Add scrolling to a platformer in pygame

You need to apply an offset to the position of your entities when drawing them. Let's call that offset a camera, since this is the effect we want to achieve with this.

First of all, we can't use the draw function of the sprite group, since the sprites don't need to know that their position (rect) is not the position they are going to be drawn on the screen (At the end, we'll subclass the Group class and reimplement the it's draw to be aware of the camera, but let's start slow).

Let's start by creating a Camera class to hold the state of the offset we want to apply to the position of our entities:

class Camera(object):    def __init__(self, camera_func, width, height):        self.camera_func = camera_func        self.state = Rect(0, 0, width, height)            def apply(self, target):        return target.rect.move(self.state.topleft)            def update(self, target):        self.state = self.camera_func(self.state, target.rect)

some things to note here:

We need to store the position of the camera, and the width and height of the level in pixels (since we want to stop scrolling at the edges of the level). I used a Rect to store all these informations, but you could easily just use some fields.

Using Rect comes in handy in the apply function. This is where we re-calculate the position of an entity on the screen to apply the scrolling.

Once per iteration of the main loop, we need to update the position of the camera, hence there's the update function. It just alters the state by calling the camera_func function, which will do all the hard work for us. We implement it later.

Let's create an instace of the camera:

for row in level:    ...total_level_width  = len(level[0])*32 # calculate size of level in pixelstotal_level_height = len(level)*32    # maybe make 32 an constantcamera = Camera(*to_be_implemented*, total_level_width, total_level_height)entities.add(player)... 

and alter our main loop:

# draw backgroundfor y in range(32):    ...camera.update(player) # camera follows player. Note that we could also follow any other sprite# update player, draw everything elseplayer.update(up, down, left, right, running, platforms)for e in entities:    # apply the offset to each entity.    # call this for everything that should scroll,    # which is basically everything other than GUI/HUD/UI    screen.blit(e.image, camera.apply(e)) pygame.display.update()

Our camera class is already very flexible and yet dead simple. It can use different kinds of scrolling (by providing different camera_func functions), and it can follow any arbitary sprite, not just the player. You even can change this at runtime.

Now for the implementation of camera_func. A simple approach is to just center the player (or whichever entity we want to follow) at the screen, and the implementation is straight forward:

def simple_camera(camera, target_rect):    l, t, _, _ = target_rect # l = left,  t = top    _, _, w, h = camera      # w = width, h = height    return Rect(-l+HALF_WIDTH, -t+HALF_HEIGHT, w, h)

We just take the position of our target, and add the half total screen size. You can try it by creating your camera like this:

camera = Camera(simple_camera, total_level_width, total_level_height)

So far, so good. But maybe we don't want to see the black background outside the level? How about:

def complex_camera(camera, target_rect):    # we want to center target_rect    x = -target_rect.center[0] + WIN_WIDTH/2     y = -target_rect.center[1] + WIN_HEIGHT/2    # move the camera. Let's use some vectors so we can easily substract/multiply    camera.topleft += (pygame.Vector2((x, y)) - pygame.Vector2(camera.topleft)) * 0.06 # add some smoothness coolnes    # set max/min x/y so we don't see stuff outside the world    camera.x = max(-(camera.width-WIN_WIDTH), min(0, camera.x))    camera.y = max(-(camera.height-WIN_HEIGHT), min(0, camera.y))        return camera

Here we simply use the min/max functions to ensure we don't scroll outside out level.

Try it by creating your camera like this:

camera = Camera(complex_camera, total_level_width, total_level_height)

There's a little animation of our final scrolling in action:

Here's the complete code again. Note I changed some things:

• the level is bigger and to have some more platforms
• use python 3
• use a sprite group to handle the camera
• refactored some duplicate code
• since Vector2/3 is now stable, use them for easier math
• get rid of that ugly event handling code and use pygame.key.get_pressed instead

