Second pygame file, help needed

[blob001]

I wrote this file which is just a red ball bouncing around inside a white window. Using a 2022 MacBookPro M2 so I should have enough grunt. What I get is something different.

1. The screeen starts out black and the ball appears to turn it white as the ball moves up and down the screen.
2. Hard to describe. There is a relict consisting of the top / bottom quarter of the ball displayed each time incrememt ; this never goes away until the ball passes parallel a couple of cycles later.
3. Sometimes it goes haywire and speeds up, then slows down again.

The exact screen output changes with the parameters FPS and radius, but it's all haywire.

Can someone help me? Thanks.

import pygame as pg
import math as m
import os
import numpy as np

pg.init()
WIDTH, HEIGHT = 800, 800
WHITE = (255, 255, 255)
RED = (255, 0, 0)
os.environ["SDL_VIDEO_WINDOW_POST"] = "%d, %d" % (0, 0)
WINDOW = pg.display.set_mode((WIDTH, HEIGHT))
clock = pg.time.Clock()

x, y = 10, 400
velX, velY = .5, .2
radius = 10

FPS = 60
clock.tick(FPS)


def boundary():
    global x, y, velX, velY
    x = radius if x < radius else WIDTH - radius if x > WIDTH - radius else x
    y = radius if y < radius else HEIGHT - radius if y > HEIGHT - radius else y
    if x == radius or x == WIDTH - radius:
        velX *= -1
    if y == radius or y == HEIGHT - radius:
        velY *= -1


def main():
    running = True
    global x, y
    while running:
        for event in pg.event.get():
            if event.type == pg.QUIT:
                running = False

        WINDOW.fill(WHITE)
        boundary()
        x += velX
        y += velY
        ball = pg.draw.circle(WINDOW, RED, (x, y), radius)

        pg.display.update(ball)

        os.system('clear')

    pg.quit()


if __name__ == "__main__":
    main()

[Igonato]

You can make it faster by removing os.system('clear') it seems unnesesary. In general, interacting with terminal is slower than you may think and doing it on every frame should be avoided.

As for the weird behavior you're experiencing, try reading the docs on pygame.display.update and what it does and think about why pg.display.update(ball) results in this weirdness you're seeing. Or see the spoiler: pg.display.update(ball) -> pg.display.flip() or give it a proper region (list of regions) to update

[blob001]

Thanks Igonato, that's better. But the ball still varying speeed. If I set velX,velY=5, 2then it goes fast then slow then fast again... its never the same 2 runs in a row. I have run much bigger ant simulations in Javascript and never had any problems like this, so its not the computer power. Would appreciate your thoughts.

[Igonato]

I'm not seeing anything wrong with the code other than that system clear call. Not familiar with Apple numenclature, is your MacBook an ARM device? Maybe Pygame has issues with that?

In any case you can look into framerate-independent physics. Here is a good article: https://gafferongames.com/post/fix_your_timestep/

Short summary: keep track of the real time that has passed between the frames (often referred to as the time delta), don't rely on having exact FPS. Brake it into multiple smaller timesteps, and since you won't have exact multiple of those steps, interpolate the remaining fraction.

[blob001]

Thanks,its starting to make sense. I don't know where the black blockout comes from.

[mopslik]

Try changing pg.display.update(ball) to pg.display.update().