How do I prevent the player from moving through the walls in a maze in pygame?

Question

I have a maze organized in a grid. Each cell of the grid stores the information about the walls to its right and bottom neighboring cell. The player is an object of a certain size whose bounding box is known. I want to move the player smoothly through the maze with the walls preventing them from going through.

Minimal and reproducible example:

import pygame, random

class Maze:
    def __init__(self, rows = 9, columns = 9):
        self.size = (columns, rows)
        self.walls = [[[True, True] for _ in range(self.size[1])] for __ in range(self.size[0])]
        visited = [[False for _ in range(self.size[1])] for __ in range(self.size[0])]
        i, j = (self.size[0]+1) // 2, (self.size[1]+1) // 2
        visited[i][j] = True
        stack = [(i, j)]
        while stack:
            current = stack.pop()
            i, j = current
            nl = [n for n in [(i-1, j), (i+1, j), (i, j-1), (i, j+1)] 
                  if 0 <= n[0] < self.size[0] and 0 <= n[1] < self.size[1] and not visited[n[0]][n[1]]]
            if nl:
                stack.insert(0, current)
                next = random.choice(nl)
                self.walls[min(next[0], current[0])][min(next[1], current[1])][abs(next[1]-current[1])] = False
                visited[next[0]][next[1]] = True
                stack.insert(0, next)

def draw_maze(surf, maze, x, y, l, color, width):
    lines = [((x, y), (x + l * len(maze.walls), y)), ((x, y), (x, y + l * len(maze.walls[0])))] 
    for i, row in enumerate(maze.walls):
        for j, cell in enumerate(row):
            if cell[0]: lines += [((x + i*l + l, y + j*l), (x + i*l + l, y + j*l + l))]
            if cell[1]: lines += [((x + i*l, y + j*l + l), (x + i*l + l, y + j*l + l))]
    for line in lines:
        pygame.draw.line(surf, color, *line, width)

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

maze = Maze()
player_rect = pygame.Rect(190, 190, 20, 20)

run = True
while run:
    clock.tick(100)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False

    keys = pygame.key.get_pressed()
    player_rect.x += (keys[pygame.K_RIGHT] - keys[pygame.K_LEFT]) * 3
    player_rect.y += (keys[pygame.K_DOWN] - keys[pygame.K_UP]) * 3

    window.fill(0)
    draw_maze(window, maze, 20, 20, 40, (196, 196, 196), 3)
    pygame.draw.circle(window, (255, 255, 0), player_rect.center, player_rect.width//2)
    pygame.display.flip()

pygame.quit()
exit()

Answer 1

Implement simple logic that tests if there is a wall in the player's path when the player moves. Discard the movement when a collision with a wall is detected.

Add methods to the Maze class that check for a wall between a cell and its neighboring cell:

class Maze:
    # [...]

    def wall_left(self, i, j):
        return i < 1 or self.walls[i-1][j][0]
    def wall_right(self, i, j):
        return i >= self.size[0] or self.walls[i][j][0]
    def wall_top(self, i, j):
        return j < 1 or self.walls[i][j-1][1]
    def wall_bottom(self, i, j):
        return j >= self.size[0] or self.walls[i][j][1]

Calculate the rows and columns of the corner points of the player's bounding box.

i0 = (player_rect.left - maze_pos[0]) // cell_size 
i1 = (player_rect.right - maze_pos[0]) // cell_size
j0 = (player_rect.top - maze_pos[1]) // cell_size  
j1 = (player_rect.bottom - maze_pos[1]) // cell_size  

As the player moves, test to see if the player is entering a new cell. Use the new methods in the Maze class to test whether there is a wall in the player's path. Skip the movement if the path is blocked by a wall:

