Why is the position of this image changing?

Question

I have a python script to simulate a wrench moving, I have calculated it's origins position in x and y as a function of time, the problem is when I try to rotate the image, my code moves the image out of frame.

import pygame, sys
import math

screen = pygame.display.set_mode((700, 400))
wrench = pygame.image.load('prueba.bmp')
white = (255,255,255)


ox = 20
oy = 32
running = True
posX = 100 - ox
posY = 100 - oy

pygame.init()
while running:
    event = pygame.event.poll()
    if event.type == pygame.QUIT:
        running=0
        screen.blit(wrench, (posX, posY))
    if(pygame.time.get_ticks() <= 20000):
            screen.fill(white)
            t = pygame.time.get_ticks() / 1000
            print(t)
            ang = math.degrees((math.pi / 2 * math.cos(math.pi * t)))
            posX = 100 + (50 * math.cos(math.pi * t)) - ox 
            posY = 100 + (20 * math.sin(math.pi * t)) - oy
            wrench = pygame.transform.rotate(wrench, ang)
            screen.blit(wrench, (posX, posY))
    pygame.display.flip()

Does anybody knows why this is happening?

Answer 1

I'm not very familiar with Python, but I believe it's because of this line:

wrench = pygame.transform.rotate(wrench, ang);

wrench is updating itself, and the origin of rotation <0,0> will be in a different place with respect to the image each time. This is because the wrench's bounding rectangle is being updated as it turns.

To fix the problem, keep track of the rotation angle, and rotate the raw image only.

rotated_wrench = pygame.transform.rotate(wrench, ang);