Pygame collision with masks

I have made a putt-putt game and now I want to add a slanted wall type. Because of this, I need to use masks for the collision (until now I have just used rects). I have spent hours learning about masks and trying to figure out why my code won’t work. There are no errors, the collision just isn’t detected.

I have simplified my code down to something much smaller just as a way for me to test it efficiently. From everything I’ve seen this seems like it should work, but it doesnt. Here it is:

import pygame

# Pygame init stuff
pygame.init()

wind_width = 1200
wind_height = 700

gameDisplay = pygame.display.set_mode((wind_width, wind_height))
pygame.display.set_caption("Mini Golf!")

pygame.display.update()

gameExit = False

clock = pygame.time.Clock()

# Class setups
class Ball:

    def __init__(self, x, y):
        self.x = x
        self.y = y

        self.image = pygame.image.load("sball.png")
        self.rect = self.image.get_rect()
        self.mask = pygame.mask.from_surface(self.image)

    def render(self):
        self.rect.topleft = (self.x, self.y)
        gameDisplay.blit(self.image, self.rect)

class Slant:

    def __init__(self, x, y):
        self.x = x
        self.y = y

        self.image = pygame.image.load("posslant.png")
        self.rect = self.image.get_rect()
        self.mask = pygame.mask.from_surface(self.image)

    def render(self):
        self.rect.topleft = (self.x, self.y)
        gameDisplay.blit(self.image, self.rect)

# Creating objects
ball = Ball(250, 250)

slant = Slant(270, 250)

# Game loop
gameExit = False
while not(gameExit):

    # Moves ball
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            gameExit = True
        elif event.type == pygame.KEYDOWN:
            if event.key == pygame.K_UP:
                ball.y -= 1
            elif event.key == pygame.K_DOWN:
                ball.y += 1
            elif event.key == pygame.K_LEFT:
                ball.x -= 1
            elif event.key == pygame.K_RIGHT:
                ball.x += 1

    # Collision detection
    offset_x, offset_y = (slant.rect.x - ball.rect.x), (slant.rect.y - ball.rect.y)
    if slant.mask.overlap(ball.mask, (offset_x, offset_y)):
        print("hit")

    # Draws everything
    gameDisplay.fill((0, 0, 0))
    ball.render()
    slant.render()

    pygame.display.update()

    clock.tick(100)

Answer

The offset parameter of the method overlap() is the relative position of the othermask in relation to the pygame.mask.Mask object.
So the offset is calculated by subtracting the coordinates of slant from the coordinates of ball:

offset_x, offset_y = (slant.rect.x - ball.rect.x), (slant.rect.y - ball.rect.y)

offset = (ball.rect.x - slant.rect.x), (ball.rect.y - slant.rect.y)
if slant.mask.overlap(ball.mask, offset):
    print("hit")
    

---

When you create the mask images, then I recommend to ensure that the image has per pixel alpha format by calling .convert_alpha():

class Ball:

    def __init__(self, x, y):
        self.x = x
        self.y = y

        self.image = pygame.image.load("sball.png")
        self.rect = self.image.get_rect()
        self.mask = pygame.mask.from_surface(self.image.convert_alpha()) # <---

class Slant:

    def __init__(self, x, y):
        self.x = x
        self.y = y

        self.image = pygame.image.load("posslant.png")
        self.rect = self.image.get_rect()
        self.mask = pygame.mask.from_surface(self.image.image.convert_alpha()) # <---

---

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

See also: Mask