PyGame旋转-矩形不透明？

Question

我已经弄清楚了如何在游戏前围绕图像的中心点旋转图像，但是这样做会导致背景透明性下降。

class Agent:

    def __init__(self,display_width,display_height):
        self.x = random.randint(0,display_width)
        self.y = random.randint(0,display_height)
        self.angle = random.randint(0,359)
        self.sprite_original = pygame.image.load('agent.png').convert()
        self.sprite = self.sprite_original


    def rot_center(self, image, angle):
        """rotate an image while keeping its center and size"""
        orig_rect = image.get_rect()
        rot_image = pygame.transform.rotate(image, angle)
        rot_rect = orig_rect.copy()
        rot_rect.center = rot_image.get_rect().center
        rot_image = rot_image.subsurface(rot_rect).copy()
        return rot_image


    def action(self,keys):

    #a rotate left
        if keys[97] == True:
            self.angle +=1 % 360
            self.sprite = self.rot_center(self.sprite_original,self.angle)

    #d rotate right
        if keys[119] == True:
            self.angle -=1 % 360
            self.sprite = self.rot_center(self.sprite_original,self.angle)     

我希望我的agent.png图像保持其背景透明性，但事实并非如此。