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A twisted strip to demonstrate back face culling E-mail
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This Java tip demonstrates back face culling. A 'twisted strip' is a continuous surface with a two twists in it. (A Mobius strip has one twist) The Class Strip creates a surface using a TriangleStripArray. A twist strip is placed in a scene graph with a RotationInterpolator so the strip spins. As the strip spins, when the back faces of the individual triangles face the image plate, they disappear.

One twisted strip is created using the inner class (Twist). This visual object is rendered as filled polygons. A second twist visual object is made from the same Geometry with a different Appearance to render as lines (only). The line-only strip helps to see where the filled polygon strip is.


Image

import java.applet.Applet;
import java.awt.BorderLayout;
import java.awt.Frame;

import javax.media.j3d.Alpha;
import javax.media.j3d.Appearance;
import javax.media.j3d.Background;
import javax.media.j3d.BoundingSphere;
import javax.media.j3d.BranchGroup;
import javax.media.j3d.Canvas3D;
import javax.media.j3d.Geometry;
import javax.media.j3d.PolygonAttributes;
import javax.media.j3d.RotationInterpolator;
import javax.media.j3d.Shape3D;
import javax.media.j3d.TransformGroup;
import javax.media.j3d.TriangleStripArray;
import javax.vecmath.Color3f;
import javax.vecmath.Point3d;

import com.sun.j3d.utils.applet.MainFrame;
import com.sun.j3d.utils.universe.SimpleUniverse;

public class TwistStripApp extends Applet {

  /////////////////////////////////////////////////
  //
  // create Twist visual object
  //
  public class Twist extends Shape3D {

    ////////////////////////////////////////////
    //
    // create twist subgraph
    //
    public Twist() {

      this.setGeometry(createGeometry());
      this.setAppearance(createAppearance());
    // end of twist constructor

    Geometry createGeometry() {

      TriangleStripArray twistStrip;
      Color3f blue = new Color3f(0.0f0.0f1.0f);

      // create triangle strip for twist
      int N = 80;
      int stripCounts[] };
      twistStrip = new TriangleStripArray(
          N,
          TriangleStripArray.COORDINATES | TriangleStripArray.COLOR_3,
          stripCounts);

      double a;
      int v;
      for (v = 0, a = 0.0; v < N; v += 2, a = v * 2.0 * Math.PI / (N - 2)) {
        twistStrip.setCoordinate(v, new Point3d(0.7 * Math.sin(a0.2
            * Math.cos(a)0.3 * Math.sin(a)0.7 * Math.cos(a)
            0.2 * Math.cos(a)));
        twistStrip.setCoordinate(v + 1new Point3d(0.7 * Math.sin(a)
            0.2 * Math.cos(a), -0.3 * Math.sin(a)0.7
            * Math.cos(a0.2 * Math.cos(a)));
        twistStrip.setColor(v, blue);
        twistStrip.setColor(v + 1, blue);
      }

      return twistStrip;

    }

    // create Appearance for Twist Strip
    //
    // this method creates the default Appearance for the
    // twist strip. The commented line of code containting
    // the setCullFace will fix the problem of half of the
    // Twisted Strip disappearing.

    Appearance createAppearance() {

      Appearance twistAppear = new Appearance();
      PolygonAttributes polyAttrib = new PolygonAttributes();
      // polyAttrib.setCullFace(PolygonAttributes.CULL_NONE);
      twistAppear.setPolygonAttributes(polyAttrib);

      return twistAppear;
    }

  // end of class Twist

  /////////////////////////////////////////////////
  //
  // create scene graph branch group
  //
  public BranchGroup createSceneGraph() {

    BranchGroup contentRoot = new BranchGroup();

    // Create the transform group node and initialize it to the
    // identity. Add it to the root of the subgraph.
    TransformGroup objSpin = new TransformGroup();
    objSpin.setCapability(TransformGroup.ALLOW_TRANSFORM_WRITE);
    contentRoot.addChild(objSpin);

    Shape3D twist = new Twist();
    objSpin.addChild(twist);

    // Duplicate the twist strip geometry and set the
    // appearance of the new Shape3D object to line mode
    // without culling.
    // Add the POLYGON_FILLED and POLYGON_LINE strips
    // in the scene graph at the same point.
    // This will show the triangles of the original Mobius strip that
    // are clipped. The PolygonOffset is set to prevent stitching.
    PolygonAttributes polyAttrib = new PolygonAttributes();
    polyAttrib.setCullFace(PolygonAttributes.CULL_NONE);
    polyAttrib.setPolygonMode(PolygonAttributes.POLYGON_LINE);
    polyAttrib.setPolygonOffset(0.001f);
    Appearance polyAppear = new Appearance();
    polyAppear.setPolygonAttributes(polyAttrib);
    objSpin.addChild(new Shape3D(twist.getGeometry(), polyAppear));

    Alpha rotationAlpha = new Alpha(-116000);

    RotationInterpolator rotator = new RotationInterpolator(rotationAlpha,
        objSpin);

    // a bounding sphere specifies a region a behavior is active
    // create a sphere centered at the origin with radius of 1
    BoundingSphere bounds = new BoundingSphere();
    rotator.setSchedulingBounds(bounds);
    objSpin.addChild(rotator);

    // make background white
    Background background = new Background(1.0f1.0f1.0f);
    background.setApplicationBounds(bounds);
    contentRoot.addChild(background);

    // Let Java 3D perform optimizations on this scene graph.
    contentRoot.compile();

    return contentRoot;
  // end of CreateSceneGraph method of TwistStripApp

  // Create a simple scene and attach it to the virtual universe

  public TwistStripApp() {
    setLayout(new BorderLayout());
    Canvas3D canvas3D = new Canvas3D(null);
    add("Center", canvas3D);

    BranchGroup scene = createSceneGraph();

    // SimpleUniverse is a Convenience Utility class
    SimpleUniverse simpleU = new SimpleUniverse(canvas3D);

    // This will move the ViewPlatform back a bit so the
    // objects in the scene can be viewed.
    simpleU.getViewingPlatform().setNominalViewingTransform();

    simpleU.addBranchGraph(scene);

  // end of TwistStripApp constructor

  //  The following method allows this to be run as an application

  public static void main(String[] args) {
    System.out.println("TwistStripApp - Java 3D API version 1.1");
    System.out.print("See \"Getting Started with the Java 3D API\"");
    System.out.println(" (section 2.6)");
    System.out.println("This program demonstrates back face culling.");
    System.out.print("In this program two visual objects rotate, ");
    System.out.println("one wireframe and one solid surface.");
    System.out.print("The wire frame is visible only when components");
    System.out.println(" of the surface are culled.");

    Frame frame = new MainFrame(new TwistStripApp()256256);
  // end of main method of TwistStripApp

// end of class TwistStripApp

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