keys = pygame.key.get_pressed()
if keys[pygame.K_LEFT]:
    new_rect = player_rect.move(-3, 0)
    ni = (new_rect.left - maze_pos[0]) // cell_size
    if i0 == ni or not (maze.wall_left(i0, j0) or maze.wall_left(i0, j1) or (j0 != j1 and maze.wall_bottom(ni, j0))):
        player_rect = new_rect
if keys[pygame.K_RIGHT]:
    new_rect = player_rect.move(3, 0)
    ni = (new_rect.right - maze_pos[0]) // cell_size
    if i1 == ni or not (maze.wall_right(i1, j0) or maze.wall_right(i1, j1) or (j0 != j1 and maze.wall_bottom(ni, j0))):
        player_rect = new_rect
keys = pygame.key.get_pressed()
if keys[pygame.K_UP]:
    new_rect = player_rect.move(0, -3)
    nj = (new_rect.top - maze_pos[1]) // cell_size
    if j0 == nj or not (maze.wall_top(i0, j0) or maze.wall_top(i1, j0) or (i0 != i1 and maze.wall_right(i0, nj))):
        player_rect = new_rect
if keys[pygame.K_DOWN]:
    new_rect = player_rect.move(0, 3)
    nj = (new_rect.bottom - maze_pos[1]) // cell_size
    if j1 == nj or not (maze.wall_bottom(i0, j1) or maze.wall_bottom(i1, j1) or (i0 != i1 and maze.wall_right(i0, nj))):
        player_rect = new_rect

See also Maze collision detection

---

Minimal example: repl.it/@Rabbid76/PyGame-Maze-CollisionLogic

import pygame, random

class Maze:
    def __init__(self, rows = 9, columns = 9):
        self.size = (columns, rows)
        self.walls = [[[True, True] for _ in range(self.size[1])] for __ in range(self.size[0])]
        visited = [[False for _ in range(self.size[1])] for __ in range(self.size[0])]
        i, j = (self.size[0]+1) // 2, (self.size[1]+1) // 2
        visited[i][j] = True
        stack = [(i, j)]
        while stack:
            current = stack.pop()
            i, j = current
            nl = [n for n in [(i-1, j), (i+1, j), (i, j-1), (i, j+1)] 
                  if 0 <= n[0] < self.size[0] and 0 <= n[1] < self.size[1] and not visited[n[0]][n[1]]]
            if nl:
                stack.insert(0, current)
                next = random.choice(nl)
                self.walls[min(next[0], current[0])][min(next[1], current[1])][abs(next[1]-current[1])] = False
                visited[next[0]][next[1]] = True
                stack.insert(0, next)

    def wall_left(self, i, j):
        return i < 1 or self.walls[i-1][j][0]
    def wall_right(self, i, j):
        return i >= self.size[0] or self.walls[i][j][0]
    def wall_top(self, i, j):
        return j < 1 or self.walls[i][j-1][1]
    def wall_bottom(self, i, j):
        return j >= self.size[0] or self.walls[i][j][1]

def draw_maze(surf, maze, x, y, l, color, width):
    lines = [((x, y), (x + l * len(maze.walls), y)), ((x, y), (x, y + l * len(maze.walls[0])))] 
    for i, row in enumerate(maze.walls):
        for j, cell in enumerate(row):
            if cell[0]: lines += [((x + i*l + l, y + j*l), (x + i*l + l, y + j*l + l))]
            if cell[1]: lines += [((x + i*l, y + j*l + l), (x + i*l + l, y + j*l + l))]
    for line in lines:
        pygame.draw.line(surf, color, *line, width)

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

maze = Maze()
player_rect = pygame.Rect(190, 190, 20, 20)

run = True
while run:
    clock.tick(100)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False

    maze_pos = 20, 20
    cell_size = 40
    i0 = (player_rect.left - maze_pos[0]) // cell_size 
    i1 = (player_rect.right - maze_pos[0]) // cell_size
    j0 = (player_rect.top - maze_pos[1]) // cell_size  
    j1 = (player_rect.bottom - maze_pos[1]) // cell_size  

    keys = pygame.key.get_pressed()
    if keys[pygame.K_LEFT]:
        new_rect = player_rect.move(-3, 0)
        ni = (new_rect.left - maze_pos[0]) // cell_size
        if i0 == ni or not (maze.wall_left(i0, j0) or maze.wall_left(i0, j1) or (j0 != j1 and maze.wall_bottom(ni, j0))):
            player_rect = new_rect
    if keys[pygame.K_RIGHT]:
        new_rect = player_rect.move(3, 0)
        ni = (new_rect.right - maze_pos[0]) // cell_size
        if i1 == ni or not (maze.wall_right(i1, j0) or maze.wall_right(i1, j1) or (j0 != j1 and maze.wall_bottom(ni, j0))):
            player_rect = new_rect
    keys = pygame.key.get_pressed()
    if keys[pygame.K_UP]:
        new_rect = player_rect.move(0, -3)
        nj = (new_rect.top - maze_pos[1]) // cell_size
        if j0 == nj or not (maze.wall_top(i0, j0) or maze.wall_top(i1, j0) or (i0 != i1 and maze.wall_right(i0, nj))):
            player_rect = new_rect
    if keys[pygame.K_DOWN]:
        new_rect = player_rect.move(0, 3)
        nj = (new_rect.bottom - maze_pos[1]) // cell_size
        if j1 == nj or not (maze.wall_bottom(i0, j1) or maze.wall_bottom(i1, j1) or (i0 != i1 and maze.wall_right(i0, nj))):
            player_rect = new_rect

