我需要一个基本函数来求点到线段的最短距离。你可以随意用任何你想要的语言来编写解决方案;我可以把它翻译成什么我正在使用(Javascript)。
编辑:线段由两个端点定义。线段AB由两点A (x1,y1)和B (x2,y2)定义。我要求的是这条线段到点C (x3,y3)的距离。我的几何技能生疏了,所以我看到的例子让我很困惑,我很遗憾地承认。
我需要一个基本函数来求点到线段的最短距离。你可以随意用任何你想要的语言来编写解决方案;我可以把它翻译成什么我正在使用(Javascript)。
编辑:线段由两个端点定义。线段AB由两点A (x1,y1)和B (x2,y2)定义。我要求的是这条线段到点C (x3,y3)的距离。我的几何技能生疏了,所以我看到的例子让我很困惑,我很遗憾地承认。
当前回答
下面是HSQLDB的SQL实现:
CREATE FUNCTION dist_to_segment(px double, py double, vx double, vy double, wx double, wy double)
RETURNS double
BEGIN atomic
declare l2 double;
declare t double;
declare nx double;
declare ny double;
set l2 =(vx - wx)*(vx - wx) + (vy - wy)*(vy - wy);
IF l2 = 0 THEN
RETURN sqrt((vx - px)*(vx - px) + (vy - py)*(vy - py));
ELSE
set t = ((px - vx) * (wx - vx) + (py - vy) * (wy - vy)) / l2;
set t = GREATEST(0, LEAST(1, t));
set nx=vx + t * (wx - vx);
set ny=vy + t * (wy - vy);
RETURN sqrt((nx - px)*(nx - px) + (ny - py)*(ny - py));
END IF;
END;
Postgres的实现:
CREATE FUNCTION dist_to_segment(px numeric, py numeric, vx numeric, vy numeric, wx numeric, wy numeric)
RETURNS numeric
AS $$
declare l2 numeric;
declare t numeric;
declare nx numeric;
declare ny numeric;
BEGIN
l2 := (vx - wx)*(vx - wx) + (vy - wy)*(vy - wy);
IF l2 = 0 THEN
RETURN sqrt((vx - px)*(vx - px) + (vy - py)*(vy - py));
ELSE
t := ((px - vx) * (wx - vx) + (py - vy) * (wy - vy)) / l2;
t := GREATEST(0, LEAST(1, t));
nx := vx + t * (wx - vx);
ny := vy + t * (wy - vy);
RETURN sqrt((nx - px)*(nx - px) + (ny - py)*(ny - py));
END IF;
END;
$$ LANGUAGE plpgsql;
其他回答
以下是Grumdrig解决方案的一个更完整的说明。这个版本还返回最近的点本身。
#include "stdio.h"
#include "math.h"
class Vec2
{
public:
float _x;
float _y;
Vec2()
{
_x = 0;
_y = 0;
}
Vec2( const float x, const float y )
{
_x = x;
_y = y;
}
Vec2 operator+( const Vec2 &v ) const
{
return Vec2( this->_x + v._x, this->_y + v._y );
}
Vec2 operator-( const Vec2 &v ) const
{
return Vec2( this->_x - v._x, this->_y - v._y );
}
Vec2 operator*( const float f ) const
{
return Vec2( this->_x * f, this->_y * f );
}
float DistanceToSquared( const Vec2 p ) const
{
const float dX = p._x - this->_x;
const float dY = p._y - this->_y;
return dX * dX + dY * dY;
}
float DistanceTo( const Vec2 p ) const
{
return sqrt( this->DistanceToSquared( p ) );
}
float DotProduct( const Vec2 p ) const
{
return this->_x * p._x + this->_y * p._y;
}
};
// return minimum distance between line segment vw and point p, and the closest point on the line segment, q
float DistanceFromLineSegmentToPoint( const Vec2 v, const Vec2 w, const Vec2 p, Vec2 * const q )
{
const float distSq = v.DistanceToSquared( w ); // i.e. |w-v|^2 ... avoid a sqrt
if ( distSq == 0.0 )
{
// v == w case
(*q) = v;
return v.DistanceTo( p );
}
// consider the line extending the segment, parameterized as v + t (w - v)
// we find projection of point p onto the line
// it falls where t = [(p-v) . (w-v)] / |w-v|^2
const float t = ( p - v ).DotProduct( w - v ) / distSq;
if ( t < 0.0 )
{
// beyond the v end of the segment
(*q) = v;
return v.DistanceTo( p );
}
else if ( t > 1.0 )
{
// beyond the w end of the segment
(*q) = w;
return w.DistanceTo( p );
}
// projection falls on the segment
const Vec2 projection = v + ( ( w - v ) * t );
(*q) = projection;
return p.DistanceTo( projection );
}
float DistanceFromLineSegmentToPoint( float segmentX1, float segmentY1, float segmentX2, float segmentY2, float pX, float pY, float *qX, float *qY )
{
Vec2 q;
float distance = DistanceFromLineSegmentToPoint( Vec2( segmentX1, segmentY1 ), Vec2( segmentX2, segmentY2 ), Vec2( pX, pY ), &q );
(*qX) = q._x;
(*qY) = q._y;
return distance;
}
void TestDistanceFromLineSegmentToPoint( float segmentX1, float segmentY1, float segmentX2, float segmentY2, float pX, float pY )
{
float qX;
float qY;
float d = DistanceFromLineSegmentToPoint( segmentX1, segmentY1, segmentX2, segmentY2, pX, pY, &qX, &qY );
