(P1; P2; P3; Q):
if (pointInTriangleBoundingBox(P1; P2; P3; Q) | not) then false
elif naivePointInTriangle(P1; P2; P3; Q) then true
elif distanceSquarePointToSegment(P1; P2; Q) <= EPS_SQUARE then true
elif distanceSquarePointToSegment(P2; P3; Q) <= EPS_SQUARE then true
elif distanceSquarePointToSegment(P3; P1; Q) <= EPS_SQUARE then true
else false
end;Examples
def task1:
def pts: [ [0, 0], [0, 1], [3, 1]];
"Triangle is \(.)",
(. as [$P1, $P2, $P3]
| pts[] as $Q
| accuratePointInTriangle($P1; $P2; $P3; $Q) as $within
| "Point \($Q) is within triangle ? \($within)"
);
def task2:
"Triangle is \(.)",
(. as [$P1, $P2, $P3]
| [ $P1[0] + (3/7)*($P2[0] - $P1[0]), $P1[1] + (3/7)*($P2[1] - $P1[1]) ] as $Q
| accuratePointInTriangle($P1; $P2; $P3; $Q) as $within
| "Point \($Q) is within triangle ? \($within)"
);
([ [3/2, 12/5], [51/10, -31/10], [-19/5, 1.2] ] | task1), "",
([ [1/10, 1/9], [100/8, 100/3], [100/4, 100/9] ] | task2), "",
([ [1/10, 1/9], [100/8, 100/3], [-100/8, 100/6] ] | task2)- Output:
Triangle is [[1.5,2.4],[5.1,-3.1],[-3.8,1.2]] Point [0,0] is within triangle ? true Point [0,1] is within triangle ? true Point [3,1] is within triangle ? false Triangle is [[0.1,0.1111111111111111],[12.5,33.333333333333336],[25,11.11111111111111]] Point [5.414285714285714,14.349206349206348] is within triangle ? true Triangle is [[0.1,0.1111111111111111],[12.5,33.333333333333336],[-12.5,16.666666666666668]] Point [5.414285714285714,14.349206349206348] is within triangle ? true
Julia
Using the Wren examples.
Point(x, y) = [x, y]
Triangle(a, b, c) = [a, b, c]
LEzero(x) = x < 0 || isapprox(x, 0, atol=0.00000001)
GEzero(x) = x > 0 || isapprox(x, 0, atol=0.00000001)
""" Determine which side of plane cut by line (p2, p3) p1 is on """
side(p1, p2, p3) = (p1[1] - p3[1]) * (p2[2] - p3[2]) - (p2[1] - p3[1]) * (p1[2] - p3[2])
"""
Determine if point is within triangle formed by points p1, p2, p3.
If so, the point will be on the same side of each of the half planes
defined by vectors p1p2, p2p3, and p3p1. Each z is positive if outside,
negative if inside such a plane. All should be positive or all negative
if point is within the triangle.
"""
function iswithin(point, p1, p2, p3)
z1 = side(point, p1, p2)
z2 = side(point, p2, p3)
z3 = side(point, p3, p1)
notanyneg = GEzero(z1) && GEzero(z2) && GEzero(z3)
notanypos = LEzero(z1) && LEzero(z2) && LEzero(z3)
return notanyneg || notanypos
end
const POINTS = [Point(0 // 1, 0 // 1), Point(0 // 1, 1 // 1), Point(3 // 1, 1 // 1),
Point(1 // 10 + (3 // 7) * (100 // 8 - 1 // 10), 1 // 9 + (3 // 7) * (100 // 3 - 1 // 9)),
Point(3 // 2, 12 // 5), Point(51 // 100, -31 // 100), Point(-19 // 50, 6 // 5),
Point(1 // 10, 1 // 9), Point(25 / 2, 100 // 3), Point(25, 100 // 9),
Point(-25 // 2, 50 // 3)
]
const TRI = [
Triangle(POINTS[5], POINTS[6], POINTS[7]),
Triangle(POINTS[8], POINTS[9], POINTS[10]),
Triangle(POINTS[8], POINTS[9], POINTS[11])
]
for tri in TRI
pstring(pt) = "[$(Float32(pt[1])), $(Float32(pt[2]))]"
println("\nUsing triangle [", join([pstring(x) for x in tri], ", "), "]:")
a, b, c = tri[1], tri[2], tri[3]
for p in POINTS[1:4]
isornot = iswithin(p, a, b, c) ? "is" : "is not"
println("Point $(pstring(p)) $isornot within the triangle.")
end
end
- Output:
Using triangle [[1.5, 2.4], [0.51, -0.31], [-0.38, 1.2]]: Point [0.0, 0.0] is not within the triangle. Point [0.0, 1.0] is within the triangle. Point [3.0, 1.0] is not within the triangle. Point [5.4142857, 14.349206] is not within the triangle. Using triangle [[0.1, 0.11111111], [12.5, 33.333332], [25.0, 11.111111]]: Point [0.0, 0.0] is not within the triangle. Point [0.0, 1.0] is not within the triangle. Point [3.0, 1.0] is not within the triangle. Point [5.4142857, 14.349206] is within the triangle. Using triangle [[0.1, 0.11111111], [12.5, 33.333332], [-12.5, 16.666666]]: Point [0.0, 0.0] is not within the triangle. Point [0.0, 1.0] is within the triangle. Point [3.0, 1.0] is not within the triangle. Point [5.4142857, 14.349206] is within the triangle.
