Invisible Plane?

Question

I found something very wierd... i have a program that calculates a polynomical function from some points and draws it. It should represent a dome and i rotate it. But it looks like if there was a black plane at z=-20 and parallel with both x and y. When the dome rotates behind this plane the part of it dissapears. I didnt add this plane anywhere!!! Where did it come from.
Image of problem http://img523.imageshack.us/img523/8202/invisibleif6.png
class DomeUniverse extends JFrame{
     private class Arch extends Shape3D{
          private BranchGroup arch;
          public Arch( ArrayList def, int height){
              arch = new BranchGroup();
              //Set the overall appearance
               Appearance app = new Appearance();
             PolygonAttributes poly = new PolygonAttributes();
             poly.setCullFace(PolygonAttributes.CULL_NONE);
             app.setPolygonAttributes(poly);
             app.setLineAttributes(new LineAttributes( 2.0f, LineAttributes.PATTERN_SOLID, false));
             app.setColoringAttributes(new ColoringAttributes( new Color3f(1.0f,0.0f,0.0f), ColoringAttributes.SHADE_GOURAUD));
              // rotate object has composited transformation matrix
              Transform3D rotate = new Transform3D();
              Transform3D translate = new Transform3D();
              translate.set(new Vector3f(0.1f, 0.0f, 0.0f));
              TransformGroup archTGT1 = new TransformGroup(translate);
              arch.addChild(archTGT1);
                   rotate.rotZ(Math.PI/2.0d);
              TransformGroup archTGR1 = new TransformGroup(rotate);
              //help variables
              double r = (Math.tan(Math.PI/2-Math.atan(height/1500.0))*1500+1000)/2;
              double terciarWidth = Math.sqrt(r*r-(height/3+r-height)*(height/3+r-height));
              double sekTerciarWidth = Math.sqrt(r*r-(height*2/3+r-height)*(height*2/3+r-height));
              double sekundarHeight = Math.sqrt(r*r-3000.0*3000.0/16.0)-r+height;
              //The default values of the dome
              double[] xs = {0,1500-terciarWidth,1500-sekTerciarWidth,750,1500,2250,1500+sekTerciarWidth,1500+terciarWidth,3000};
              double[] fs = {0,height/3,height*2/3,sekundarHeight,height,sekundarHeight,height*2/3,height/3,0};
              //These x values and y values will be further edited. Only testing version.
              double[] xs1 = {0,xs[1]+C,xs[2]+D,xs[3]  ,xs[4]  ,xs[5]  ,xs[6]-E,xs[7]-F,3000};
              double[] fs1 = {0,fs[1]  ,fs[2]  ,fs[3]+J,fs[4]+K,fs[5]+L,fs[6]  ,fs[7]  ,0   };
              double[] xs2 = {0,xs[1]+C,xs[2]+B,xs[3]  ,xs[4]  ,xs[5]  ,xs[6]-A,xs[7]  ,3000};
              double[] fs2 = {0,fs[1]  ,fs[2]  ,fs[3]+I,fs[4]+H,fs[5]+G,fs[6]  ,fs[7]-F,0   };
              //In these variables there are stored the last y values.
              float lastY1 = 0;
              float lastY2 = 0;
               for( float x = BLEND; x <= 3000; x+=BLEND){
                    float y1 = 0.0f;
                    float y2 = 0.0f;
                    //polynomical function
                    for( int i = 0; i < xs.length; i++){
                         double li1 = 1.0;
                         double li2 = 1.0;
                         for( int j = 0; j < xs.length; j++){
                              if( j != i){
                                   li1 *= (x - xs1[j])/( xs1[i]-xs1[j]);
                                   li2 *= (x - xs2[j])/( xs2[i]-xs2[j]);
                         y1+=li1*fs1;
                         y2+=li2*fs2[i];
                    //polygon down
          TriangleArray archPolygon1 = new TriangleArray(3, QuadArray.COORDINATES|QuadArray.COLOR_3);
          archPolygon1.setCoordinate(0, new Point3f( lastY1*4/300,-(x-BLEND)*4/300+20,-10.0f));
          archPolygon1.setCoordinate(1, new Point3f( lastY2*4/300,-(x-BLEND)*4/300+20,10.0f));
          archPolygon1.setCoordinate(2, new Point3f( y2*4/300,-x*4/300+20,10.0f));
          archPolygon1.setColor(0, yellow);
          archPolygon1.setColor(1, green);
          archPolygon1.setColor(2, blue);
          Shape3D archP = new Shape3D( archPolygon1);
          archP.setAppearance(app);
          archTGR1.addChild(archP);
          //polygon up
          TriangleArray archPolygon2 = new TriangleArray(3, QuadArray.COORDINATES|QuadArray.COLOR_3);
          archPolygon2.setCoordinate(0, new Point3f( lastY1*4/300,-(x-BLEND)*4/300+20,-10.0f));
          archPolygon2.setCoordinate(1, new Point3f( y1*4/300,-x*4/300+20,-10.0f));
          archPolygon2.setCoordinate(2, new Point3f( y2*4/300,-x*4/300+20,10.0f));
          archPolygon2.setColor(0, yellow);
          archPolygon2.setColor(1, red);
          archPolygon2.setColor(2, blue);
          Shape3D archP2 = new Shape3D( archPolygon2);
          archP2.setAppearance(app);
          archTGR1.addChild(archP2);
          lastY1 = y1;
          lastY2 = y2;
     //Add it all
     archTGT1.addChild(zs);
     archTGT1.addChild(ys);
     archTGT1.addChild(xos);
               archTGT1.addChild(archTGR1);
          public BranchGroup getArch(){
               return arch;
     public ArchUniverse( Main main, boolean animate, String[] startStrength, String[] endStrength, int height){
//....... deleted code for better preview
Canvas3D canvas3D = new Canvas3D(SimpleUniverse.getPreferredConfiguration());
add("Center", canvas3D);
BranchGroup scene = createSceneGraph();
SimpleUniverse simpleU = new SimpleUniverse(canvas3D);
Transform3D tr = new Transform3D();
tr.setTranslation(new Vector3d(0,0,50));
simpleU.getViewingPlatform().getViewPlatformTransform().setTransform(tr);
simpleU.addBranchGraph(scene);
public BranchGroup createSceneGraph() {
     BranchGroup objRoot = new BranchGroup();
     TransformGroup objSpin = new TransformGroup();
     objSpin.setCapability(TransformGroup.ALLOW_TRANSFORM_WRITE);
     objRoot.addChild(objSpin);
     objSpin.addChild(new Arch(main.getDeformation(startStrength[0].substring(0,startStrength[0].indexOf(" kN")), Integer.parseInt(startStrength[1]), main.getCP()), height).getArch());
     Alpha rotationAlpha = new Alpha(-1, 10000);
     RotationInterpolator rotator =
          new RotationInterpolator(rotationAlpha, objSpin);
// isnt this it? some sphere? does it exist real or only imaginar
     BoundingSphere bounds =
          new BoundingSphere(new Point3d(0.0,0.0,0.0), 1.0);
     rotator.setSchedulingBounds(bounds);
     objSpin.addChild(rotator);
objRoot.compile();
return objRoot;

Answer 1

5 dukes for that one who explains the problem