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c# - 检测两条重合线段的重合子集

转载 作者:太空狗 更新时间:2023-10-29 17:35:29 26 4
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这个问题是关于:

但是请注意,一个有趣的子问题在大多数解决方案中都被完全掩盖了,即使存在三个子情况,它们也只是为重合情况返回 null:

  • 重合但不重叠
  • 仅点和重合
  • 重叠/重合线子段

例如,我们可以像这样设计一个 C# 函数:

public static PointF[] Intersection(PointF a1, PointF a2, PointF b1, PointF b2)

其中 (a1,a2) 是一条线段,(b1,b2) 是另一条线段。

此函数需要涵盖大多数实现或解释所掩盖的所有奇怪情况。为了解决重合线的奇怪问题,该函数可以返回一个 PointF 数组:

  • 如果线平行或不相交(无限线相交但线段不相交,或线平行),则结果点为零(或空)<
  • 一个结果点(包含相交位置),如果它们相交或者如果它们在一点重合
  • 两个结果点(对于线段的重叠部分)如果两条线重合

最佳答案

    // port of this JavaScript code with some changes:
// http://www.kevlindev.com/gui/math/intersection/Intersection.js
// found here:
// http://stackoverflow.com/questions/563198/how-do-you-detect-where-two-line-segments-intersect/563240#563240

public class Intersector
{
static double MyEpsilon = 0.00001;

private static float[] OverlapIntervals(float ub1, float ub2)
{
float l = Math.Min(ub1, ub2);
float r = Math.Max(ub1, ub2);
float A = Math.Max(0, l);
float B = Math.Min(1, r);
if (A > B) // no intersection
return new float[] { };
else if (A == B)
return new float[] { A };
else // if (A < B)
return new float[] { A, B };
}

// IMPORTANT: a1 and a2 cannot be the same, e.g. a1--a2 is a true segment, not a point
// b1/b2 may be the same (b1--b2 is a point)
private static PointF[] OneD_Intersection(PointF a1, PointF a2, PointF b1, PointF b2)
{
//float ua1 = 0.0f; // by definition
//float ua2 = 1.0f; // by definition
float ub1, ub2;

float denomx = a2.X - a1.X;
float denomy = a2.Y - a1.Y;

if (Math.Abs(denomx) > Math.Abs(denomy))
{
ub1 = (b1.X - a1.X) / denomx;
ub2 = (b2.X - a1.X) / denomx;
}
else
{
ub1 = (b1.Y - a1.Y) / denomy;
ub2 = (b2.Y - a1.Y) / denomy;
}

List<PointF> ret = new List<PointF>();
float[] interval = OverlapIntervals(ub1, ub2);
foreach (float f in interval)
{
float x = a2.X * f + a1.X * (1.0f - f);
float y = a2.Y * f + a1.Y * (1.0f - f);
PointF p = new PointF(x, y);
ret.Add(p);
}
return ret.ToArray();
}

private static bool PointOnLine(PointF p, PointF a1, PointF a2)
{
float dummyU = 0.0f;
double d = DistFromSeg(p, a1, a2, MyEpsilon, ref dummyU);
return d < MyEpsilon;
}

private static double DistFromSeg(PointF p, PointF q0, PointF q1, double radius, ref float u)
{
// formula here:
//http://mathworld.wolfram.com/Point-LineDistance2-Dimensional.html
// where x0,y0 = p
// x1,y1 = q0
// x2,y2 = q1
double dx21 = q1.X - q0.X;
double dy21 = q1.Y - q0.Y;
double dx10 = q0.X - p.X;
double dy10 = q0.Y - p.Y;
double segLength = Math.Sqrt(dx21 * dx21 + dy21 * dy21);
if (segLength < MyEpsilon)
throw new Exception("Expected line segment, not point.");
double num = Math.Abs(dx21 * dy10 - dx10 * dy21);
double d = num / segLength;
return d;
}

// this is the general case. Really really general
public static PointF[] Intersection(PointF a1, PointF a2, PointF b1, PointF b2)
{
if (a1.Equals(a2) && b1.Equals(b2))
{
// both "segments" are points, return either point
if (a1.Equals(b1))
return new PointF[] { a1 };
else // both "segments" are different points, return empty set
return new PointF[] { };
}
else if (b1.Equals(b2)) // b is a point, a is a segment
{
if (PointOnLine(b1, a1, a2))
return new PointF[] { b1 };
else
return new PointF[] { };
}
else if (a1.Equals(a2)) // a is a point, b is a segment
{
if (PointOnLine(a1, b1, b2))
return new PointF[] { a1 };
else
return new PointF[] { };
}

// at this point we know both a and b are actual segments

float ua_t = (b2.X - b1.X) * (a1.Y - b1.Y) - (b2.Y - b1.Y) * (a1.X - b1.X);
float ub_t = (a2.X - a1.X) * (a1.Y - b1.Y) - (a2.Y - a1.Y) * (a1.X - b1.X);
float u_b = (b2.Y - b1.Y) * (a2.X - a1.X) - (b2.X - b1.X) * (a2.Y - a1.Y);

