理論根據(jù):
假設(shè)一個(gè)光的方向是(-1,-1,-1) , 投影到XZ平面

一個(gè)是直線方程，一個(gè)是平面方程，求交
而且平面方程還比較非凡，經(jīng)過(guò)原點(diǎn)，法向量是 0 1 0
簡(jiǎn)化后就簡(jiǎn)單了， 假定v是直線的方向

  x - vertex.x       y - vertex.y        z-vertex.z
----------------  = --------------- = --------------         直線方程
     v.x               v.y                 v.z

平面方程 y = 0

帶入就得到了
x = vertex.x + v.x / v.y * (-vertex.y)
z = vertex.z + v.x / v.z * (-vertex.z)

源程序:
import java.applet.Applet;
import java.awt.BorderLayout;
import java.awt.GraphicsConfiguration;
import java.awt.Label;
import com.sun.j3d.utils.applet.MainFrame;
import com.sun.j3d.utils.universe.*;
import com.sun.j3d.utils.geometry.*;
import javax.media.j3d.*;
import javax.vecmath.*;
public class SimpleShadow extends Applet{
 //三角平面類
 public class Triplane extends Shape3D{
  TriPlane(Point3f A,Point3f B,Point3f C){
   this.setGeometry(this.createGeometry3(A,B,C));
   this.setAppearance(this.createAppearance());
  }
  //建立三角平面
  
  Geometry createGeometry3(Point3f A,Point3f B,Point3f C){
   TriangleArray plane=new TriangleArray(3,GeometryArray.COORDINATESGeometryArray.NORMALS);
   //設(shè)置平面3個(gè)頂點(diǎn)的坐標(biāo)
   plane.setCoordinate(0,A);
   plane.setCoordinate(1,B);
   plane.setCoordinate(2,C);
   
   //計(jì)算平面法向量
   Vector3f a=new Vector3f(A.x-B.x,A.y-B.y,A.z-B.z);
   Vector3f b=new Vector3f(C.x-B.x,C.y-B.y,C.z-B.z);
   Vector3f n=new Vector3f();
   n.cross(b,a);
   //法向量單位化
   n.normalize();
   //設(shè)置平面3個(gè)頂點(diǎn)的法向量
   plane.setNormal(0,n);
   plane.setNormal(1,n);
      plane.setNormal(2,n);
   return plane;
  }
  
  //創(chuàng)建Material不為空的外觀
  
  Appearance createAppearance(){
   Appearance appear=new Appearance();
   Material material=new Material();
   appear.setMaterial(material);
   return appear;
  }
  
 }
 
 
 //四邊平面類
 public class QuadPlane extends Shape3D{
  
  QuadPlane(Point3f A,Point3f B,Point3f C,Point3f D){
   this.setGeometry(this.createGeometry4(A,B,C,D));
   this.setAppearance(this.createAppearance());
  }
  
  //創(chuàng)建四邊性平面
  Geometry createGeometry4(Point3f A,Point3f B,Point3f C,Point3f D){
   QuadArray plane=new QuadArray(4,GeometryArray.COORDINATESGeometryArray.NORMALS);
   //設(shè)置平面3個(gè)頂點(diǎn)的坐標(biāo)
   plane.setCoordinate(0,A);
   plane.setCoordinate(1,B);
   plane.setCoordinate(2,C);
   plane.setCoordinate(3,D);
   //計(jì)算平面法向量
   Vector3f a=new Vector3f(A.x-B.x,A.y-B.y,A.z-B.z);
   Vector3f b=new Vector3f(C.x-B.x,C.y-B.y,C.z-B.z);
   Vector3f n=new Vector3f();
   n.cross(b,a);
   //法向量單位化
   n.normalize();
   //設(shè)置平面4個(gè)頂點(diǎn)的法向量
   plane.setNormal(0,n);
   plane.setNormal(1,n);
   plane.setNormal(2,n);
   plane.setNormal(3,n);
   return plane;
  }