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 pixels
total_level_height = len(level)*32 # maybe make 32 an constant
camera = Camera(*to_be_implemented*, total_level_width, total_level_height)
entities.add(player)
...
and alter our main loop:
# draw background
for y in range(32):
...
camera.update(player) # camera follows player. Note that we could also follow any other sprite
# update player, draw everything else
player.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_pressedinstead
#! /usr/bin/python
import pygame
from 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.bottom
class 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()