pygame 增量时间导致运动不一致

Question

I am trying to understand delta time and wrote this simple example:

import pygame, sys, time

pygame.init()
screen = pygame.display.set_mode((1280,720))
clock = pygame.time.Clock()

rect1 = pygame.Rect(0,150,100,100)
rect2 = pygame.Rect(1180,500,100,100)
speed = 300

last_time = time.time()

while True:
    dt = time.time() - last_time
    last_time = time.time()

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

    screen.fill('white')
    rect1.x += speed * dt
    rect2.x -= speed * dt

    pygame.draw.rect(screen,'red',rect1)
    pygame.draw.rect(screen,'green',rect2)

    pygame.display.update()
    clock.tick(60)

The problem I have with it is that the left movement is faster than the right movement. So in the code snippet the green rectangle (rect2) reaches the end of the screen noticeably faster than the red rectangle.

I also tried to use pygame.clock to get delta time:

import pygame, sys, time

pygame.init()
screen = pygame.display.set_mode((1280,720))
clock = pygame.time.Clock()

rect1 = pygame.Rect(0,150,100,100)
rect2 = pygame.Rect(1180,500,100,100)
speed = 300

while True:

    dt = clock.tick(60) / 1000

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

    screen.fill('white')
    rect1.x += speed * dt
    rect2.x -= speed * dt

    pygame.draw.rect(screen,'red',rect1)
    pygame.draw.rect(screen,'green',rect2)

    pygame.display.update()

But the result remains the same.

I am really confused by this, am I doing something wrong?

Edit: Someone closed this and linked to deltatime not working at higher framerates. The framerate here is a constant 60fps.

Answer 1

Since pygame.Rect is supposed to represent an area on the screen, a pygame.Rect object can only store integral data.

The coordinates for Rect objects are all integers. [...]

The fraction part of the coordinates gets lost when the new position of the object is assigned to the Rect object. If this is done every frame, the position error will accumulate over time.

If you want to store object positions with floating point accuracy, you have to store the location of the object in separate variables and you have to synchronize the pygame.Rect object. round the coordinate and assign it to the position of the rectangle:

rect1 = pygame.Rect(0,150,100,100)
rect2 = pygame.Rect(1180,500,100,100)
pos1_x = rect1.x
pos2_x = rect2.x  

while True:
    # [...]

    pos1_x += speed * dt
    pos2_x -= speed * dt
    rect1.x = round(pos1_x)
    rect2.x = round(pos2_x)
 
    # [...]

See also Pygame doesn't let me use float for rect.move, but I need it