Kotlin
import kotlin.math.max
import kotlin.math.min
private const val EPS = 0.001
private const val EPS_SQUARE = EPS * EPS
private fun test(t: Triangle, p: Point) {
println(t)
println("Point $p is within triangle ? ${t.within(p)}")
}
fun main() {
var p1 = Point(1.5, 2.4)
var p2 = Point(5.1, -3.1)
var p3 = Point(-3.8, 1.2)
var tri = Triangle(p1, p2, p3)
test(tri, Point(0.0, 0.0))
test(tri, Point(0.0, 1.0))
test(tri, Point(3.0, 1.0))
println()
p1 = Point(1.0 / 10, 1.0 / 9)
p2 = Point(100.0 / 8, 100.0 / 3)
p3 = Point(100.0 / 4, 100.0 / 9)
tri = Triangle(p1, p2, p3)
val pt = Point(p1.x + 3.0 / 7 * (p2.x - p1.x), p1.y + 3.0 / 7 * (p2.y - p1.y))
test(tri, pt)
println()
p3 = Point(-100.0 / 8, 100.0 / 6)
tri = Triangle(p1, p2, p3)
test(tri, pt)
}
class Point(val x: Double, val y: Double) {
override fun toString(): String {
return "($x, $y)"
}
}
class Triangle(private val p1: Point, private val p2: Point, private val p3: Point) {
private fun pointInTriangleBoundingBox(p: Point): Boolean {
val xMin = min(p1.x, min(p2.x, p3.x)) - EPS
val xMax = max(p1.x, max(p2.x, p3.x)) + EPS
val yMin = min(p1.y, min(p2.y, p3.y)) - EPS
val yMax = max(p1.y, max(p2.y, p3.y)) + EPS
return !(p.x < xMin || xMax < p.x || p.y < yMin || yMax < p.y)
}
private fun nativePointInTriangle(p: Point): Boolean {
val checkSide1 = side(p1, p2, p) >= 0
val checkSide2 = side(p2, p3, p) >= 0
val checkSide3 = side(p3, p1, p) >= 0
return checkSide1 && checkSide2 && checkSide3
}
private fun distanceSquarePointToSegment(p1: Point, p2: Point, p: Point): Double {
val p1P2SquareLength = (p2.x - p1.x) * (p2.x - p1.x) + (p2.y - p1.y) * (p2.y - p1.y)
val dotProduct = ((p.x - p1.x) * (p2.x - p1.x) + (p.y - p1.y) * (p2.y - p1.y)) / p1P2SquareLength
if (dotProduct < 0) {
return (p.x - p1.x) * (p.x - p1.x) + (p.y - p1.y) * (p.y - p1.y)
}
if (dotProduct <= 1) {
val pP1SquareLength = (p1.x - p.x) * (p1.x - p.x) + (p1.y - p.y) * (p1.y - p.y)
return pP1SquareLength - dotProduct * dotProduct * p1P2SquareLength
}
return (p.x - p2.x) * (p.x - p2.x) + (p.y - p2.y) * (p.y - p2.y)
}
private fun accuratePointInTriangle(p: Point): Boolean {
if (!pointInTriangleBoundingBox(p)) {
return false
}
if (nativePointInTriangle(p)) {
return true
}
if (distanceSquarePointToSegment(p1, p2, p) <= EPS_SQUARE) {
return true
}
return if (distanceSquarePointToSegment(p2, p3, p) <= EPS_SQUARE) {
true
} else distanceSquarePointToSegment(p3, p1, p) <= EPS_SQUARE
}
fun within(p: Point): Boolean {
return accuratePointInTriangle(p)
}
override fun toString(): String {
return "Triangle[$p1, $p2, $p3]"
}
companion object {
private fun side(p1: Point, p2: Point, p: Point): Double {
return (p2.y - p1.y) * (p.x - p1.x) + (-p2.x + p1.x) * (p.y - p1.y)
}
}
}
- Output:
Triangle[(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0.0, 0.0) is within triangle ? true Triangle[(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0.0, 1.0) is within triangle ? true Triangle[(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (3.0, 1.0) is within triangle ? false Triangle[(0.1, 0.1111111111111111), (12.5, 33.333333333333336), (25.0, 11.11111111111111)] Point (5.414285714285714, 14.349206349206348) is within triangle ? true Triangle[(0.1, 0.1111111111111111), (12.5, 33.333333333333336), (-12.5, 16.666666666666668)] Point (5.414285714285714, 14.349206349206348) is within triangle ? true
Lua
EPS = 0.001
EPS_SQUARE = EPS * EPS
function side(x1, y1, x2, y2, x, y)
return (y2 - y1) * (x - x1) + (-x2 + x1) * (y - y1)
end
function naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)
local checkSide1 = side(x1, y1, x2, y2, x, y) >= 0
local checkSide2 = side(x2, y2, x3, y3, x, y) >= 0
local checkSide3 = side(x3, y3, x1, y1, x, y) >= 0
return checkSide1 and checkSide2 and checkSide3
end
function pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y)
local xMin = math.min(x1, x2, x3) - EPS
local xMax = math.max(x1, x2, x3) + EPS
local yMin = math.min(y1, y2, y3) - EPS
local yMax = math.max(y1, y2, y3) + EPS
return not (x < xMin or xMax < x or y < yMin or yMax < y)
end
function distanceSquarePointToSegment(x1, y1, x2, y2, x, y)
local p1_p2_squareLength = (x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1)
local dotProduct = ((x - x1) * (x2 - x1) + (y - y1) * (y2 - y1)) / p1_p2_squareLength
if dotProduct < 0 then
return (x - x1) * (x - x1) + (y - y1) * (y - y1)
end
if dotProduct <= 1 then
local p_p1_squareLength = (x1 - x) * (x1 - x) + (y1 - y) * (y1 - y)
return p_p1_squareLength - dotProduct * dotProduct * p1_p2_squareLength
end
return (x - x2) * (x - x2) + (y - y2) * (y - y2)
end
function accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)
if not pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y) then
return false
end
if naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y) then
return true
end
if distanceSquarePointToSegment(x1, y1, x2, y2, x, y) <= EPS_SQUARE then
return true
end
if distanceSquarePointToSegment(x2, y2, x3, y3, x, y) <= EPS_SQUARE then
return true
end
if distanceSquarePointToSegment(x3, y3, x1, y1, x, y) <= EPS_SQUARE then
return true
end
return false
end
function printPoint(x, y)
io.write('('..x..", "..y..')')