// Infinite lines intersect somewhere
if (!(-MyEpsilon < u_b && u_b < MyEpsilon)) // e.g. u_b != 0.0
{
float ua = ua_t / u_b;
float ub = ub_t / u_b;
if (0.0f <= ua && ua <= 1.0f && 0.0f <= ub && ub <= 1.0f)
{
// Intersection
return new PointF[] {
new PointF(a1.X + ua * (a2.X - a1.X),
a1.Y + ua * (a2.Y - a1.Y)) };
}
else
{
// No Intersection
return new PointF[] { };
}
}
else // lines (not just segments) are parallel or the same line
{
// Coincident
// find the common overlapping section of the lines
// first find the distance (squared) from one point (a1) to each point
if ((-MyEpsilon < ua_t && ua_t < MyEpsilon)
|| (-MyEpsilon < ub_t && ub_t < MyEpsilon))
{
if (a1.Equals(a2)) // danger!
return OneD_Intersection(b1, b2, a1, a2);
else // safe
return OneD_Intersection(a1, a2, b1, b2);
}
else
{
// Parallel
return new PointF[] { };
}
}
}


}

测试代码如下:

    public class IntersectTest
{
public static void PrintPoints(PointF[] pf)
{
if (pf == null || pf.Length < 1)
System.Console.WriteLine("Doesn't intersect");
else if (pf.Length == 1)
{
System.Console.WriteLine(pf[0]);
}
else if (pf.Length == 2)
{
System.Console.WriteLine(pf[0] + " -- " + pf[1]);
}
}

public static void TestIntersect(PointF a1, PointF a2, PointF b1, PointF b2)
{
System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("Does " + a1 + " -- " + a2);
System.Console.WriteLine("intersect " + b1 + " -- " + b2 + " and if so, where?");
System.Console.WriteLine("");
PointF[] result = Intersect.Intersection(a1, a2, b1, b2);
PrintPoints(result);
}

public static void Main()
{
System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("line segments intersect");
TestIntersect(new PointF(0, 0),
new PointF(100, 100),
new PointF(100, 0),
new PointF(0, 100));
TestIntersect(new PointF(5, 17),
new PointF(100, 100),
new PointF(100, 29),
new PointF(8, 100));
System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("");

System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("just touching points and lines cross");
TestIntersect(new PointF(0, 0),
new PointF(25, 25),
new PointF(25, 25),
new PointF(100, 75));
System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("");

System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("parallel");
TestIntersect(new PointF(0, 0),
new PointF(0, 100),
new PointF(100, 0),
new PointF(100, 100));
System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("");

System.Console.WriteLine("----");
System.Console.WriteLine("lines cross but segments don't intersect");
TestIntersect(new PointF(50, 50),
new PointF(100, 100),
new PointF(0, 25),
new PointF(25, 0));
System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("");

System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("coincident but do not overlap!");
TestIntersect(new PointF(0, 0),
new PointF(25, 25),
new PointF(75, 75),
new PointF(100, 100));
System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("");

System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("touching points and coincident!");
TestIntersect(new PointF(0, 0),
new PointF(25, 25),
new PointF(25, 25),
new PointF(100, 100));
System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("");

System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("overlap/coincident");
TestIntersect(new PointF(0, 0),
new PointF(75, 75),
new PointF(25, 25),
new PointF(100, 100));
TestIntersect(new PointF(0, 0),
new PointF(100, 100),
new PointF(0, 0),
new PointF(100, 100));
System.Console.WriteLine("----------------------------------------------------------");
System.Console.WriteLine("");

while (!System.Console.KeyAvailable) { }
}

}

这是输出:

----------------------------------------------------------line segments intersect----------------------------------------------------------Does      {X=0, Y=0} -- {X=100, Y=100}intersect {X=100, Y=0} -- {X=0, Y=100} and if so, where?{X=50, Y=50}----------------------------------------------------------Does      {X=5, Y=17} -- {X=100, Y=100}intersect {X=100, Y=29} -- {X=8, Y=100} and if so, where?{X=56.85001, Y=62.30054}--------------------------------------------------------------------------------------------------------------------just touching points and lines cross----------------------------------------------------------Does      {X=0, Y=0} -- {X=25, Y=25}intersect {X=25, Y=25} -- {X=100, Y=75} and if so, where?{X=25, Y=25}--------------------------------------------------------------------------------------------------------------------parallel----------------------------------------------------------Does      {X=0, Y=0} -- {X=0, Y=100}intersect {X=100, Y=0} -- {X=100, Y=100} and if so, where?Doesn't intersect--------------------------------------------------------------lines cross but segments don't intersect----------------------------------------------------------Does      {X=50, Y=50} -- {X=100, Y=100}intersect {X=0, Y=25} -- {X=25, Y=0} and if so, where?Doesn't intersect--------------------------------------------------------------------------------------------------------------------coincident but do not overlap!----------------------------------------------------------Does      {X=0, Y=0} -- {X=25, Y=25}intersect {X=75, Y=75} -- {X=100, Y=100} and if so, where?Doesn't intersect--------------------------------------------------------------------------------------------------------------------touching points and coincident!----------------------------------------------------------Does      {X=0, Y=0} -- {X=25, Y=25}intersect {X=25, Y=25} -- {X=100, Y=100} and if so, where?{X=25, Y=25}--------------------------------------------------------------------------------------------------------------------overlap/coincident----------------------------------------------------------Does      {X=0, Y=0} -- {X=75, Y=75}intersect {X=25, Y=25} -- {X=100, Y=100} and if so, where?{X=25, Y=25} -- {X=75, Y=75}----------------------------------------------------------Does      {X=0, Y=0} -- {X=100, Y=100}intersect {X=0, Y=0} -- {X=100, Y=100} and if so, where?{X=0, Y=0} -- {X=100, Y=100}----------------------------------------------------------

关于c# - 检测两条重合线段的重合子集,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/2255842/

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