以下是解决方案的概述:
计算正方形的平面方程(假设四个点共面),
进行射线/平面相交,这将给出无结果(射线与正方形平行,并且我忽略了射线嵌入平面的情况)或一个点,
一旦您获得交点,请在正方形所在平面上的本地2D基础上对其进行投影,这将给出平面上点的2D坐标(u,v),
检查2D坐标(u,v)是否在正方形内(假设四个点构成平行四边形,并且您选择了两条相邻边用于本地2D基础),如果是,则存在交点(并且您具有u/v坐标)。
现在来看实际的方程式,假设四个正方形顶点如下放置:
S1 +------+ S2
| |
| |
S3 +------+ S4
平面的法向量为:n = (S2 - S1) x (S3 - S1)
点M属于该平面,当且仅当它满足这个方程:n . ( M - S1 ) = 0
点M属于射线,当且仅当它可以表示为:M = R1 + t * dR,其中dR = R2 - R1
计算射线与平面的交点(等同于上述两个方程):
n . ( M - S1 ) = 0 = n . ( R1 + t * dR - S1 ) = n . (R1 - S1) + t * n . dR
如果n . dR为0,则平面与射线平行,无交点(忽略射线嵌入平面的情况)。
否则,t = -n . (R1 - S1) / n . dR,并将此结果代入前一个方程M = R1 + t * dR,即可得到交点M的三维坐标。
将向量M - S1投影到两个向量S2 - S1和S3 - S1(从S1开始的正方形边缘),得到两个数字(u,v):
u = (M - S1) . (S2 - S1)
v = (M - S1) . (S3 - S1)
如果0 <= u <= |S2 - S1|^2并且0 <= v <= |S3 - S1|^2,则交点M位于正方形内部,否则在外部。
最后附上一个Java实现示例(为了阅读便利进行了优化...):
public class Test {
static class Vector3 {
public float x, y, z;
public Vector3(float x, float y, float z) {
this.x = x;
this.y = y;
this.z = z;
}
public Vector3 add(Vector3 other) {
return new Vector3(x + other.x, y + other.y, z + other.z);
}
public Vector3 sub(Vector3 other) {
return new Vector3(x - other.x, y - other.y, z - other.z);
}
public Vector3 scale(float f) {
return new Vector3(x * f, y * f, z * f);
}
public Vector3 cross(Vector3 other) {
return new Vector3(y * other.z - z * other.y,
z - other.x - x * other.z,
x - other.y - y * other.x);
}
public float dot(Vector3 other) {
return x * other.x + y * other.y + z * other.z;
}
}
public static boolean intersectRayWithSquare(Vector3 R1, Vector3 R2,
Vector3 S1, Vector3 S2, Vector3 S3) {
Vector3 dS21 = S2.sub(S1);
Vector3 dS31 = S3.sub(S1);
Vector3 n = dS21.cross(dS31);
Vector3 dR = R1.sub(R2);
float ndotdR = n.dot(dR);
if (Math.abs(ndotdR) < 1e-6f) {
return false;
}
float t = -n.dot(R1.sub(S1)) / ndotdR;
Vector3 M = R1.add(dR.scale(t));
Vector3 dMS1 = M.sub(S1);
float u = dMS1.dot(dS21);
float v = dMS1.dot(dS31);
return (u >= 0.0f && u <= dS21.dot(dS21)
&& v >= 0.0f && v <= dS31.dot(dS31));
}
public static void main(String... args) {
Vector3 R1 = new Vector3(0.0f, 0.0f, -1.0f);
Vector3 R2 = new Vector3(0.0f, 0.0f, 1.0f);
Vector3 S1 = new Vector3(-1.0f, 1.0f, 0.0f);
Vector3 S2 = new Vector3( 1.0f, 1.0f, 0.0f);
Vector3 S3 = new Vector3(-1.0f,-1.0f, 0.0f);
boolean b = intersectRayWithSquare(R1, R2, S1, S2, S3);
assert b;
R1 = new Vector3(1.5f, 1.5f, -1.0f);
R2 = new Vector3(1.5f, 1.5f, 1.0f);
b = intersectRayWithSquare(R1, R2, S1, S2, S3);
assert !b;
}
}