end
function printTriangle(x1, y1, x2, y2, x3, y3)
io.write("Triangle is [")
printPoint(x1, y1)
io.write(", ")
printPoint(x2, y2)
io.write(", ")
printPoint(x3, y3)
print("]")
end
function test(x1, y1, x2, y2, x3, y3, x, y)
printTriangle(x1, y1, x2, y2, x3, y3)
io.write("Point ")
printPoint(x, y)
print(" is within triangle? " .. tostring(accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)))
end
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 0)
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0, 1)
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 3, 1)
print()
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, 25, 11.11111111111111, 5.414285714285714, 14.349206349206348)
print()
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, -12.5, 16.666666666666668, 5.414285714285714, 14.349206349206348)
print()
- Output:
Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0, 0) is within triangle? true Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0, 1) is within triangle? true Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (3, 1) is within triangle? false Triangle is [(0.1, 0.11111111111111), (12.5, 33.333333333333), (25, 11.111111111111)] Point (5.4142857142857, 14.349206349206) is within triangle? true Triangle is [(0.1, 0.11111111111111), (12.5, 33.333333333333), (-12.5, 16.666666666667)] Point (5.4142857142857, 14.349206349206) is within triangle? true
Mathematica /Wolfram Language
RegionMember[Polygon[{{1, 2}, {3, 1}, {2, 4}}], {2, 2}]
- Output:
True
Nim
import strformat
const
Eps = 0.001
Eps2 = Eps * Eps
type
Point = tuple[x, y: float]
Triangle = object
p1, p2, p3: Point
func initTriangle(p1, p2, p3: Point): Triangle =
Triangle(p1: p1, p2: p2, p3: p3)
func side(p1, p2, p: Point): float =
(p2.y - p1.y) * (p.x - p1.x) + (-p2.x + p1.x) * (p.y - p1.y)
func distanceSquarePointToSegment(p1, p2, p: Point): float =
let p1P2SquareLength = (p2.x - p1.x) * (p2.x - p1.x) + (p2.y - p1.y) * (p2.y - p1.y)
let dotProduct = ((p.x - p1.x) * (p2.x - p1.x) + (p.y - p1.y) * (p2.y - p1.y)) / p1P2SquareLength
if dotProduct < 0:
return (p.x - p1.x) * (p.x - p1.x) + (p.y - p1.y) * (p.y - p1.y)
if dotProduct <= 1:
let pP1SquareLength = (p1.x - p.x) * (p1.x - p.x) + (p1.y - p.y) * (p1.y - p.y)
return pP1SquareLength - dotProduct * dotProduct * p1P2SquareLength
result = (p.x - p2.x) * (p.x - p2.x) + (p.y - p2.y) * (p.y - p2.y)
func pointInTriangleBoundingBox(t: Triangle; p: Point): bool =
let xMin = min(t.p1.x, min(t.p2.x, t.p3.x)) - EPS
let xMax = max(t.p1.x, max(t.p2.x, t.p3.x)) + EPS
let yMin = min(t.p1.y, min(t.p2.y, t.p3.y)) - EPS
let yMax = max(t.p1.y, max(t.p2.y, t.p3.y)) + EPS
result = p.x in xMin..xMax and p.y in yMin..yMax
func nativePointInTriangle(t: Triangle; p: Point): bool =
let checkSide1 = side(t.p1, t.p2, p) >= 0
let checkSide2 = side(t.p2, t.p3, p) >= 0
let checkSide3 = side(t.p3, t.p1, p) >= 0
result = checkSide1 and checkSide2 and checkSide3
func accuratePointInTriangle(t: Triangle; p: Point): bool =
if not t.pointInTriangleBoundingBox(p):
return false
if t.nativePointInTriangle(p):
return true
if distanceSquarePointToSegment(t.p1, t.p2, p) <= Eps2 or
distanceSquarePointToSegment(t.p3, t.p1, p) <= Eps2:
return true
func `$`(p: Point): string = &"({p.x}, {p.y})"
func `$`(t: Triangle): string = &"Triangle[{t.p1}, {t.p2}, {t.p3}]"
func contains(t: Triangle; p: Point): bool = t.accuratePointInTriangle(p)
when isMainModule:
proc test(t: Triangle; p: Point) =
echo t
echo &"Point {p} is within triangle ? {p in t}"
var p1: Point = (1.5, 2.4)
var p2: Point = (5.1, -3.1)
var p3: Point = (-3.8, 1.2)
var tri = initTriangle(p1, p2, p3)
test(tri, (0.0, 0.0))
test(tri, (0.0, 1.0))
test(tri, (3.0, 1.0))
echo()
p1 = (1 / 10, 1 / 9)
p2 = (100 / 8, 100 / 3)
p3 = (100 / 4, 100 / 9)
tri = initTriangle(p1, p2, p3)
let pt = (p1.x + 3.0 / 7 * (p2.x - p1.x), p1.y + 3.0 / 7 * (p2.y - p1.y))
test(tri, pt)
echo()
p3 = (-100 / 8, 100 / 6)
tri = initTriangle(p1, p2, p3)
test(tri, pt)
- Output:
Triangle[(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0.0, 0.0) is within triangle ? true Triangle[(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0.0, 1.0) is within triangle ? true Triangle[(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (3.0, 1.0) is within triangle ? false Triangle[(0.1, 0.1111111111111111), (12.5, 33.33333333333334), (25.0, 11.11111111111111)] Point (5.414285714285714, 14.34920634920635) is within triangle ? true Triangle[(0.1, 0.1111111111111111), (12.5, 33.33333333333334), (-12.5, 16.66666666666667)] Point (5.414285714285714, 14.34920634920635) is within triangle ? true
PascalABC.NET
type Point = auto class
x,y: real;
end;
function Sign(pt1,pt2,pt3: Point)
:= (pt1.x - pt3.x) * (pt2.y - pt3.y) - (pt2.x - pt3.x) * (pt1.y - pt3.y);
function InTriangle(p,pt1,pt2,pt3: Point): boolean;
begin
var val1 := Sign(p, pt1, pt2);
var val2 := Sign(p, pt2, pt3);
var val3 := Sign(p, pt3, pt1);
var notanyneg := (val1 >= 0) and (val2 >= 0) and (val3 >= 0);
var notanypos := (val1 <= 0) and (val2 <= 0) and (val3 <= 0);
Result := notanyneg or notanypos;
end;
begin
var p1 := new Point(1.5, 2.4);
var p2 := new Point(5.1, -3.1);
var p3 := new Point(-3.8, 0.5);
for var x := 0 to 5 do
begin
var p := new Point(x,0);
Println($'{p} is in triangle ({p1},{p2},{p3}): {InTriangle(p,p1,p2,p3)}');
end
end.
- Output:
(0,0) is in triangle ((1.5,2.4),(5.1,-3.1),(-3.8,0.5)): True (1,0) is in triangle ((1.5,2.4),(5.1,-3.1),(-3.8,0.5)): True (2,0) is in triangle ((1.5,2.4),(5.1,-3.1),(-3.8,0.5)): True (3,0) is in triangle ((1.5,2.4),(5.1,-3.1),(-3.8,0.5)): True (4,0) is in triangle ((1.5,2.4),(5.1,-3.1),(-3.8,0.5)): False (5,0) is in triangle ((1.5,2.4),(5.1,-3.1),(-3.8,0.5)): False
Perl
Translate the Java program at this blog post and data set is taken from the Raku entry.
