在 Pygame 中填充两个圆的交集区域

Question

我正在做一个 pygame 项目，但遇到了问题。我有两个圆圈 A 和 B，如下所示。重叠的现在我只想填充两个圆圈共有的圆圈的一部分。有什么方法可以实现吗？

我知道我可以用红色填充圆 B，用白色填充圆 A，但是有一个特定区域，圆 A 超出了该区域。所以我无法实现。

这张图片中提到了圆圈和填充位置的图像：

Answer 1

使用每个像素的 alpha (pygame.SRCALPHA) 创建 2 个 pygame.Surface 对象：

surf1 = pygame.Surface((500, 500), pygame.SRCALPHA)
surf2 = pygame.Surface((500, 500), pygame.SRCALPHA)

定义两个圆的中心点和半径：

pos1, rad1 = (250, 200), 100
pos2, rad2 = (250, 250), 80

在两个表面上用相同的颜色绘制圆圈：

pygame.draw.circle(surf1, (255, 0, 0, 255), pos1, rad1)
pygame.draw.circle(surf2, (255, 0, 0, 255), pos2, rad2)

使用混合模式pygame.BLEND_RGBA_MIN 将一个表面混合到另一个表面：

surf1.blit(surf2, (0, 0), special_flags = pygame.BLEND_RGBA_MIN)

此时surf1包含了2个圆的交集区域。

要获得外部部分，还需要另一个步骤。在surf2 上混合surf1，使用混合模式pygame.BLEND_RGBA_MIN：

surf2.blit(surf1, (0, 0), special_flags = pygame.BLEND_RGBA_SUB)

现在surf2 包含 circle B 的部分，如果 circle A 被减去，则剩下的部分：

---

最小示例： repl.it/@Rabbid76/PyGame-CircleIntersection

import pygame

pygame.init()
window = pygame.display.set_mode((500, 500))
clock = pygame.time.Clock()

pos1, rad1 = (250, 200), 100
pos2, rad2 = (250, 250), 80

surf1 = pygame.Surface((500, 500), pygame.SRCALPHA)
surf2 = pygame.Surface((500, 500), pygame.SRCALPHA)

pygame.draw.circle(surf1, (255, 0, 0, 255), pos1, rad1)
pygame.draw.circle(surf2, (255, 0, 0, 255), pos2, rad2)

surf1.blit(surf2, (0, 0), special_flags = pygame.BLEND_RGBA_MIN)
surf2.blit(surf1, (0, 0), special_flags = pygame.BLEND_RGBA_SUB)

run = True
while run:
    clock.tick(60)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
        if event.type == pygame.KEYDOWN:
            print(pygame.key.name(event.key))
 
    window.fill((255, 255, 255))

    window.blit(surf2, (0, 0))     
    pygame.draw.circle(window, (128, 128, 128), pos1, rad1+1, 3)
    pygame.draw.circle(window, (128, 128, 128), pos2, rad2+1, 3)

    pygame.display.flip()

Answer 2

我找不到使用 pygame 函数的方法，所以我在这里做了一个关于如何自己做的快速演示：

import pygame
import math
import time

# Screen dimensions
WIDTH = 600
HEIGHT = 400

# Circle and screen colours
WHITE = (255, 255, 255)
RED = (255, 0, 0)
GREEN = (0, 255, 0)
BLUE = (0, 0, 255)

# Circle data
BIGGER_POS = (300, 150)
SMALLER_POS = (300, 220)
BIGGER_RADIUS = 130
SMALLER_RADIUS = 100
BIGGER_COLOUR = GREEN
SMALLER_COLOUR = BLUE
INTERSECTION_COLOUR = RED


def prepare_screen():
    """
    Create the initial screen.
    """
    pygame.init()
    screen = pygame.display.set_mode((WIDTH, HEIGHT))
    screen.fill(WHITE)
    return screen


def draw_circles(screen):
    """
    Draw the circles and wait to visually inspect what they look like.
    """
    bigger = pygame.draw.circle(screen, BIGGER_COLOUR, BIGGER_POS, BIGGER_RADIUS)
    smaller = pygame.draw.circle(screen, SMALLER_COLOUR, SMALLER_POS, SMALLER_RADIUS)
    pygame.display.update()
    time.sleep(3)
    return bigger, smaller


def calculate_distance(p1: tuple, p2: tuple):
    """
    Calculate distance between two points p1 and p2.
    """
    return math.sqrt(((p1[0] - p2[0])**2) + ((p1[1] - p2[1])**2))


def draw_intersection(screen, bigger, smaller):
    """
    Animation to mark intersecting points with red.
    
    Ideally this should only go through colliding points, but I'm lazy and made it iterate through all pixels
    instead.
    """
    for x in range(WIDTH):
        for y in range(HEIGHT):
            print("Checking point", x, y)
            
            # A point is in the intersection if it's within the radius-es of both circles
            if calculate_distance(bigger.center, (x, y)) <= BIGGER_RADIUS and \
                    calculate_distance(smaller.center, (x, y)) <= SMALLER_RADIUS:
                screen.set_at((x, y), INTERSECTION_COLOUR)
                pygame.display.update()


# Get screen to draw on
screen = prepare_screen()

# Draw some initial circles
bigger, smaller = draw_circles(screen)

# Draw the intersection
draw_intersection(screen, bigger, smaller)

# Finally sleep some to allow visual inspection, and quit
time.sleep(3)
pygame.quit()

根据需要随意调整 - 请注意，可以优化各个地方，使其更具可读性等。

结果：

如果有什么不清楚的地方请告诉我。