Question

I teach Computer Programming. For first course students I’m looking for most easy algorithm to change direction on collide (with window frame). This is my actual code:

import sys, pygame
pygame.init()
speed_x = 1
speed_y = 1

black = (0, 0, 0)
width, height = 320, 240
size = ( width, height )

screen = pygame.display.set_mode(size)
display_surface = pygame.display.get_surface()
display_rectangle = display_surface.get_rect()

ball_img = pygame.image.load("data/ball.gif")
ball = ball_img.convert_alpha()
ballrect = ball.get_rect()

while 1:
        for event in pygame.event.get():
            if event.type == pygame.QUIT: sys.exit()

        if not display_rectangle.contains( ballrect.move([speed_x, 0]) ):
            speed_x *= -1
        else:
            ballrect = ballrect.move([speed_x, 0])

        if not display_rectangle.contains( ballrect.move([0, speed_y]) ):
            speed_y *= -1
        else:
            ballrect = ballrect.move([0, speed_y])  

        screen.fill(black)
        screen.blit(ball, ballrect)
        pygame.display.flip()
        pygame.time.delay(30)

This code works fine, my question is if someone know about a easy algorithm or clear algorithm to reverse direction using contains or any other collide pygame test.

Some times I think that an approach is most easy for students and a new clear approach appears.

All suggestions are welcome

Some suggestions? Thanks!

Answer 1

I think your approach is fine. Your code models the velocity of the ball using the 2-vector [speed_x, speed_y]. Bouncing off a (horizontal or vertical) wall involves negating the appropriate component of the vector.

As long as your students have a background in introductory physics (velocity, momentum, collisions) then your code should be understandable.