printf( "line segment = ( ( %f, %f ), ( %f, %f ) ), p = ( %f, %f ), distance = %f, q = ( %f, %f )\n",
segmentX1, segmentY1, segmentX2, segmentY2, pX, pY, d, qX, qY );
}
void TestDistanceFromLineSegmentToPoint()
{
TestDistanceFromLineSegmentToPoint( 0, 0, 1, 1, 1, 0 );
TestDistanceFromLineSegmentToPoint( 0, 0, 20, 10, 5, 4 );
TestDistanceFromLineSegmentToPoint( 0, 0, 20, 10, 30, 15 );
TestDistanceFromLineSegmentToPoint( 0, 0, 20, 10, -30, 15 );
TestDistanceFromLineSegmentToPoint( 0, 0, 10, 0, 5, 1 );
TestDistanceFromLineSegmentToPoint( 0, 0, 0, 10, 1, 5 );
}
GLSL版:
// line (a -> b ) point p[enter image description here][1]
float distanceToLine(vec2 a, vec2 b, vec2 p) {
float aside = dot((p - a),(b - a));
if(aside< 0.0) return length(p-a);
float bside = dot((p - b),(a - b));
if(bside< 0.0) return length(p-b);
vec2 pointOnLine = (bside*a + aside*b)/pow(length(a-b),2.0);
return length(p - pointOnLine);
}
我需要一个Godot (GDscript)的实现,所以我写了一个基于grumdrig接受的答案:
func minimum_distance(v: Vector2, w: Vector2, p: Vector2):
# Return minimum distance between line segment vw and point p
var l2: float = (v - w).length_squared() # i.e. |w-v|^2 - avoid a sqrt
if l2 == 0.0:
return p.distance_to(v) # v == w case
# Consider the line extending the segment, parameterized as v + t (w - v).
# We find projection of point p onto the line.
# It falls where t = [(p-v) . (w-v)] / |w-v|^2
# We clamp t from [0,1] to handle points outside the segment vw.
var t: float = max(0, min(1, (p - v).dot(w - v) / l2))
var projection: Vector2 = v + t * (w - v) # Projection falls on the segment
return p.distance_to(projection)
看起来几乎每个人都在StackOverflow上贡献了一个答案(目前为止有23个答案),所以这里是我对c#的贡献。这主要是基于M. Katz的回答,而Katz的回答又基于Grumdrig的回答。
public struct MyVector
{
private readonly double _x, _y;
// Constructor
public MyVector(double x, double y)
{
_x = x;
_y = y;
}
// Distance from this point to another point, squared
private double DistanceSquared(MyVector otherPoint)
{
double dx = otherPoint._x - this._x;
double dy = otherPoint._y - this._y;
return dx * dx + dy * dy;
}
// Find the distance from this point to a line segment (which is not the same as from this
// point to anywhere on an infinite line). Also returns the closest point.
public double DistanceToLineSegment(MyVector lineSegmentPoint1, MyVector lineSegmentPoint2,
out MyVector closestPoint)
{
return Math.Sqrt(DistanceToLineSegmentSquared(lineSegmentPoint1, lineSegmentPoint2,
out closestPoint));
}
// Same as above, but avoid using Sqrt(), saves a new nanoseconds in cases where you only want
// to compare several distances to find the smallest or largest, but don't need the distance
public double DistanceToLineSegmentSquared(MyVector lineSegmentPoint1,
MyVector lineSegmentPoint2, out MyVector closestPoint)
{
// Compute length of line segment (squared) and handle special case of coincident points
double segmentLengthSquared = lineSegmentPoint1.DistanceSquared(lineSegmentPoint2);
if (segmentLengthSquared < 1E-7f) // Arbitrary "close enough for government work" value
{
closestPoint = lineSegmentPoint1;
return this.DistanceSquared(closestPoint);
}
// Use the magic formula to compute the "projection" of this point on the infinite line
MyVector lineSegment = lineSegmentPoint2 - lineSegmentPoint1;
double t = (this - lineSegmentPoint1).DotProduct(lineSegment) / segmentLengthSquared;
// Handle the two cases where the projection is not on the line segment, and the case where
// the projection is on the segment
if (t <= 0)
closestPoint = lineSegmentPoint1;
else if (t >= 1)
closestPoint = lineSegmentPoint2;
else
closestPoint = lineSegmentPoint1 + (lineSegment * t);
return this.DistanceSquared(closestPoint);
}
public double DotProduct(MyVector otherVector)
{
return this._x * otherVector._x + this._y * otherVector._y;
}
public static MyVector operator +(MyVector leftVector, MyVector rightVector)
{
