Pygame -画线。双摆的轨迹是由摆本身画出来的--我该如何防止这种情况？

import pygame

# 初始化Pygame
pygame.init()

# 设置窗口大小
width, height = 800, 600
screen = pygame.display.set_mode((width, height))

# 设置颜色
BLACK = (0, 0, 0)
WHITE = (255, 255, 255)

# 设置双缓冲
back_buffer = pygame.Surface((width, height))

# 设置摆的初始位置和速度
x1, y1 = width // 2, height // 2
x2, y2 = width // 2, height // 2
length1, length2 = 200, 200
angle1, angle2 = 0, 0
angular_velocity1, angular_velocity2 = 0.01, 0.1

# 游戏主循环
running = True
while running:
    # 处理事件
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    # 更新摆的位置和角度
    angle1 += angular_velocity1
    angle2 += angular_velocity2

    x1 = int(width // 2 + length1 * pygame.sin(angle1))
    y1 = int(height // 2 + length1 * pygame.cos(angle1))
    x2 = int(x1 + length2 * pygame.sin(angle2))
    y2 = int(y1 + length2 * pygame.cos(angle2))

    # 绘制背景
    back_buffer.fill(BLACK)

    # 绘制摆的轨迹
    pygame.draw.line(back_buffer, WHITE, (width // 2, height // 2), (x1, y1), 2)
    pygame.draw.line(back_buffer, WHITE, (x1, y1), (x2, y2), 2)

    # 将缓冲区绘制到屏幕上
    screen.blit(back_buffer, (0, 0))

    # 更新屏幕显示
    pygame.display.flip()

# 退出Pygame
pygame.quit()