如何将活动的tkinter画布保存为png文件？

from tkinter import *
from tkinter import messagebox
from PIL import ImageGrab

root = Tk()
canvas = Canvas(root, width=500, height=500)
canvas.pack()

canvas.create_rectangle(50, 50, 200, 200, fill='red')
canvas.create_oval(100, 100, 250, 250, fill='blue')

def save_canvas():
    x = root.winfo_rootx() + canvas.winfo_x()
    y = root.winfo_rooty() + canvas.winfo_y()
    x1 = x + canvas.winfo_width()
    y1 = y + canvas.winfo_height()
    ImageGrab.grab().crop((x, y, x1, y1)).save("canvas.png")
    messagebox.showinfo("保存成功", "画布已保存为canvas.png")

save_button = Button(root, text="保存画布", command=save_canvas)
save_button.pack()

root.mainloop()