3.18: Frames Per Second and pygame.time.Clock Objects

The frame rate or refresh rate is the number of pictures that the program draws per second, and is measured in FPS or frames per second. (On computer monitors, the common name for FPS is hertz. Many monitors have a frame rate of 60 hertz, or 60 frames per second.) A low frame rate in video games can make the game look choppy or jumpy. If the program has too much code to run to draw to the screen frequently enough, then the FPS goes down. But the games in this book are simple enough that this won’t be issue even on old computers.

A pygame.time.Clock object can help us make sure our program runs at a certain maximum FPS. This Clock object will ensure that our game programs don’t run too fast by putting in small pauses on each iteration of the game loop. If we didn’t have these pauses, our game program would run as fast as the computer could run it. This is often too fast for the player, and as computers get faster they would run the game faster too. A call to the tick() method of a Clock object in the game loop can make sure the game runs at the same speed no matter how fast of a computer it runs on. The Clock object is created on line 7 of the catanimation.py program:

import pygame, sys
from pygame.locals import *

pygame.init()

FPS = 30 # frames per second setting
fpsClock = pygame.time.Clock()

# set up the window
DISPLAYSURF = pygame.display.set_mode((400, 300), 0, 32)
pygame.display.set_caption('Animation')

WHITE = (255, 255, 255)
catImg = pygame.image.load('cat.png')
catx = 10
caty = 10
direction = 'right'

while True: # the main game loop
    DISPLAYSURF.fill(WHITE)

    if direction == 'right':
        catx += 5
        if catx == 280:
            direction = 'down'
    elif direction == 'down':
        caty += 5
        if caty == 220:
            direction = 'left'
    elif direction == 'left':
        catx -= 5
        if catx == 10:
            direction = 'up'
    elif direction == 'up':
        caty -= 5
        if caty == 10:
            direction = 'right'

    DISPLAYSURF.blit(catImg, (catx, caty))

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

    pygame.display.update()
    fpsClock.tick(FPS)

The Clock object’s tick() method should be called at the very end of the game loop, after the call to pygame.display.update(). The length of the pause is calculated based on how long it has been since the previous call to tick(), which would have taken place at the end of the previous iteration of the game loop. (The first time the tick() method is called, it doesn’t pause at all.) In the animation program, is it run on line 47 as the last instruction in the game loop.

All you need to know is that you should call the tick() method once per iteration through the game loop at the end of the loop. Usually this is right after the call to pygame.display.update().

Try modifying the FPS constant variable to run the same program at different frame rates. Setting it to a lower value would make the program run slower. Setting it to a higher value would make the program run faster.