I’m trying to write a little physics simulation using only standart Pygame. I wanna to have a trapezoid a draw it using 4 dots. So I wanna to rotate every single of this dots independetly (not use pygame.transform.rotate ). I write a code that calculate a new coordinates of dot, after rotation around a pivot, using trigonometric functions.

fps = 15
clock = pygame.time.Clock()

x1 = 150
y1 = 150

x = x1
y = y1

p = 250
q = 250

angle = 10
theta = math.radians (angle)

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

    clock.tick(fps)

    screen.fill ((0,0,0))
    screen.set_at ((p,q),(255,0,0))
    pygame.draw.circle (screen, (255,0,0), (x,y), 10)

    x = p + math.cos(theta) * (x - p) - math.sin(theta) * (y - q)
    y = q + math.sin(theta) * (x - p) + math.cos(theta) * (y - q)

    pygame.display.update()
    pygame.display.flip()

But when I run it, dot perform not circular move, but goes a spiral down to a pivot. I look at coordinates in first frame, and they just a little bit different from what I expect.


How to fix this? I wanna a circular movement.