# 20201123 added Perl programming solution
use strict;
use warnings;
use List::AllUtils qw(min max natatime);
use constant EPSILON => 0.001;
use constant EPSILON_SQUARE => EPSILON*EPSILON;
sub side {
my ($x1, $y1, $x2, $y2, $x, $y) = @_;
return ($y2 - $y1)*($x - $x1) + (-$x2 + $x1)*($y - $y1);
}
sub naivePointInTriangle {
my ($x1, $y1, $x2, $y2, $x3, $y3, $x, $y) = @_;
my $checkSide1 = side($x1, $y1, $x2, $y2, $x, $y) >= 0 ;
my $checkSide2 = side($x2, $y2, $x3, $y3, $x, $y) >= 0 ;
my $checkSide3 = side($x3, $y3, $x1, $y1, $x, $y) >= 0 ;
return $checkSide1 && $checkSide2 && $checkSide3 || 0 ;
}
sub pointInTriangleBoundingBox {
my ($x1, $y1, $x2, $y2, $x3, $y3, $x, $y) = @_;
my $xMin = min($x1, min($x2, $x3)) - EPSILON;
my $xMax = max($x1, max($x2, $x3)) + EPSILON;
my $yMin = min($y1, min($y2, $y3)) - EPSILON;
my $yMax = max($y1, max($y2, $y3)) + EPSILON;
( $x < $xMin || $xMax < $x || $y < $yMin || $yMax < $y ) ? 0 : 1
}
sub distanceSquarePointToSegment {
my ($x1, $y1, $x2, $y2, $x, $y) = @_;
my $p1_p2_squareLength = ($x2 - $x1)**2 + ($y2 - $y1)**2;
my $dotProduct = ($x-$x1)*($x2-$x1)+($y-$y1)*($y2-$y1) ;
if ( $dotProduct < 0 ) {
return ($x - $x1)**2 + ($y - $y1)**2;
} elsif ( $dotProduct <= $p1_p2_squareLength ) {
my $p_p1_squareLength = ($x1 - $x)**2 + ($y1 - $y)**2;
return $p_p1_squareLength - $dotProduct**2 / $p1_p2_squareLength;
} else {
return ($x - $x2)**2 + ($y - $y2)**2;
}
}
sub accuratePointInTriangle {
my ($x1, $y1, $x2, $y2, $x3, $y3, $x, $y) = @_;
return 0 unless pointInTriangleBoundingBox($x1,$y1,$x2,$y2,$x3,$y3,$x,$y);
return 1 if ( naivePointInTriangle($x1, $y1, $x2, $y2, $x3, $y3, $x, $y)
or distanceSquarePointToSegment($x1, $y1, $x2, $y2, $x, $y) <= EPSILON_SQUARE
or distanceSquarePointToSegment($x2, $y2, $x3, $y3, $x, $y) <= EPSILON_SQUARE
or distanceSquarePointToSegment($x3, $y3, $x1, $y1, $x, $y) <= EPSILON_SQUARE);
return 0
}
my @DATA = (1.5, 2.4, 5.1, -3.1, -3.8, 0.5);
for my $point ( [0,0] , [0,1] ,[3,1] ) {
print "Point (", join(',',@$point), ") is within triangle ";
my $iter = natatime 2, @DATA;
while ( my @vertex = $iter->()) { print '(',join(',',@vertex),') ' }
print ': ',naivePointInTriangle (@DATA, @$point) ? 'True' : 'False', "\n" ;
}
- Output:
Point (0,0) is within triangle (1.5,2.4) (5.1,-3.1) (-3.8,0.5) : True Point (0,1) is within triangle (1.5,2.4) (5.1,-3.1) (-3.8,0.5) : True Point (3,1) is within triangle (1.5,2.4) (5.1,-3.1) (-3.8,0.5) : False
Phix
Both the following as well as some further experiments can be found in demo\rosetta\Within_triangle.exw
using convex_hull
Using convex_hull() from Convex_hull#Phix
with javascript_semantics constant p0 = {0,0}, p1 = {0,1}, p2 = {3,1}, triangle = {{3/2, 12/5}, {51/10, -31/10}, {-19/5, 1/2}} function inside(sequence p) return sort(convex_hull({p}&triangle))==sort(deep_copy(triangle)) end function printf(1,"Point %v is with triangle %v?:%t\n",{p0,triangle,inside(p0)}) printf(1,"Point %v is with triangle %v?:%t\n",{p1,triangle,inside(p1)}) printf(1,"Point %v is with triangle %v?:%t\n",{p2,triangle,inside(p2)})
- Output:
Point {0,0} is with triangle {{1.5,2.4},{5.1,-3.1},{-3.8,0.5}}?:true
Point {0,1} is with triangle {{1.5,2.4},{5.1,-3.1},{-3.8,0.5}}?:true
Point {3,1} is with triangle {{1.5,2.4},{5.1,-3.1},{-3.8,0.5}}?:false
trans python
(same output)
with javascript_semantics constant p0 = {0,0}, p1 = {0,1}, p2 = {3,1}, triangle = {{3/2, 12/5}, {51/10, -31/10}, {-19/5, 1/2}} function side(sequence p1, p2, p3) -- which side of plane cut by line (p2, p3) is p1 on? atom {x1, y1} = p1, {x2, y2} = p2, {x3, y3} = p3 return (x1 - x3) * (y2 - y3) - (x2 - x3) * (y1 - y3) end function function iswithin(sequence point, triangle) -- -- Determine if point is within triangle. -- If so, the point will be on the same side of each of the half planes -- defined by vectors p1p2, p2p3, and p3p1. side is positive if outside, -- negative if inside such a plane. All should be non-negative or all -- non-positive if the point is within the triangle. -- sequence {pt1, pt2, pt3} = triangle atom side12 = side(point, pt1, pt2), side23 = side(point, pt2, pt3), side31 = side(point, pt3, pt1) bool all_non_neg = side12 >= 0 and side23 >= 0 and side31 >= 0, all_non_pos = side12 <= 0 and side23 <= 0 and side31 <= 0 return all_non_neg or all_non_pos end function printf(1,"point %v is with triangle %v?:%t\n",{p0,triangle,iswithin(p0,triangle)}) printf(1,"point %v is with triangle %v?:%t\n",{p1,triangle,iswithin(p1,triangle)}) printf(1,"point %v is with triangle %v?:%t\n",{p2,triangle,iswithin(p2,triangle)})
Python
""" find if point is in a triangle """
from sympy.geometry import Point, Triangle
def sign(pt1, pt2, pt3):
""" which side of plane cut by line (pt2, pt3) is pt1 on? """
return (pt1.x - pt3.x) * (pt2.y - pt3.y) - (pt2.x - pt3.x) * (pt1.y - pt3.y)
def iswithin(point, pt1, pt2, pt3):
"""
Determine if point is within triangle formed by points p1, p2, p3.
If so, the point will be on the same side of each of the half planes
defined by vectors p1p2, p2p3, and p3p1. zval is positive if outside,
negative if inside such a plane. All should be positive or all negative
if point is within the triangle.
"""
zval1 = sign(point, pt1, pt2)
zval2 = sign(point, pt2, pt3)
zval3 = sign(point, pt3, pt1)
notanyneg = zval1 >= 0 and zval2 >= 0 and zval3 >= 0
notanypos = zval1 <= 0 and zval2 <= 0 and zval3 <= 0
return notanyneg or notanypos
if __name__ == "__main__":
POINTS = [Point(0, 0)]
TRI = Triangle(Point(1.5, 2.4), Point(5.1, -3.1), Point(-3.8, 0.5))
for pnt in POINTS:
a, b, c = TRI.vertices
isornot = "is" if iswithin(pnt, a, b, c) else "is not"
print("Point", pnt, isornot, "within the triangle", TRI)
- Output:
Point Point2D(0, 0) is within the triangle Triangle(Point2D(3/2, 12/5), Point2D(51/10, -31/10), Point2D(-19/5, 1/2))
Racket
Racket has exact numbers in its numerical tower... so I don't see much motivation to accomodate rounding errors. This is why the implementation _fails_ the second imprecise test, whereas other implementations pass it. That point is very close to the edge of the triange. If your edge is fat enough (epsilon), it will fall inside. If it is infinitessimal (i.e. exact), it is on the outside.