    window.fill(0)
    draw_maze(window, maze, 20, 20, cell_size, (196, 196, 196), 3)
    pygame.draw.circle(window, (255, 255, 0), player_rect.center, player_rect.width//2)
    pygame.display.flip()

pygame.quit()
exit()

Answer 2

Both @Rabbid76 solutions are good, but I would like to provide another approach.

In this approach, you would need two variables x and y to store the previous position, position before the collision. You would then need to store all the maze line's rect in a list.

Now, check if the player's rect has collided with any of the rect in the list either by iterating through the list and using Rect.colliderect or use pygame's Rect.collidelist(note: collidelist will return -1 if there is no collision). If they have collided reset the current position to the previous position.

code:

import pygame, random

class Maze:
    def __init__(self, rows = 9, columns = 9):
        self.size = (columns, rows)
        self.walls = [[[True, True] for _ in range(self.size[1])] for __ in range(self.size[0])]
        visited = [[False for _ in range(self.size[1])] for __ in range(self.size[0])]
        i, j = (self.size[0]+1) // 2, (self.size[1]+1) // 2
        visited[i][j] = True
        stack = [(i, j)]
        while stack:
            current = stack.pop()
            i, j = current
            nl = [n for n in [(i-1, j), (i+1, j), (i, j-1), (i, j+1)]
                  if 0 <= n[0] < self.size[0] and 0 <= n[1] < self.size[1] and not visited[n[0]][n[1]]]
            if nl:
                stack.insert(0, current)
                next = random.choice(nl)
                self.walls[min(next[0], current[0])][min(next[1], current[1])][abs(next[1]-current[1])] = False
                visited[next[0]][next[1]] = True
                stack.insert(0, next)


def draw_maze(surf, maze, x, y, l, color, width):
    lines = maze_lines(maze, x, y, l)
    for line in lines:
        pygame.draw.line(surf, color, *line, width)


def maze_lines(maze, x, y, l):
    lines = [((x, y), (x + l * len(maze.walls), y)), ((x, y), (x, y + l * len(maze.walls[0])))]

    for i, row in enumerate(maze.walls):
        for j, cell in enumerate(row):
            if cell[0]: lines += [((x + i * l + l, y + j * l), (x + i * l + l, y + j * l + l))]
            if cell[1]: lines += [((x + i * l, y + j * l + l), (x + i * l + l, y + j * l + l))]

    return lines

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

maze = Maze()
player_rect = pygame.Rect(190, 190, 20, 20)

line_rects = [pygame.draw.line(window, (0, 0, 0), *line) for line in maze_lines(maze, 20, 20, 40)]  # first store all the line's rect in the list
prev_x, prev_y = 0, 0

run = True
while run:
    clock.tick(100)
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False

    keys = pygame.key.get_pressed()

    prev_x, prev_y = player_rect.x, player_rect.y
    player_rect.x += (keys[pygame.K_RIGHT] - keys[pygame.K_LEFT]) * 3
    player_rect.y += (keys[pygame.K_DOWN] - keys[pygame.K_UP]) * 3

    # if any(x.colliderect(player_rect) for x in line_rects):
    if player_rect.collidelist(line_rects) != -1:
        player_rect.x = prev_x
        player_rect.y = prev_y

    window.fill(0)
    draw_maze(window, maze, 20, 20, 40, (196, 196, 196), 3)

    pygame.draw.circle(window, (255, 255, 0), player_rect.center, player_rect.width//2)
    pygame.display.flip()

pygame.quit()
exit()

Output: