you can use this
Graphics g = frameBuffer.getGraphics();
Camera cam = world.getCamera();
//use whatever source vectors you want. if you want to show lines between children and parents
// use Object3D.getTransformedCenter() instead of a new SimpleVector()
SimpleVector pointA = Interact2D.project3D2D(cam, frameBuffer, new SimpleVector(0,0,0)); // start
SimpleVector pointB = Interact2D.project3D2D(cam, frameBuffer, new SimpleVector(1,1,1)); // to
g.drawLine((int)pointA.x, (int)pointA.y, (int)pointB.x, (int)pointB.y);