I would probably use the dot-product version, if only because it requires less (no) division.
#lang racket/base
(define-syntax-rule (all-between-0..1? x ...)
(and (<= 0 x 1) ...))
(define (point-in-triangle?/barycentric x1 y1 x2 y2 x3 y3)
(let* ((y2-y3 (- y2 y3))
(x1-x3 (- x1 x3))
(x3-x2 (- x3 x2))
(y1-y3 (- y1 y3))
(d (+ (* y2-y3 x1-x3) (* x3-x2 y1-y3))))
(λ (x y)
(define a (/ (+ (* x3-x2 (- y y3)) (* y2-y3 (- x x3))) d))
(define b (/ (- (* x1-x3 (- y y3)) (* y1-y3 (- x x3))) d))
(define c (- 1 a b))
(all-between-0..1? a b c))))
(define (point-in-triangle?/parametric x1 y1 x2 y2 x3 y3)
(let ((dp (+ (* x1 (- y2 y3)) (* y1 (- x3 x2)) (* x2 y3) (- (* y2 x3)))))
(λ (x y)
(define t1 (/ (+ (* x (- y3 y1)) (* y (- x1 x3)) (- (* x1 y3)) (* y1 x3)) dp))
(define t2 (/ (+ (* x (- y2 y1)) (* y (- x1 x2)) (- (* x1 y2)) (* y1 x2)) (- dp)))
(all-between-0..1? t1 t2 (+ t1 t2)))))
(define (point-in-triangle?/dot-product X1 Y1 X2 Y2 X3 Y3)
(λ (x y)
(define (check-side x1 y1 x2 y2)
(>= (+ (* (- y2 y1) (- x x1)) (* (- x1 x2) (- y y1))) 0))
(and
(check-side X1 Y1 X2 Y2)
(check-side X2 Y2 X3 Y3)
(check-side X3 Y3 X1 Y1))))
(module+ main
(require rackunit)
(define (run-tests point-in-triangle?)
(define pit?-1 (point-in-triangle? #e1.5 #e2.4 #e5.1 #e-3.1 #e-3.8 #e1.2))
(check-true (pit?-1 0 0))
(check-true (pit?-1 0 1))
(check-false (pit?-1 3 1))
(check-true ((point-in-triangle? 1/10 1/9 25/2 100/3 25 10/9) #e5.414285714285714 #e14.349206349206348))
; exactly speaking, point is _not_ in the triangle
(check-false ((point-in-triangle? 1/10 1/9 25/2 100/3 -25/2 50/3) #e5.414285714285714 #e14.349206349206348)))
(run-tests point-in-triangle?/barycentric)
(run-tests point-in-triangle?/parametric)
(run-tests point-in-triangle?/dot-product))
- Output:
no output means all tests passed
Raku
Reusing code from the Convex hull task and some logic from the Determine if two triangles overlap task.
class Point {
has Real $.x is rw;
has Real $.y is rw;
method gist { [~] '(', self.x,', ', self.y, ')' };
}
sub sign (Point $a, Point $b, Point $c) {
($b.x - $a.x)*($c.y - $a.y) - ($b.y - $a.y)*($c.x - $a.x);
}
sub triangle (*@points where *.elems == 6) {
@points.batch(2).map: { Point.new(:x(.[0]),:y(.[1])) };
}
sub is-within ($point, @triangle is copy) {
my @signs = sign($point, |(@triangle.=rotate)[0,1]) xx 3;
so (all(@signs) >= 0) or so(all(@signs) <= 0);
}
my @triangle = triangle((1.5, 2.4), (5.1, -3.1), (-3.8, 0.5));
for Point.new(:x(0),:y(0)),
Point.new(:x(0),:y(1)),
Point.new(:x(3),:y(1))
-> $point {
say "Point {$point.gist} is within triangle {join ', ', @triangle».gist}: ",
$point.&is-within: @triangle
}
- Output:
Point (0, 0) is within triangle (1.5, 2.4), (5.1, -3.1), (-3.8, 0.5): True Point (0, 1) is within triangle (1.5, 2.4), (5.1, -3.1), (-3.8, 0.5): True Point (3, 1) is within triangle (1.5, 2.4), (5.1, -3.1), (-3.8, 0.5): False
REXX
Extra certification code was added to verify that the X,Y coördinates for the points are not missing and are numeric.
/*REXX program determines if a specified point is within a specified triangle. */
parse arg p a b c . /*obtain optional arguments from the CL*/
if p=='' | p=="," then p= '(0,0)' /*Not specified? Then use the default.*/
if a=='' | a=="," then a= '(1.5,2.4)' /* " " " " " " */
if b=='' | b=="," then b= '(5.1,-3.1)' /* " " " " " " */
if c=='' | c=="," then c= '(-3.8,0.5)' /* " " " " " " */
if ?(p, a, b, c) then @= ' is ' /*Is the point inside the triangle ? */
else @= " isn't " /* " " " outside " " */
say 'point' p @ " within the triangle " a ',' b "," c
exit 0 /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
cert: parse arg z,W; if datatype(z,'N') then return z; call serr z /*return coördinate.*/
serr: say W 'data point ' z " isn't numeric or missing."; exit 13 /*tell error message*/
x: procedure; parse arg "(" x ',' ; return cert(x,"X") /*return the X coördinate.*/
y: procedure; parse arg ',' y ")"; return cert(y,"Y") /* " " Y " */
$: parse arg aa,bb,cc; return (x(aa)-x(cc)) *(y(bb)-y(cc)) - (x(bb)-x(cc)) *(y(aa)-y(cc))
?: #1=$(p,a,b); #2=$(p,b,c); #3=$(p,c,a); return (#1>=0>=0>=0) | (#1<=0<=0<=0)
- output when using the default triangle and the point at: (0,0)
point (0,0) is within the triangle (1.5,2.4) , (5.1,-3.1) , (-3.8,0.5)
- output when using the default triangle and the point at: (0,1)
point (0,1) is within the triangle (1.5,2.4) , (5.1,-3.1) , (-3.8,0.5)
- output when using the default triangle and the point at: (3,1)
point (3,1) isn't within the triangle (1.5,2.4) , (5.1,-3.1) , (-3.8,0.5)
RPL
≪ { } → points
≪ 1 4 START C→R 1 →V3 'points' STO+ NEXT
1 3 FOR j points j GET V→ NEXT
{ 3 3 } →ARRY DET ABS
1 3 FOR j
points j GET V→
points j 1 + 4 MOD 1 MAX GET V→
points 4 GET V→
{ 3 3 } →ARRY DET ABS
NEXT
+ + ==
≫ ≫ 'INTRI?' STO
(1 0) (2 0) (0 2) (0 0) INTRI? (-1 0) (-1 -1) (2 2) (0 0) INTRI?
