How to display a moving ball trajectory

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I have a solid-coloured ball moving in a tkinter canvas, using the move(), and update() method to refresh the canvas. I want to keep track of the ball trajectory onscreen. i.e. - the ball moves to its new position but a line - series of points showing its trajectory - remains onscreen. How do I do this? (two canvases, one of them transparent??)

tkinter python
1 Answers

Just as suggested, you can paint line that will start from the last position of the ball and end in the current new ball position.

I've made simple app doing so in tkinter, that you can modify to your needs. It's using only one canvas and you can move ball just by clicking on the canvas. Right mouse click resets the canvas and ball position.

from tkinter import Tk, Canvas, Frame, BOTH


class MovingBall(Frame):
    ball_r = 25
    x, y = 0, 0  # ball last coords

    def __init__(self):
        super().__init__()
        self.ball = None
        self.initUI()

    def initUI(self):
        self.master.title("Moving Ball")
        self.pack(fill=BOTH, expand=1)

        self.canvas = Canvas(self)
            
        self.ball = self.paint_ball(self.x, self.y)
        self.canvas.pack(fill=BOTH, expand=1)
        # Left click on canvas moves the ball
        self.canvas.bind("<Button-1>", self.move)
        # Right click on canvas reset ball position
        self.canvas.bind("<Button-3>", self.reset)

    def paint_ball(self, x, y):
        return self.canvas.create_oval(x - self.ball_r, y - self.ball_r, x + self.ball_r, y + self.ball_r, fill="red",
                                       outline="silver", width=1)

    def paint_path(self, x, y):
        return self.canvas.create_line(self.x, self.y, x, y, fill="silver", width=1)

    def move(self, event):
        # Remove last painted ball
        self.canvas.delete(self.ball)
        # Add new line to the path
        self.paint_path(event.x, event.y)
        # Paint new ball at new position
        self.ball = self.paint_ball(event.x, event.y)
        self.canvas.pack(fill=BOTH, expand=1)
        # Store current ball coords
        self.x, self.y = event.x, event.y

    def reset(self, event):
        """Reset whole scene and put ball to 0,0"""
        self.x, self.y = 0, 0
        self.canvas.delete("all")
        self.ball = self.paint_ball(self.x, self.y)
        self.canvas.pack(fill=BOTH, expand=1)


def main():
    root = Tk()
    MovingBall()
    root.geometry("1024x768")
    root.mainloop()


if __name__ == '__main__':
    main()
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