Reverse Direction

Detect when we hit the right edge then reverse direction.


import arcade

class MyGame(arcade.Window):
    def __init__(self, width, height, title, bg_color):
        super().__init__(width, height, title)
        self.width = width
        self.height = height
        self.position = 0
        self.velocity = 200
        self.radius = 30

    def on_draw(self):
        y = self.height / 2
        arcade.draw_circle_filled(self.position, y, self.radius, arcade.color.RED)

    def update(self, delta_time):
        self.position += self.velocity * delta_time

        # Did the circle hit the right side of screen?
        is_at_right = self.position > self.width - self.radius
        if is_at_right:
            self.velocity *= -1

def main():
    game1 = MyGame(600, 600, 'Drawing Example', arcade.color.WHEAT)
    game1.position = 10

if __name__ == '__main__':


  1. We put the starting position at 10.

  2. We introduce concept of velocity in the constructor.

  3. Velocity is two things:

    • How fast we are moving
    • Which direction we are moving
    • A negative number means we are moving that fast, backwards
    • Note: This number is a LOT bigger for Arcade 2.0
  4. No changes to on_draw…shows the point that we change the data and let the drawing take care of itself.

  5. Important changes in update

  6. On each tick, move self.position (really, center_x) based on:

    • Velocity
    • delta_time meaning, how long since we were last called
    • Makes sense
  7. Introduce a conditional, aka the if

  8. How do we know if we are at the right edge?

    • The right edge is a certain number of pixels from the origin
    • The number is…the window width
    • We care about the edge of the circle, not the center
    • So we have to use the radius
    • If center + radius is at the edge, turn around
  9. How do we turn around? Change the velocity to reverse.


  1. Try making the circle bigger. Does the formula still work?
  2. Same for the window width.


  1. We could have skipped a separate line for is_at_right and just done that formula in the if. What are some reasons for doing it separately?