- Output:
2: 0 1: 1
Ruby
EPS = 0.001
EPS_SQUARE = EPS * EPS
def side(x1, y1, x2, y2, x, y)
return (y2 - y1) * (x - x1) + (-x2 + x1) * (y - y1)
end
def naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)
checkSide1 = side(x1, y1, x2, y2, x, y) >= 0
checkSide2 = side(x2, y2, x3, y3, x, y) >= 0
checkSide3 = side(x3, y3, x1, y1, x, y) >= 0
return checkSide1 && checkSide2 && checkSide3
end
def pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y)
xMin = [x1, x2, x3].min - EPS
xMax = [x1, x2, x3].max + EPS
yMin = [y1, y2, y3].min - EPS
yMax = [y1, y2, y3].max + EPS
return !(x < xMin || xMax < x || y < yMin || yMax < y)
end
def distanceSquarePointToSegment(x1, y1, x2, y2, x, y)
p1_p2_squareLength = (x2 - x1) * (x2 - x1) + (y2 - y1) * (y2 - y1)
dotProduct = ((x - x1) * (x2 - x1) + (y - y1) * (y2 - y1)) / p1_p2_squareLength
if dotProduct < 0 then
return (x - x1) * (x - x1) + (y - y1) * (y - y1)
end
if dotProduct <= 1 then
p_p1_squareLength = (x1 - x) * (x1 - x) + (y1 - y) * (y1 - y)
return p_p1_squareLength - dotProduct * dotProduct * p1_p2_squareLength
end
return (x - x2) * (x - x2) + (y - y2) * (y - y2)
end
def accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)
if !pointInTriangleBoundingBox(x1, y1, x2, y2, x3, y3, x, y) then
return false
end
if naivePointInTriangle(x1, y1, x2, y2, x3, y3, x, y) then
return true
end
if distanceSquarePointToSegment(x1, y1, x2, y2, x, y) <= EPS_SQUARE then
return true
end
if distanceSquarePointToSegment(x2, y2, x3, y3, x, y) <= EPS_SQUARE then
return true
end
if distanceSquarePointToSegment(x3, y3, x1, y1, x, y) <= EPS_SQUARE then
return true
end
return false
end
def main
pts = [[0, 0], [0, 1], [3, 1]]
tri = [[1.5, 2.4], [5.1, -3.1], [-3.8, 1.2]]
print "Triangle is ", tri, "\n"
x1, y1 = tri[0][0], tri[0][1]
x2, y2 = tri[1][0], tri[1][1]
x3, y3 = tri[2][0], tri[2][1]
for pt in pts
x, y = pt[0], pt[1]
within = accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)
print "Point ", pt, " is within triangle? ", within, "\n"
end
print "\n"
tri = [[0.1, 1.0 / 9.0], [12.5, 100.0 / 3.0], [25.0, 100.0 / 9.0]]
print "Triangle is ", tri, "\n"
x1, y1 = tri[0][0], tri[0][1]
x2, y2 = tri[1][0], tri[1][1]
x3, y3 = tri[2][0], tri[2][1]
x = x1 + (3.0 / 7.0) * (x2 - x1)
y = y1 + (3.0 / 7.0) * (y2 - y1)
pt = [x, y]
within = accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)
print "Point ", pt, " is within triangle? ", within, "\n"
print "\n"
tri = [[0.1, 1.0 / 9.0], [12.5, 100.0 / 3.0], [-12.5, 100.0 / 6.0]]
print "Triangle is ", tri, "\n"
x3, y3 = tri[2][0], tri[2][1]
within = accuratePointInTriangle(x1, y1, x2, y2, x3, y3, x, y)
print "Point ", pt, " is within triangle? ", within, "\n"
end
main()
- Output:
Triangle is [[1.5, 2.4], [5.1, -3.1], [-3.8, 1.2]] Point [0, 0] is within triangle? true Point [0, 1] is within triangle? true Point [3, 1] is within triangle? false Triangle is [[0.1, 0.1111111111111111], [12.5, 33.333333333333336], [25.0, 11.11111111111111]] Point [5.414285714285714, 14.349206349206348] is within triangle? true Triangle is [[0.1, 0.1111111111111111], [12.5, 33.333333333333336], [-12.5, 16.666666666666668]] Point [5.414285714285714, 14.349206349206348] is within triangle? true
Rust
const EPS: f64 = 0.001;
const EPS_SQUARE: f64 = EPS * EPS;
fn side(x1: f64, y1: f64, x2: f64, y2: f64, x: f64, y: f64) -> f64 {
(y2 - y1) * (x - x1) + (-x2 + x1) * (y - y1)
}
fn naive_point_in_triangle(x1: f64, y1: f64, x2: f64, y2: f64, x3: f64, y3: f64, x: f64, y: f64) -> bool {
let check_side1 = side(x1, y1, x2, y2, x, y) >= 0.0;
let check_side2 = side(x2, y2, x3, y3, x, y) >= 0.0;
let check_side3 = side(x3, y3, x1, y1, x, y) >= 0.0;
check_side1 && check_side2 && check_side3
}
fn point_in_triangle_bounding_box(x1: f64, y1: f64, x2: f64, y2: f64, x3: f64, y3: f64, x: f64, y: f64) -> bool {
let x_min = f64::min(x1, f64::min(x2, x3)) - EPS;
let x_max = f64::max(x1, f64::max(x2, x3)) + EPS;
let y_min = f64::min(y1, f64::min(y2, y3)) - EPS;
let y_max = f64::max(y1, f64::max(y2, y3)) + EPS;
!(x < x_min || x_max < x || y < y_min || y_max < y)
}
fn distance_square_point_to_segment(x1: f64, y1: f64, x2: f64, y2: f64, x: f64, y: f64) -> f64 {
let p1_p2_square_length = (x2 - x1).powi(2) + (y2 - y1).powi(2);
let dot_product = ((x - x1) * (x2 - x1) + (y - y1) * (y2 - y1)) / p1_p2_square_length;
if dot_product < 0.0 {
(x - x1).powi(2) + (y - y1).powi(2)
} else if dot_product <= 1.0 {
let p_p1_square_length = (x1 - x).powi(2) + (y1 - y).powi(2);
p_p1_square_length - dot_product.powi(2) * p1_p2_square_length
} else {
(x - x2).powi(2) + (y - y2).powi(2)
}
}
fn accurate_point_in_triangle(x1: f64, y1: f64, x2: f64, y2: f64, x3: f64, y3: f64, x: f64, y: f64) -> bool {
if !point_in_triangle_bounding_box(x1, y1, x2, y2, x3, y3, x, y) {
return false;
}
if naive_point_in_triangle(x1, y1, x2, y2, x3, y3, x, y) {
return true;
}
if distance_square_point_to_segment(x1, y1, x2, y2, x, y) <= EPS_SQUARE {
return true;
}
if distance_square_point_to_segment(x2, y2, x3, y3, x, y) <= EPS_SQUARE {
return true;
}
if distance_square_point_to_segment(x3, y3, x1, y1, x, y) <= EPS_SQUARE {
return true;
}
false
}
fn print_point(x: f64, y: f64) {
