Rotating a Red Box

rotating_box.py

import pygame
pygame.init()


# 1000 ms in a second, cycle length of 30 seconds
CYCLE = 1000. * 30


def add_vectors(vec1, vec2):
    return vec1[0] + vec2[0], vec1[1] + vec2[1]


def negate_vector(vec):
    return -vec[0], -vec[1]


def rotate_box(box, degrees):
    rotated = pygame.transform.rotate(box, degrees)
    # Find the offset o
    offset = add_vectors(negate_vector(rotated.get_rect().center), 
                         box.get_rect().center)
    return rotated, offset


def degrees_from_time(t):
    """
    Get the degrees to rotate, which is:
    percentage_of_cycle_complete = time_elapsed_in_cycle / cycle_length
    degrees_to_rotate = percentage_of_cycle_complete * 360
    """
    t = t % CYCLE
    return t / CYCLE * 360


def main():
    screen = pygame.display.set_mode((400, 400))
    box = pygame.Surface((200, 200), pygame.SRCALPHA)
    box.fill((255, 0, 0))
    # 400 - 200 / 2
    base_offset = (100, 100)
    time_elapsed = 0
    clock = pygame.time.Clock()
    
    while True:
        for e in pygame.event.get():
            if e.type == pygame.QUIT or (e.type == pygame.KEYDOWN and 
                                         e.key == pygame.K_ESCAPE):
                pygame.quit()
                return
        
        ms = clock.tick(60)
        time_elapsed += ms
        to_rotate = degrees_from_time(time_elapsed)
        rotated, offset = rotate_box(box, to_rotate)
        
        screen.fill((0, 0, 0))
        screen.blit(rotated, add_vectors(base_offset, offset))
        pygame.display.update()


if __name__ == '__main__':
    main()