Pygame mask collision

I’m trying to get proper collision detection with rotating surfaces in pygame. I decided to try using masks. It somewhat works, but it is not as precise as I’d liked/thought. I tried updating the mask at the end of the cycle to get a “fresh” hitbox for the next frame, but it didn’t work as expected. What is my mistake?

import pygame
import random

WHITE = [255, 255, 255]
RED = [255, 0, 0]

pygame.init()

FPS = pygame.time.Clock()
fps = 6

winW = 1000
winH = 500
BGCOLOR = WHITE
win = pygame.display.set_mode((winW, winH))
win.fill(WHITE)
pygame.display.set_caption('')
pygame.display.set_icon(win)


class Box(pygame.sprite.Sprite):
    def __init__(self, x, y, w, h):
        pygame.sprite.Sprite.__init__(self)
        self.image = pygame.Surface([w, h], pygame.SRCALPHA)
        self.image.fill(random_color())
        self.mask = pygame.mask.from_surface(self.image)
        self.rect = pygame.Rect(x, y, w, h)
        self.angle = 0

    def move(self):
        self.rect.center = pygame.mouse.get_pos()

    def draw(self):
        blits = self.rotate()
        win.blit(blits[0], blits[1])
        self.mask = pygame.mask.from_surface(blits[0])

    def rotate(self):
        self.angle += 3
        new_img = pygame.transform.rotate(self.image, self.angle)
        new_rect = new_img.get_rect(center=self.rect.center)
        return new_img, new_rect


def update_display():
    win.fill(BGCOLOR)
    player.draw()
    for p in platforms:
        p.draw()
    pygame.display.update()


def collision():
    return pygame.sprite.spritecollide(player, plat_collide, False, pygame.sprite.collide_mask)


def random_color():
    return [random.randint(0, 255), random.randint(0, 255), random.randint(0, 255)]


player = Box(100, 400, 50, 50)

platforms = [Box(300, 400, 100, 50)]
plat_collide = pygame.sprite.Group(platforms)

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

    hits = collision()
    if hits:
        BGCOLOR = RED
    else:
        BGCOLOR = WHITE

    player.move()

    update_display()

    FPS.tick(fps)

pygame.quit()

Answers:

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Method 1

Your application works fine. But note, pygame.sprite.collide_mask() use the .rect and .mask attribute of the sprite object for the collision detection.
You have to update self.rect after rotating the image:

class Box(pygame.sprite.Sprite):
    # [...]

    def rotate(self):
        self.angle += 3
        new_img = pygame.transform.rotate(self.image, self.angle)
        new_rect = new_img.get_rect(center=self.rect.center)
        
        # update .rect attribute
        self.rect = new_rect # <------
        
        return new_img, new_rect

See also Sprite mask

Minimal example: repl.it/@Rabbid76/PyGame-SpriteMask

import pygame

class SpriteObject(pygame.sprite.Sprite):
    def __init__(self, x, y, w, h, color):
        pygame.sprite.Sprite.__init__(self)
        self.angle = 0
        self.original_image = pygame.Surface([w, h], pygame.SRCALPHA)
        self.original_image.fill(color)
        self.image = self.original_image
        self.rect = self.image.get_rect(center = (x, y))
        self.mask = pygame.mask.from_surface(self.image )
    def update(self):
        self.rotate()
    def rotate(self):
        self.angle += 0.3
        self.image = pygame.transform.rotate(self.original_image, self.angle)
        self.rect = self.image.get_rect(center = self.rect.center)
        self.mask = pygame.mask.from_surface(self.image )

pygame.init()
clock = pygame.time.Clock()
window = pygame.display.set_mode((400, 400))
size = window.get_size()

moving_object = SpriteObject(0, 0, 50, 50, (128, 0, 255))
static_objects = [
    SpriteObject(size[0] // 2, size[1] // 3, 100, 50, (128, 128, 128)),
    SpriteObject(size[0] // 4, size[1] * 2 // 3, 100, 50, (128, 128, 128)),
    SpriteObject(size[0] * 3 // 4, size[1] * 2 // 3, 100, 50, (128, 128, 128))
]
all_sprites = pygame.sprite.Group([moving_object] + static_objects)
static_sprites = pygame.sprite.Group(static_objects)

run = True
while run:
    clock.tick(60)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False

    moving_object.rect.center = pygame.mouse.get_pos()
    all_sprites.update() 
    collide = pygame.sprite.spritecollide(moving_object, static_sprites, False, pygame.sprite.collide_mask)
    
    window.fill((255, 0, 0) if collide else (255, 255, 255))
    all_sprites.draw(window)
    pygame.display.update()

pygame.quit()
exit()

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0