print!("({}, {})", x, y);
}
fn print_triangle(x1: f64, y1: f64, x2: f64, y2: f64, x3: f64, y3: f64) {
print!("Triangle is [");
print_point(x1, y1);
print!(", ");
print_point(x2, y2);
print!(", ");
print_point(x3, y3);
println!("]");
}
fn test(x1: f64, y1: f64, x2: f64, y2: f64, x3: f64, y3: f64, x: f64, y: f64) {
print_triangle(x1, y1, x2, y2, x3, y3);
print!("Point ");
print_point(x, y);
print!(" is within triangle? ");
println!("{}", accurate_point_in_triangle(x1, y1, x2, y2, x3, y3, x, y));
}
fn main() {
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0.0, 0.0);
println!();
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 0.0, 1.0);
println!();
test(1.5, 2.4, 5.1, -3.1, -3.8, 1.2, 3.0, 1.0);
println!();
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, 25.0, 11.11111111111111, 5.414285714285714, 14.349206349206348);
println!();
test(0.1, 0.1111111111111111, 12.5, 33.333333333333336, -12.5, 16.666666666666668, 5.414285714285714, 14.349206349206348);
println!();
}
- Output:
Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0, 0) is within triangle? true Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (0, 1) is within triangle? true Triangle is [(1.5, 2.4), (5.1, -3.1), (-3.8, 1.2)] Point (3, 1) is within triangle? false Triangle is [(0.1, 0.1111111111111111), (12.5, 33.333333333333336), (25, 11.11111111111111)] Point (5.414285714285714, 14.349206349206348) is within triangle? true Triangle is [(0.1, 0.1111111111111111), (12.5, 33.333333333333336), (-12.5, 16.666666666666668)] Point (5.414285714285714, 14.349206349206348) is within triangle? true
V (Vlang)
import math
const eps = 0.001
const eps_square = eps * eps
fn side(x1 f64, y1 f64, x2 f64, y2 f64, x f64, y f64) f64 {
return (y2-y1)*(x-x1) + (-x2+x1)*(y-y1)
}
fn native_point_in_triangle(x1 f64, y1 f64, x2 f64, y2 f64, x3 f64, y3 f64, x f64, y f64) bool {
check_side1 := side(x1, y1, x2, y2, x, y) >= 0
check_side2 := side(x2, y2, x3, y3, x, y) >= 0
check_side3 := side(x3, y3, x1, y1, x, y) >= 0
return check_side1 && check_side2 && check_side3
}
fn point_in_triangle_bounding_box(x1 f64, y1 f64, x2 f64, y2 f64, x3 f64, y3 f64, x f64, y f64) bool {
x_min := math.min(x1, math.min(x2, x3)) - eps
x_max := math.max(x1, math.max(x2, x3)) + eps
y_min := math.min(y1, math.min(y2, y3)) - eps
y_max := math.max(y1, math.max(y2, y3)) + eps
return !(x < x_min || x_max < x || y < y_min || y_max < y)
}
fn distance_square_point_to_segment(x1 f64, y1 f64, x2 f64, y2 f64, x f64, y f64) f64 {
pq_p2_square_length := (x2-x1)*(x2-x1) + (y2-y1)*(y2-y1)
dot_product := ((x-x1)*(x2-x1) + (y-y1)*(y2-y1)) / pq_p2_square_length
if dot_product < 0 {
return (x-x1)*(x-x1) + (y-y1)*(y-y1)
} else if dot_product <= 1 {
p_p1_square_length := (x1-x)*(x1-x) + (y1-y)*(y1-y)
return p_p1_square_length - dot_product*dot_product*pq_p2_square_length
} else {
return (x-x2)*(x-x2) + (y-y2)*(y-y2)
}
}
fn accurate_point_in_triangle(x1 f64, y1 f64, x2 f64, y2 f64, x3 f64, y3 f64, x f64, y f64) bool {
if !point_in_triangle_bounding_box(x1, y1, x2, y2, x3, y3, x, y) {
return false
}
if native_point_in_triangle(x1, y1, x2, y2, x3, y3, x, y) {
return true
}
if distance_square_point_to_segment(x1, y1, x2, y2, x, y) <= eps_square {
return true
}
if distance_square_point_to_segment(x2, y2, x3, y3, x, y) <= eps_square {
return true
}
if distance_square_point_to_segment(x3, y3, x1, y1, x, y) <= eps_square {
return true
}
return false
}
fn main() {
pts := [[f64(0), 0], [f64(0), 1], [f64(3), 1]]
mut tri := [[3.0 / 2, 12.0 / 5], [51.0 / 10, -31.0 / 10], [-19.0 / 5, 1.2]]
println("Triangle is $tri")
mut x1, mut y1 := tri[0][0], tri[0][1]
mut x2, mut y2 := tri[1][0], tri[1][1]
mut x3, mut y3 := tri[2][0], tri[2][1]
for pt in pts {
x, y := pt[0], pt[1]
within := accurate_point_in_triangle(x1, y1, x2, y2, x3, y3, x, y)
println("Point $pt is within triangle? $within")
}
println('')
tri = [[1.0 / 10, 1.0 / 9], [100.0 / 8, 100.0 / 3], [100.0 / 4, 100.0 / 9]]
println("Triangle is $tri")
x1, y1 = tri[0][0], tri[0][1]
x2, y2 = tri[1][0], tri[1][1]
x3, y3 = tri[2][0], tri[2][1]
x := x1 + (3.0/7)*(x2-x1)
y := y1 + (3.0/7)*(y2-y1)
pt := [x, y]
mut within := accurate_point_in_triangle(x1, y1, x2, y2, x3, y3, x, y)
println("Point $pt is within triangle ? $within")
println('')
tri = [[1.0 / 10, 1.0 / 9], [100.0 / 8, 100.0 / 3], [-100.0 / 8, 100.0 / 6]]
println("Triangle is $tri")
x3 = tri[2][0]
y3 = tri[2][1]
within = accurate_point_in_triangle(x1, y1, x2, y2, x3, y3, x, y)
println("Point $pt is within triangle ? $within")
}- Output:
Triangle is [[1.5, 2.4], [5.1, -3.1], [-3.8, 1.2]] Point [0, 0] is within triangle? true Point [0, 1] is within triangle? true Point [3, 1] is within triangle? false Triangle is [[0.1, 0.1111111111111111], [12.5, 33.333333333333336], [25, 11.11111111111111]] Point [5.414285714285714, 14.349206349206348] is within triangle ? true Triangle is [[0.1, 0.1111111111111111], [12.5, 33.333333333333336], [-12.5, 16.666666666666668]] Point [5.414285714285714, 14.349206349206348] is within triangle ? true
Wren
This is a translation of the ActionScript code for the 'accurate' method in the first referenced article above.
