Circle Hitboxes

Part of the Sprites and Images section of Coddy's Pygame journey. Lesson 24 of 36.

A pixel test is exact, but exact is not always what feels fair. A spike grazing the tip of a wing counts as a hit. Many games test a smaller, simpler shape instead, a hitbox, and let near misses go.

For round things, test circles. Give each sprite a radius, and pygame.sprite.collide_circle calls it a hit when the centres are closer than the two radii added together:

self.radius = 24  # in Player.__init__
self.radius = 22  # in Ball.__init__

hits = pygame.sprite.spritecollide(player, balls, False, pygame.sprite.collide_circle)

Here that is 24 + 22 = 46 pixels between the centres. The circles are smaller than the pictures, so the very edges can brush without a hit. A circle test is also cheap: one distance, no pixels.

Without a radius, collide_circle uses a circle around the whole Rect, bigger than the Rect itself. For a box-shaped sprite, a smaller Rect does the same job: self.rect.inflate(-16, -16) is the Rect 16 pixels narrower and shorter, with the same centre.

Challenge

Medium

Give Player self.radius = 24 and Ball self.radius = 22.
Test with pygame.sprite.collide_circle instead of collide_mask.

Try it yourself

import pygame

pygame.init()
screen = pygame.display.set_mode((640, 480))
pygame.display.set_caption("Spike Field")
clock = pygame.time.Clock()

ship_image = pygame.image.load("assets/images/ship.png").convert_alpha()
ball_image = pygame.image.load("assets/images/spike_ball.png").convert_alpha()
font = pygame.font.Font(None, 40)


class Player(pygame.sprite.Sprite):
    def __init__(self):
        super().__init__()
        self.image = ship_image
        self.rect = self.image.get_rect(topleft=(420, 320))
        self.mask = pygame.mask.from_surface(self.image)

    def update(self):
        keys = pygame.key.get_pressed()
        if keys[pygame.K_LEFT]:
            self.rect.x -= 4
        if keys[pygame.K_RIGHT]:
            self.rect.x += 4
        if keys[pygame.K_UP]:
            self.rect.y -= 4
        if keys[pygame.K_DOWN]:
            self.rect.y += 4


class Ball(pygame.sprite.Sprite):
    def __init__(self, x, y):
        super().__init__()
        self.image = ball_image
        self.rect = self.image.get_rect(center=(x, y))
        self.mask = pygame.mask.from_surface(self.image)


player = Player()
balls = pygame.sprite.Group(Ball(160, 240), Ball(320, 240), Ball(480, 240))

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

    player.update()
    hits = pygame.sprite.spritecollide(player, balls, False, pygame.sprite.collide_mask)

    screen.fill((20, 22, 40))
    balls.draw(screen)
    screen.blit(player.image, player.rect)
    if hits:
        text = font.render("Hit!", True, (240, 240, 240))
        screen.blit(text, (10, 10))
    pygame.display.flip()
    clock.tick(60)

pygame.quit()