Попробуйте этот код ...
BranchGroup group = new BranchGroup(); //Content branch.
PolygonAttributes p = new PolygonAttributes(); //Not sure how to make it transparent/try code above.
Appearance planeAppearance = new Appearance();
planeAppearance.setPolygonAttributes (p);
Color3f planeColor = new Color3f (1.0f, 1.0f, 1.0f); //This makes it white.
ColoringAttributes planeCA = new ColoringAttributes (planeColor, 1);
planeAppearance.setColoringAttributes(planeCA);
QuadArray plane = new QuadArray (4, QuadArray.COORDINATES); //This makes the plane.
plane.setCoordinate(0, new Point3f(-5f, -5f, -15f)); //You specify your own cornerpoints...
plane.setCoordinate(1, new Point3f(5f, -5f, -15f));
plane.setCoordinate(2, new Point3f(5f, 5f, -15f));
plane.setCoordinate(3, new Point3f(-5f, 5f, -15f));
group.addChild(new Shape3D(plane, planeAppearance)); //Add plane to content branch.