Java 3D渲染器 - 边缘消失

Question

我正在使用Java Graphics制作一个简单的3D渲染器，它目前可以渲染一组构成简单平面的多边形。问题是，渲染几秒后，边缘上的一些线条消失了。有什么问题？

package com.zove.renderer;

import java.awt.Graphics;
import java.awt.Point;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

import javax.swing.JFrame;
import javax.swing.JPanel;

public final class Frame extends JPanel implements KeyListener{
volatile int x1 = 0;
volatile int x2 = 0;
volatile int y1 = 0;
volatile int y2 = 550;
volatile int x3 = 0;
volatile int y3 = 0;
volatile int x4 = 800;
volatile int y4 = 0;
volatile int angle = 0;
volatile int i;
public JFrame f;
    public Frame(){
        f = new JFrame();
        f.setVisible(true);
        f.add(this);
        f.setResizable(false);
        f.setSize(800,550);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.addKeyListener(this);



    }
    public void paint(Graphics g){
        super.paint(g);
        for(i=0; i<2000;){
            if(x1+i+angle<2600){
            //Vertical
            g.drawLine(x1+i+angle, y1,x2+i-angle*2,y2);
            }
            else{
                g.drawLine(x1+i+angle, y1-i/200,x2+i-angle,y2+angle);

            }



            //Horizontal
            g.drawLine(x3, y3+i-angle/2,x4,y4+i+angle/2);
            i +=10*angle/100;




        }
    }
    public static void main(String[] args){
        Thread s = new Thread(new repainter());
        s.start();
    }
    @Override
    public void keyPressed(KeyEvent arg0) {
    i -=20;
    angle -=20;

    }
    @Override
    public void keyReleased(KeyEvent arg0) {
        // TODO Auto-generated method stub

    }
    @Override
    public void keyTyped(KeyEvent arg0) {
        // TODO Auto-generated method stub

    }

}
class repainter implements Runnable{


    public void run() {
        Frame f = new Frame();
    while(true){

         f.repaint();
         try {
            Thread.sleep(30);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();

        }
         System.out.println(f.x1+f.i+f.angle);
         f.angle +=10;

    }

    }

}