 #! /usr/bin/pythonimport pygamefrom pygame import *SCREEN_SIZE = pygame.Rect((0, 0, 800, 640))TILE_SIZE = 32 GRAVITY = pygame.Vector2((0, 0.3))class CameraAwareLayeredUpdates(pygame.sprite.LayeredUpdates):    def __init__(self, target, world_size):        super().__init__()        self.target = target        self.cam = pygame.Vector2(0, 0)        self.world_size = world_size        if self.target:            self.add(target)    def update(self, *args):        super().update(*args)        if self.target:            x = -self.target.rect.center[0] + SCREEN_SIZE.width/2            y = -self.target.rect.center[1] + SCREEN_SIZE.height/2            self.cam += (pygame.Vector2((x, y)) - self.cam) * 0.05            self.cam.x = max(-(self.world_size.width-SCREEN_SIZE.width), min(0, self.cam.x))            self.cam.y = max(-(self.world_size.height-SCREEN_SIZE.height), min(0, self.cam.y))    def draw(self, surface):        spritedict = self.spritedict        surface_blit = surface.blit        dirty = self.lostsprites        self.lostsprites = []        dirty_append = dirty.append        init_rect = self._init_rect        for spr in self.sprites():            rec = spritedict[spr]            newrect = surface_blit(spr.image, spr.rect.move(self.cam))            if rec is init_rect:                dirty_append(newrect)            else:                if newrect.colliderect(rec):                    dirty_append(newrect.union(rec))                else:                    dirty_append(newrect)                    dirty_append(rec)            spritedict[spr] = newrect        return dirty                        def main():    pygame.init()    screen = pygame.display.set_mode(SCREEN_SIZE.size)    pygame.display.set_caption("Use arrows to move!")    timer = pygame.time.Clock()    level = [        "PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP",        "P                                          P",        "P                                          P",        "P                                          P",        "P                    PPPPPPPPPPP           P",        "P                                          P",        "P                                          P",        "P                                          P",        "P    PPPPPPPP                              P",        "P                                          P",        "P                          PPPPPPP         P",        "P                 PPPPPP                   P",        "P                                          P",        "P         PPPPPPP                          P",        "P                                          P",        "P                     PPPPPP               P",        "P                                          P",        "P   PPPPPPPPPPP                            P",        "P                                          P",        "P                 PPPPPPPPPPP              P",        "P                                          P",        "P                                          P",        "P                                          P",        "P                                          P",        "PPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPP",]        platforms = pygame.sprite.Group()    player = Player(platforms, (TILE_SIZE, TILE_SIZE))    level_width  = len(level[0])*TILE_SIZE    level_height = len(level)*TILE_SIZE    entities = CameraAwareLayeredUpdates(player, pygame.Rect(0, 0, level_width, level_height))        # build the level    x = y = 0    for row in level:        for col in row:            if col == "P":                Platform((x, y), platforms, entities)            if col == "E":                ExitBlock((x, y), platforms, entities)            x += TILE_SIZE        y += TILE_SIZE        x = 0        while 1:        for e in pygame.event.get():            if e.type == QUIT:                 return            if e.type == KEYDOWN and e.key == K_ESCAPE:                return        entities.update()        screen.fill((0, 0, 0))        entities.draw(screen)        pygame.display.update()        timer.tick(60)class Entity(pygame.sprite.Sprite):    def __init__(self, color, pos, *groups):        super().__init__(*groups)        self.image = Surface((TILE_SIZE, TILE_SIZE))        self.image.fill(color)        self.rect = self.image.get_rect(topleft=pos)class Player(Entity):    def __init__(self, platforms, pos, *groups):        super().__init__(Color("#0000FF"), pos)        self.vel = pygame.Vector2((0, 0))        self.onGround = False        self.platforms = platforms        self.speed = 8        self.jump_strength = 10            def update(self):        pressed = pygame.key.get_pressed()        up = pressed[K_UP]        left = pressed[K_LEFT]        right = pressed[K_RIGHT]        running = pressed[K_SPACE]                if up:            # only jump if on the ground            if self.onGround: self.vel.y = -self.jump_strength        if left:            self.vel.x = -self.speed        if right:            self.vel.x = self.speed        if running:            self.vel.x *= 1.5        if not self.onGround:            # only accelerate with gravity if in the air            self.vel += GRAVITY            # max falling speed            if self.vel.y > 100: self.vel.y = 100        print(self.vel.y)        if not(left or right):            self.vel.x = 0        # increment in x direction        self.rect.left += self.vel.x        # do x-axis collisions        self.collide(self.vel.x, 0, self.platforms)        # increment in y direction        self.rect.top += self.vel.y        # assuming we're in the air        self.onGround = False;        # do y-axis collisions        self.collide(0, self.vel.y, self.platforms)    def collide(self, xvel, yvel, platforms):        for p in platforms:            if pygame.sprite.collide_rect(self, p):                if isinstance(p, ExitBlock):                    pygame.event.post(pygame.event.Event(QUIT))                if xvel > 0:                    self.rect.right = p.rect.left                if xvel < 0:                    self.rect.left = p.rect.right                if yvel > 0:                    self.rect.bottom = p.rect.top                    self.onGround = True                    self.vel.y = 0                if yvel < 0:                    self.rect.top = p.rect.bottomclass Platform(Entity):    def __init__(self, pos, *groups):        super().__init__(Color("#DDDDDD"), pos, *groups)class ExitBlock(Entity):    def __init__(self, pos, *groups):        super().__init__(Color("#0033FF"), pos, *groups)if __name__ == "__main__":    main()

Since right know, you have a static background, and the player that you control, is blitted in the position he is in, you have 2 options to always show the character in the middle.

1. If you map is small enought, you can have a big img A, and derive a rectangle, based on the position of the player that will be the size of the screen. That way, the player will always be in the middle. A Rect.clamp(Rect) or Rect.clamp_ip(Rect) will aid you in that.

2. Another approach is to have a different tuple for position on screen. The player will have a constant value in the center of the screen, while the backgrounds position will be the negative of the player position.

The only way to do that is to separate logical positions in the map, from physical positions on the screen .

Any code related to actually drawing your map on the screen - in your case all the .rect attributes of your sprites - have to do so based on an offset of what part of yor map the screen is actually using.

For example, your screen might be showing your map starting with position (10,10) on the top left - all display related code them (which in the case above are the .rectattributes) should subtract the screen offset from the current logical position - (say the character is at map coords(12,15) - so, it should be drawn at (12,15) - (10, 10) -> (2, 5) * BLOCK_SIZE) In your example above BLOCK_SIZE is hardcoded to 32,32, so you want to draw it at physical pixel position (2 * 32, 5 * 32) on the display)

(hint: avoid hardcoding things this way, make it a constant declaration at the beginning of your code)