rotating a rectangle in pygame

Question

I'm trying to make a white rectangle rotate like a the hands of a clock in pygame using this code,

import random, pygame, math, sys
from pygame.locals import *
 
Blue = (0,0,255)
Black = (0, 0, 0) 
Green = (0,255,0)
White = (255,255,255)
  
pygame.init()
DISPLAYSURF = pygame.display.set_mode((400, 300))
pygame.display.set_caption('Sailing!')
  
FPS = 30
fpsClock = pygame.time.Clock()
  
Sail = pygame.Surface([100,10])
Sail.set_colorkey (Black)
Sail.fill(White)
  
degrees = 0
hyp = 100
x = 200
y = 150


while True:
    for event in pygame.event.get():
        if event.type == QUIT:
            pygame.quit()
            sys.exit()
    adj = 10 * math.cos(degrees)
    opp = 10 * math.sin(degrees)
    dx = adj + 200
    dy = opp + 150
    
    rotatedSail = pygame.transform.rotate(Sail, degrees)
    Sail_rect = Sail.get_rect(topleft = (dx, dy))
    DISPLAYSURF.fill(Blue)
    DISPLAYSURF.blit(rotatedSail, Sail_rect)
    pygame.display.flip()
  
    fpsClock.tick(FPS)
  
    degrees += 1

but the rectangle rotates in a weird way. I would appreciate it if you could keep the suggestion as simple and as close to my code as possible, because I'm just starting to learn. Plus i know it's easier to do it using an image of a rectangle, but I'm trying to use a surface. can anyone help?

Rabbid76 · Answer 1 · 2021-08-25T19:35:35.103

You need to get the bounding rectangle of the rotated rectangle and set the center of the rectangle by (x, y) (see also How do I rotate an image around its center using PyGame?):

while True:
    for event in pygame.event.get():
        if event.type == QUIT:
            pygame.quit()
            sys.exit()
       
    rotatedSail = pygame.transform.rotate(Sail, degrees)
    rotatedSail_rect = rotatedSail.get_rect(center = (x, y))
    DISPLAYSURF.fill(Blue)
    DISPLAYSURF.blit(rotatedSail, rotatedSail_rect)
    pygame.display.flip()
  
    fpsClock.tick(FPS)
  
    degrees += 1

To rotate the object around another point than the center point is much more complicate. A general solution is described in the answer to How can you rotate an image around an off center pivot in PyGame.

Complete Example:

import pygame, math, sys
from pygame.locals import *
 
Blue = (0,0,255)
Black = (0, 0, 0) 
Green = (0,255,0)
White = (255,255,255)
  
pygame.init()
DISPLAYSURF = pygame.display.set_mode((400, 300))
pygame.display.set_caption('Sailing!')
  
FPS = 30
fpsClock = pygame.time.Clock()
  
Sail = pygame.Surface([100,10])
Sail.set_colorkey (Black)
Sail.fill(White)
  
degrees = 0
hyp = 100
x = 200
y = 150

def blitRotate(surf, image, pos, originPos, angle):

    # calcaulate the axis aligned bounding box of the rotated image
    w, h         = image.get_size()
    sin_a, cos_a = math.sin(math.radians(angle)), math.cos(math.radians(angle)) 
    min_x, min_y = min([0, sin_a*h, cos_a*w, sin_a*h + cos_a*w]), max([0, sin_a*w, -cos_a*h, sin_a*w - cos_a*h])

    # calculate the translation of the pivot 
    pivot        = pygame.math.Vector2(originPos[0], -originPos[1])
    pivot_rotate = pivot.rotate(angle)
    pivot_move   = pivot_rotate - pivot

    # calculate the upper left origin of the rotated image
    origin = (pos[0] - originPos[0] + min_x - pivot_move[0], pos[1] - originPos[1] - min_y + pivot_move[1])

    # get a rotated image
    rotated_image = pygame.transform.rotate(image, angle)

    # rotate and blit the image
    surf.blit(rotated_image, origin)

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

    DISPLAYSURF.fill(Blue)
    blitRotate(DISPLAYSURF, Sail, (x, y), (0, 5), degrees)   
    pygame.display.flip()
    fpsClock.tick(FPS)
    degrees += 1

rotating a rectangle in pygame

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