理论根据:
假设一个光的方向是(-1,-1,-1) , 投影到xz平面

一个是直线方程，一个是平面方程，求交
而且平面方程还比较特殊，经过原点，法向量是 0 1 0
简化后就简单了， 假定v是直线的方向

  x - vertex.x       y - vertex.y        z-vertex.z
----------------  = --------------- = --------------         直线方程
     v.x               v.y                 v.z

平面方程 y = 0

带入就得到了
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{
 //三角平面类
 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.coordinates|geometryarray.normals);
   //设置平面3个顶点的坐标
   plane.setcoordinate(0,a);
   plane.setcoordinate(1,b);
   plane.setcoordinate(2,c);
   
   //计算平面法向量
   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);
   //法向量单位化
   n.normalize();
   //设置平面3个顶点的法向量
   plane.setnormal(0,n);
   plane.setnormal(1,n);
      plane.setnormal(2,n);
   return plane;
  }
  
  //创建material不为空的外观
  
  appearance createappearance(){
   appearance appear=new appearance();
   material material=new material();
   appear.setmaterial(material);
   return appear;
  }
  
 }
 
 
 //四边平面类
 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());
  }
  
  //创建四边性平面
  geometry creategeometry4(point3f a,point3f b,point3f c,point3f d){
   quadarray plane=new quadarray(4,geometryarray.coordinates|geometryarray.normals);
   //设置平面3个顶点的坐标
   plane.setcoordinate(0,a);
   plane.setcoordinate(1,b);
   plane.setcoordinate(2,c);
   plane.setcoordinate(3,d);
   //计算平面法向量
   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);
   //法向量单位化
   n.normalize();
   //设置平面4个顶点的法向量
   plane.setnormal(0,n);
   plane.setnormal(1,n);
   plane.setnormal(2,n);
   plane.setnormal(3,n);
   return plane;
  }
  
 

 //创建material不为空的外观
  
  appearance createappearance(){
   appearance appear=new appearance();
   material material=new material();
   appear.setmaterial(material);
   return appear;
  }
 }
 
 
 //阴影类
 public class shadow3d extends shape3d{
  shadow3d(geometryarray geom,vector3f direction,color3f col,float height){
   int vcount=geom.getvertexcount();
   trianglearray poly=new trianglearray(vcount,geometryarray.coordinates|geometryarray.color_3);
   int v;
   point3f vertex=new point3f();
   point3f shadow=new point3f();
   
   for(v=0;v<vcount;v++){
    //计算可见面顶点在投影面上的投影坐标
    geom.getcoordinate(v,vertex);
    system.out.println(vertex.y-height);
          shadow.set(vertex.x-(vertex.y-height),height+0.0001f,vertex.z-(vertex.y-height));
          poly.setcoordinate(v,shadow);
          poly.setcolor(v,col);

   }
   this.setgeometry(poly);
   
  }
 }
 
 
 public simpleshadow(){
  this.setlayout(new borderlayout());
   //graphicsconfiguration config =simpleuniverse.getpreferredconfiguration();
  canvas3d c=new canvas3d(null);
  this.add("center",c);
  //创建分支节点
  branchgroup scene=new branchgroup();
  //创建三角形可见平面
  shape3d plane=new triplane(new point3f(0.0f,0.6f,-0.2f),new point3f(-0.3f,-0.3f,0.2f),new point3f(0.6f,-0.3f,0.2f));
  //添加到根分支节点
  scene.addchild(plane);
  //创建四边形投影平面
  shape3d floor=new quadplane(new point3f(-1.0f,-0.5f,-1.0f),new point3f(-1.0f,-0.5f,1.0f),new point3f(1.0f,-0.5f,1.0f),new point3f(1.0f,-0.5f,-1.0f));
  //添加到根分支节点
  scene.addchild(floor);
  //添加到环境光
  ambientlight lighta=new ambientlight();
  lighta.setinfluencingbounds(new boundingsphere());
  scene.addchild(lighta);
  //添加平行光
  directionallight lightd1=new directionallight();
  lightd1.setinfluencingbounds(new boundingsphere());
  vector3f direction=new vector3f(-1.0f,-1.0f,-1.0f);
  //方向矢量单位化
   direction.normalize();
  lightd1.setdirection(direction);
  scene.addchild(lightd1);
  //创建阴影物体
  shape3d shadow=new shadow3d((geometryarray)plane.getgeometry(),direction,new color3f(0.2f,0.2f,0.2f),-0.5f);
  scene.addchild(shadow);
  simpleuniverse u=new simpleuniverse(c);
  u.getviewingplatform().setnominalviewingtransform();
  u.addbranchgraph(scene);
 }
 
 public static void main(string argv[]){
  new mainframe(new simpleshadow(),256,256);
 }

}