import pygame

### initialization

width = 640
height = 480
screen = pygame.display.set_mode((width,height))

# loading the balloon picture; gif file needs to be in same
# folder as this file (one_balloon.py)
balloon = pygame.image.load("red_balloon.gif")

# get the dimensions of the balloon image
b_width = balloon.get_width()
b_height = balloon.get_height()

# convert graphics format (in this case gif) to Pygame's internal
# format
balloon = balloon.convert()

# initial x and y coordinates of balloon
b_x = 100.0
b_y = 100.0

# speed of balloon in x and y direction
speed_x = 0.2
speed_y = 0.1


# set up clock
clock = pygame.time.Clock()

### the game loop

keepGoing = True

while (keepGoing):

    # deal with events
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            keepGoing = False

    # background color
    screen.fill(pygame.color.Color("darkblue"))

    # number of milliseconds since last time
    dt = clock.tick()

    # update position based on speed and time passed
    b_x = b_x + speed_x * dt
    b_y = b_y + speed_y * dt

    # bounce if going beyond edge of screen
    if (b_x < 0):
        b_x = 0
        speed_x = -1 * speed_x
    elif (b_x > width - b_width):
        b_x = width - b_width
        speed_x = -1 * speed_x
    if (b_y < 0):
        b_y = 0
        speed_y = -1 * speed_y
    elif (b_y > height - b_height):
        b_y = height - b_height
        speed_y = -1 * speed_y


    # draw picture
    screen.blit(balloon, (b_x,b_y))


    # update display
    pygame.display.update()


### quit

pygame.quit()