Pixels per Second

Part of the The Game Loop section of Coddy's Pygame journey. Lesson 19 of 32.

A speed of 4 pixels a frame is 240 pixels a second only at 60 frames a second. At 30 the same game runs at half speed. Give speeds in pixels per second instead, and multiply by dt to get this frame's step:

speed = 200

if keys[pygame.K_RIGHT]:
    x += speed * dt

At 60 frames a second, dt is about 0.017 and the ball moves about 3.4 pixels a frame. At 30, dt is about 0.033 and it moves about 6.7: twice as far each frame, the same 200 pixels each second.

Positions become decimals such as 243.4. That is fine: pygame accepts decimal positions and draws on whole pixels.

Challenge

Easy

Before the loop, set speed = 200.
Make each arrow move the ball by speed * dt instead of 4.

Try it yourself

import pygame

pygame.init()
screen = pygame.display.set_mode((480, 360))
pygame.display.set_caption("Pixels per Second")
clock = pygame.time.Clock()

x, y = 240, 180
dt = 0

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False

    keys = pygame.key.get_pressed()
    if keys[pygame.K_LEFT]:
        x -= 4
    if keys[pygame.K_RIGHT]:
        x += 4
    if keys[pygame.K_UP]:
        y -= 4
    if keys[pygame.K_DOWN]:
        y += 4

    screen.fill((30, 30, 46))
    pygame.draw.circle(screen, (255, 196, 0), (x, y), 16)
    pygame.display.flip()
    dt = clock.tick(60) / 1000

pygame.quit()