var EPS = 0.001
var EPS_SQUARE = EPS * EPS
var side = Fn.new { |x1, y1, x2, y2, x, y|
return (y2 - y1)*(x - x1) + (-x2 + x1)*(y - y1)
}
var naivePointInTriangle = Fn.new { |x1, y1, x2, y2, x3, y3, x, y|
var checkSide1 = side.call(x1, y1, x2, y2, x, y) >= 0
var checkSide2 = side.call(x2, y2, x3, y3, x, y) >= 0
var checkSide3 = side.call(x3, y3, x1, y1, x, y) >= 0
return checkSide1 && checkSide2 && checkSide3
}
var pointInTriangleBoundingBox = Fn.new { |x1, y1, x2, y2, x3, y3, x, y|
var xMin = x1.min(x2.min(x3)) - EPS
var xMax = x1.max(x2.max(x3)) + EPS
var yMin = y1.min(y2.min(y3)) - EPS
var yMax = y1.max(y2.max(y3)) + EPS
return !(x < xMin || xMax < x || y < yMin || yMax < y)
}
var distanceSquarePointToSegment = Fn.new { |x1, y1, x2, y2, x, y|
var p1_p2_squareLength = (x2 - x1)*(x2 - x1) + (y2 - y1)*(y2 - y1)
var dotProduct = ((x - x1)*(x2 - x1) + (y - y1)*(y2 - y1)) / p1_p2_squareLength
if (dotProduct < 0) {
return (x - x1)*(x - x1) + (y - y1)*(y - y1)
} else if (dotProduct <= 1) {
var p_p1_squareLength = (x1 - x)*(x1 - x) + (y1 - y)*(y1 - y)
return p_p1_squareLength - dotProduct * dotProduct * p1_p2_squareLength
} else {
return (x - x2)*(x - x2) + (y - y2)*(y - y2)
}
}
var accuratePointInTriangle = Fn.new { |x1, y1, x2, y2, x3, y3, x, y|
if (!pointInTriangleBoundingBox.call(x1, y1, x2, y2, x3, y3, x, y)) return false
if (naivePointInTriangle.call(x1, y1, x2, y2, x3, y3, x, y)) return true
if (distanceSquarePointToSegment.call(x1, y1, x2, y2, x, y) <= EPS_SQUARE) return true
if (distanceSquarePointToSegment.call(x2, y2, x3, y3, x, y) <= EPS_SQUARE) return true
if (distanceSquarePointToSegment.call(x3, y3, x1, y1, x, y) <= EPS_SQUARE) return true
return false
}
var pts = [ [0, 0], [0, 1], [3, 1]]
var tri = [ [3/2, 12/5], [51/10, -31/10], [-19/5, 1.2] ]
System.print("Triangle is %(tri)")
var x1 = tri[0][0]
var y1 = tri[0][1]
var x2 = tri[1][0]
var y2 = tri[1][1]
var x3 = tri[2][0]
var y3 = tri[2][1]
for (pt in pts) {
var x = pt[0]
var y = pt[1]
var within = accuratePointInTriangle.call(x1, y1, x2, y2, x3, y3, x, y)
System.print("Point %(pt) is within triangle ? %(within)")
}
System.print()
tri = [ [1/10, 1/9], [100/8, 100/3], [100/4, 100/9] ]
System.print("Triangle is %(tri)")
x1 = tri[0][0]
y1 = tri[0][1]
x2 = tri[1][0]
y2 = tri[1][1]
x3 = tri[2][0]
y3 = tri[2][1]
var x = x1 + (3/7)*(x2 - x1)
var y = y1 + (3/7)*(y2 - y1)
var pt = [x, y]
var within = accuratePointInTriangle.call(x1, y1, x2, y2, x3, y3, x, y)
System.print("Point %(pt) is within triangle ? %(within)")
System.print()
tri = [ [1/10, 1/9], [100/8, 100/3], [-100/8, 100/6] ]
System.print("Triangle is %(tri)")
x3 = tri[2][0]
y3 = tri[2][1]
within = accuratePointInTriangle.call(x1, y1, x2, y2, x3, y3, x, y)
System.print("Point %(pt) is within triangle ? %(within)")
- Output:
Triangle is [[1.5, 2.4], [5.1, -3.1], [-3.8, 1.2]] Point [0, 0] is within triangle ? true Point [0, 1] is within triangle ? true Point [3, 1] is within triangle ? false Triangle is [[0.1, 0.11111111111111], [12.5, 33.333333333333], [25, 11.111111111111]] Point [5.4142857142857, 14.349206349206] is within triangle ? true Triangle is [[0.1, 0.11111111111111], [12.5, 33.333333333333], [-12.5, 16.666666666667]] Point [5.4142857142857, 14.349206349206] is within triangle ? true
XPL0
func real Dot(W,X,Y,Z); \Return the dot product of two 2D vectors
real W,X,Y,Z; \ (W-X) dot (Y-Z)
real WX(2), YZ(2);
[WX(0):= W(0)-X(0); WX(1):= W(1)-X(1);
YZ(0):= Y(0)-Z(0); YZ(1):= Y(1)-Z(1);
return WX(0)*YZ(0) + WX(1)*YZ(1);
];
real A,B,C; \triangle
func PointInTri(P); \Return 'true' if point P is inside triangle ABC
real P;
int S0,S1,S2; \signs
[S0:= Dot(P,A,B,A) >= 0.0;
S1:= Dot(P,B,C,B) >= 0.0;
S2:= Dot(P,C,A,C) >= 0.0;
return S0=S1 & S1=S2 & S2=S0;
];
[A:= [10.5, 6.3]; B:= [13.5, 3.6]; C:= [ 3.3, -1.6];
Text(0, if PointInTri([10.0, 3.0]) then "inside" else "outside"); CrLf(0);
Text(0, if PointInTri([-5.0,-2.2]) then "inside" else "outside"); CrLf(0);
Text(0, if PointInTri([10.5, 6.3]) then "inside" else "outside"); CrLf(0);
]- Output:
inside outside inside
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