如何设置pygame.transform.rotate()的旋转中心？

Question

我想让一个矩形围绕一个不是中心的点旋转。到目前为止，我的代码是：

import pygame

pygame.init()
w = 640
h = 480
degree = 45
screen = pygame.display.set_mode((w, h))

surf = pygame.Surface((25, 100))
surf.fill((255, 255, 255))
surf.set_colorkey((255, 0, 0))
bigger = pygame.Rect(0, 0, 25, 100)
pygame.draw.rect(surf, (100, 0, 0), bigger)
rotatedSurf = pygame.transform.rotate(surf, degree)
screen.blit(rotatedSurf, (400, 300))

running = True
while running:
    event = pygame.event.poll()
    if event.type == pygame.QUIT:
        running = False
    pygame.display.flip()

我可以通过改变角度来实现不同的旋转，但现在的旋转是围绕矩形的中心进行的。我想把旋转的点设置为矩形中心以外的其他点。

Answer 1

我同意MegaIng和skrx的看法。不过我也得承认，看完他们的回答后，我还是没完全明白这个概念。后来我找到了一份关于在边缘旋转的炮台的游戏教程。

运行之后，我心里还是有疑问，但我发现他们用的炮台图片其实是个小技巧。

这张图片并不是围绕炮台的中心居中的，而是围绕支点居中的，图片的另一半是透明的。意识到这一点后，我把同样的做法应用到了我虫子的腿上，现在它们都能正常工作了。以下是我的旋转代码：

def rotatePivoted(im, angle, pivot):
    # rotate the leg image around the pivot
    image = pygame.transform.rotate(im, angle)
    rect = image.get_rect()
    rect.center = pivot
    return image, rect

希望这对你有帮助！

Answer 2

我也遇到过这个问题，找到了一种简单的解决办法：你可以创建一个更大的区域（长度和高度都加倍），然后把较小的区域放到这个更大的区域里，这样旋转的中心点就是更大的区域的中心。现在你只需要围绕中心旋转这个更大的区域就可以了。

def rotate(img, pos, angle):
    w, h = img.get_size()
    img2 = pygame.Surface((w*2, h*2), pygame.SRCALPHA)
    img2.blit(img, (w-pos[0], h-pos[1]))
    return pygame.transform.rotate(img2, angle)

（如果你画个草图，可能会更容易理解，但相信我：这个方法有效，而且在我看来比其他解决方案更简单易懂。）

Answer 3

要让一个表面围绕它的中心旋转，首先我们要旋转这个图像，然后得到一个新的矩形（rect），并把之前矩形的center坐标传给它，这样就能保持它在中心位置。如果想要围绕一个任意的点旋转，我们基本上也可以这样做，但还需要在中心位置（也就是旋转的支点）加上一个偏移向量，这样才能移动这个矩形。每次旋转图像时，这个向量也需要一起旋转。

所以我们需要保存这个支点（图像或精灵的原始中心）——可以用元组、列表、向量或者矩形来存储，还有偏移向量（也就是我们移动矩形的量），然后把它们传给rotate函数。接着我们旋转图像和偏移向量，得到一个新的矩形，把支点加上偏移作为center参数传入，最后返回旋转后的图像和新的矩形。

import pygame as pg


def rotate(surface, angle, pivot, offset):
    """Rotate the surface around the pivot point.

    Args:
        surface (pygame.Surface): The surface that is to be rotated.
        angle (float): Rotate by this angle.
        pivot (tuple, list, pygame.math.Vector2): The pivot point.
        offset (pygame.math.Vector2): This vector is added to the pivot.
    """
    rotated_image = pg.transform.rotozoom(surface, -angle, 1)  # Rotate the image.
    rotated_offset = offset.rotate(angle)  # Rotate the offset vector.
    # Add the offset vector to the center/pivot point to shift the rect.
    rect = rotated_image.get_rect(center=pivot+rotated_offset)
    return rotated_image, rect  # Return the rotated image and shifted rect.


pg.init()
screen = pg.display.set_mode((640, 480))
clock = pg.time.Clock()
BG_COLOR = pg.Color('gray12')
# The original image will never be modified.
IMAGE = pg.Surface((140, 60), pg.SRCALPHA)
pg.draw.polygon(IMAGE, pg.Color('dodgerblue3'), ((0, 0), (140, 30), (0, 60)))
# Store the original center position of the surface.
pivot = [200, 250]
# This offset vector will be added to the pivot point, so the
# resulting rect will be blitted at `rect.topleft + offset`.
offset = pg.math.Vector2(50, 0)
angle = 0

running = True
while running:
    for event in pg.event.get():
        if event.type == pg.QUIT:
            running = False

    keys = pg.key.get_pressed()
    if keys[pg.K_d] or keys[pg.K_RIGHT]:
        angle += 1
    elif keys[pg.K_a] or keys[pg.K_LEFT]:
        angle -= 1
    if keys[pg.K_f]:
        pivot[0] += 2

    # Rotated version of the image and the shifted rect.
    rotated_image, rect = rotate(IMAGE, angle, pivot, offset)

    # Drawing.
    screen.fill(BG_COLOR)
    screen.blit(rotated_image, rect)  # Blit the rotated image.
    pg.draw.circle(screen, (30, 250, 70), pivot, 3)  # Pivot point.
    pg.draw.rect(screen, (30, 250, 70), rect, 1)  # The rect.
    pg.display.set_caption('Angle: {}'.format(angle))
    pg.display.flip()
    clock.tick(30)

pg.quit()

这里有一个使用pygame.sprite.Sprite的版本：

import pygame as pg
from pygame.math import Vector2


class Entity(pg.sprite.Sprite):

    def __init__(self, pos):
        super().__init__()
        self.image = pg.Surface((122, 70), pg.SRCALPHA)
        pg.draw.polygon(self.image, pg.Color('dodgerblue1'),
                        ((1, 0), (120, 35), (1, 70)))
        # A reference to the original image to preserve the quality.
        self.orig_image = self.image
        self.rect = self.image.get_rect(center=pos)
        self.pos = Vector2(pos)  # The original center position/pivot point.
        self.offset = Vector2(50, 0)  # We shift the sprite 50 px to the right.
        self.angle = 0

    def update(self):
        self.angle += 2
        self.rotate()

    def rotate(self):
        """Rotate the image of the sprite around a pivot point."""
        # Rotate the image.
        self.image = pg.transform.rotozoom(self.orig_image, -self.angle, 1)
        # Rotate the offset vector.
        offset_rotated = self.offset.rotate(self.angle)
        # Create a new rect with the center of the sprite + the offset.
        self.rect = self.image.get_rect(center=self.pos+offset_rotated)


def main():
    screen = pg.display.set_mode((640, 480))
    clock = pg.time.Clock()
    entity = Entity((320, 240))
    all_sprites = pg.sprite.Group(entity)

    while True:
        for event in pg.event.get():
            if event.type == pg.QUIT:
                return

        keys = pg.key.get_pressed()
        if keys[pg.K_d]:
            entity.pos.x += 5
        elif keys[pg.K_a]:
            entity.pos.x -= 5

        all_sprites.update()
        screen.fill((30, 30, 30))
        all_sprites.draw(screen)
        pg.draw.circle(screen, (255, 128, 0), [int(i) for i in entity.pos], 3)
        pg.draw.rect(screen, (255, 128, 0), entity.rect, 2)
        pg.draw.line(screen, (100, 200, 255), (0, 240), (640, 240), 1)
        pg.display.flip()
        clock.tick(30)


if __name__ == '__main__':
    pg.init()
    main()
    pg.quit()