return new MyVector(leftVector._x + rightVector._x, leftVector._y + rightVector._y);
}
public static MyVector operator -(MyVector leftVector, MyVector rightVector)
{
return new MyVector(leftVector._x - rightVector._x, leftVector._y - rightVector._y);
}
public static MyVector operator *(MyVector aVector, double aScalar)
{
return new MyVector(aVector._x * aScalar, aVector._y * aScalar);
}
// Added using ReSharper due to CodeAnalysis nagging
public bool Equals(MyVector other)
{
return _x.Equals(other._x) && _y.Equals(other._y);
}
public override bool Equals(object obj)
{
if (ReferenceEquals(null, obj)) return false;
return obj is MyVector && Equals((MyVector) obj);
}
public override int GetHashCode()
{
unchecked
{
return (_x.GetHashCode()*397) ^ _y.GetHashCode();
}
}
public static bool operator ==(MyVector left, MyVector right)
{
return left.Equals(right);
}
public static bool operator !=(MyVector left, MyVector right)
{
return !left.Equals(right);
}
}
这是一个小测试程序。
public static class JustTesting
{
public static void Main()
{
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
for (int i = 0; i < 10000000; i++)
{
TestIt(1, 0, 0, 0, 1, 1, 0.70710678118654757);
TestIt(5, 4, 0, 0, 20, 10, 1.3416407864998738);
TestIt(30, 15, 0, 0, 20, 10, 11.180339887498949);
TestIt(-30, 15, 0, 0, 20, 10, 33.541019662496844);
TestIt(5, 1, 0, 0, 10, 0, 1.0);
TestIt(1, 5, 0, 0, 0, 10, 1.0);
}
stopwatch.Stop();
TimeSpan timeSpan = stopwatch.Elapsed;
}
private static void TestIt(float aPointX, float aPointY,
float lineSegmentPoint1X, float lineSegmentPoint1Y,
float lineSegmentPoint2X, float lineSegmentPoint2Y,
double expectedAnswer)
{
// Katz
double d1 = DistanceFromPointToLineSegment(new MyVector(aPointX, aPointY),
new MyVector(lineSegmentPoint1X, lineSegmentPoint1Y),
new MyVector(lineSegmentPoint2X, lineSegmentPoint2Y));
Debug.Assert(d1 == expectedAnswer);
/*
// Katz using squared distance
double d2 = DistanceFromPointToLineSegmentSquared(new MyVector(aPointX, aPointY),
new MyVector(lineSegmentPoint1X, lineSegmentPoint1Y),
new MyVector(lineSegmentPoint2X, lineSegmentPoint2Y));
Debug.Assert(Math.Abs(d2 - expectedAnswer * expectedAnswer) < 1E-7f);
*/
/*
// Matti (optimized)
double d3 = FloatVector.DistanceToLineSegment(new PointF(aPointX, aPointY),
new PointF(lineSegmentPoint1X, lineSegmentPoint1Y),
new PointF(lineSegmentPoint2X, lineSegmentPoint2Y));
Debug.Assert(Math.Abs(d3 - expectedAnswer) < 1E-7f);
*/
}
private static double DistanceFromPointToLineSegment(MyVector aPoint,
MyVector lineSegmentPoint1, MyVector lineSegmentPoint2)
{
MyVector closestPoint; // Not used
return aPoint.DistanceToLineSegment(lineSegmentPoint1, lineSegmentPoint2,
out closestPoint);
}
private static double DistanceFromPointToLineSegmentSquared(MyVector aPoint,
MyVector lineSegmentPoint1, MyVector lineSegmentPoint2)
{
MyVector closestPoint; // Not used
return aPoint.DistanceToLineSegmentSquared(lineSegmentPoint1, lineSegmentPoint2,
out closestPoint);
}
}
如您所见,我试图衡量使用避免Sqrt()方法的版本与使用普通版本之间的差异。我的测试表明你可能可以节省2.5%,但我甚至不确定——各种测试运行中的变化是相同的数量级。我还试着测量了Matti发布的版本(加上一个明显的优化),该版本似乎比基于Katz/Grumdrig代码的版本慢了大约4%。
编辑:顺便说一句,我还尝试过测量一种方法,该方法使用叉乘(和平方根())来查找到无限直线(不是线段)的距离,它大约快32%。
C#
改编自@Grumdrig
public static double MinimumDistanceToLineSegment(this Point p,
Line line)
{
var v = line.StartPoint;
var w = line.EndPoint;
double lengthSquared = DistanceSquared(v, w);
if (lengthSquared == 0.0)
return Distance(p, v);
double t = Math.Max(0, Math.Min(1, DotProduct(p - v, w - v) / lengthSquared));
var projection = v + t * (w - v);
return Distance(p, projection);
}
public static double Distance(Point a, Point b)
{
return Math.Sqrt(DistanceSquared(a, b));
}
public static double DistanceSquared(Point a, Point b)
{
var d = a - b;
return DotProduct(d, d);
}
public static double DotProduct(Point a, Point b)
{
return (a.X * b.X) + (a.Y * b.Y);
}