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);}}
去陌生的街角，去做一切我们曾经或现在也很想做的事情，
