A Camera Offset

Part of the Worlds section of Coddy's Pygame journey. Lesson 20 of 35.

This level is 48 tiles wide, three windows. Everything keeps its level position, and a camera decides which part of the level the window shows. The camera is one number, camera_x: the level x at the window's left edge.

To draw something, subtract the camera from its level x:

for wall in walls:
    screen.blit(block_img, (wall.x - camera_x, wall.y))
screen.blit(player_img, (player.x - camera_x, player.y))

With camera_x = 0 you see the start. With camera_x = 600, a wall at x = 800 is drawn at 200. Only drawing changes: the Rects in walls and player keep their level positions, so collisions work as before.

In this program the camera drifts right by itself, 2 pixels a frame, until it shows the end of the level. The starter moves camera_x but draws without it, so nothing moves.

Challenge

Easy

Draw every wall at (wall.x - camera_x, wall.y).
Draw the player at (player.x - camera_x, player.y).

Leave the drift as it is: the level should slide left on its own.

Try it yourself

import pygame

TILE = 36
WIDTH = 576
SPEED = 4
GRAVITY = 1
JUMP = -15
MAX_FALL = 12

LEVEL = [
    "................................................",
    "................................................",
    "#...........###..........####..........###.....#",
    "#..............................................#",
    "#.....####.........###.........###.............#",
    "#..............................................#",
    "#.P........#####.........######.........####...#",
    "#..............................................#",
    "#..............................................#",
    "################################################",
]

pygame.init()
screen = pygame.display.set_mode((WIDTH, 360))
pygame.display.set_caption("Camera")
clock = pygame.time.Clock()

block_img = pygame.image.load("assets/images/block.png").convert_alpha()
player_img = pygame.image.load("assets/images/player.png").convert_alpha()

walls = []
for row, line in enumerate(LEVEL):
    for col, char in enumerate(line):
        if char == "#":
            walls.append(pygame.Rect(col * TILE, row * TILE, TILE, TILE))
        elif char == "P":
            start = (col * TILE + TILE // 2, row * TILE + TILE)

level_width = len(LEVEL[0]) * TILE
player = player_img.get_rect(midbottom=start)
vel_y = 0
on_ground = False
camera_x = 0

running = True
while running:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            running = False
        elif event.type == pygame.KEYDOWN and event.key in (pygame.K_SPACE, pygame.K_UP):
            if on_ground:
                vel_y = JUMP
        elif event.type == pygame.KEYUP and event.key in (pygame.K_SPACE, pygame.K_UP):
            if vel_y < -4:
                vel_y = -4

    keys = pygame.key.get_pressed()
    dx = 0
    if keys[pygame.K_LEFT]:
        dx -= SPEED
    if keys[pygame.K_RIGHT]:
        dx += SPEED

    player.x += dx
    for wall in walls:
        if player.colliderect(wall):
            if dx > 0:
                player.right = wall.left
            elif dx < 0:
                player.left = wall.right

    vel_y = min(vel_y + GRAVITY, MAX_FALL)
    player.y += vel_y
    on_ground = False
    for wall in walls:
        if player.colliderect(wall):
            if vel_y > 0:
                player.bottom = wall.top
                on_ground = True
            elif vel_y < 0:
                player.top = wall.bottom
            vel_y = 0

    if camera_x < level_width - WIDTH:
        camera_x += 2

    screen.fill((140, 200, 240))
    for wall in walls:
        screen.blit(block_img, (wall.x, wall.y))
    screen.blit(player_img, (player.x, player.y))
    pygame.display.flip()
    clock.tick(60)

pygame.quit()