Problem with Pygame movement acceleration, platformer game
Question:
When i move right using the right key, i accelerate to a max speed. When i release it, i do decelerate to a stop so that is fine. However, when moving left using the left key, and after releasing it, i continue moving at a fixed speed and then come to an abrupt stop after a short while. Any idea what could be wrong with my code?
The original code is from http://programarcadegames.com/python_examples/show_file.php?file=platform_jumper.py
import pygame
# Global constants
# Colors
BLACK = (0, 0, 0)
WHITE = (255, 255, 255)
GREEN = (0, 255, 0)
RED = (255, 0, 0)
BLUE = (0, 0, 255)
# Screen dimensions
SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600
class Player(pygame.sprite.Sprite):
""" This class represents the bar at the bottom that the player
controls. """
# -- Methods
def __init__(self):
""" Constructor function """
# Call the parent's constructor
super().__init__()
# Create an image of the block, and fill it with a color.
# This could also be an image loaded from the disk.
width = 40
height = 60
self.image = pygame.Surface([width, height])
self.image.fill(RED)
# Set a referance to the image rect.
self.rect = self.image.get_rect()
# Set speed vector of player
self.xVel = 0
self.yVel = 0
# List of sprites we can bump against
self.level = None
def update(self):
""" Move the player. """
# Gravity
self.calc_grav()
# Move left/right
# See if we hit anything
block_hit_list = pygame.sprite.spritecollide(self, self.level.platform_list, False)
for block in block_hit_list:
# If we are moving right,
# set our right side to the left side of the item we hit
if self.xVel > 0:
self.rect.right = block.rect.left
elif self.xVel < 0:
# Otherwise if we are moving left, do the opposite.
self.rect.left = block.rect.right
# Move up/down
self.rect.y += self.yVel
# Check and see if we hit anything
block_hit_list = pygame.sprite.spritecollide(self, self.level.platform_list, False)
for block in block_hit_list:
# Reset our position based on the top/bottom of the object.
if self.yVel > 0:
self.rect.bottom = block.rect.top
elif self.yVel < 0:
self.rect.top = block.rect.bottom
# Stop our vertical movement
self.yVel = 0
def calc_grav(self):
""" Calculate effect of gravity. """
if self.yVel == 0:
self.yVel = 1
else:
self.yVel += .35
# See if we are on the ground.
if self.rect.y >= SCREEN_HEIGHT - self.rect.height and self.yVel >= 0:
self.yVel = 0
self.rect.y = SCREEN_HEIGHT - self.rect.height
def jump(self):
""" Called when user hits 'jump' button. """
# move down a bit and see if there is a platform below us.
# Move down 2 pixels because it doesn't work well if we only move down
# 1 when working with a platform moving down.
self.rect.y += 2
platform_hit_list = pygame.sprite.spritecollide(self, self.level.platform_list, False)
self.rect.y -= 2
# If it is ok to jump, set our speed upwards
if len(platform_hit_list) > 0 or self.rect.bottom >= SCREEN_HEIGHT:
self.yVel = -10
class Platform(pygame.sprite.Sprite):
""" Platform the user can jump on """
def __init__(self, width, height):
""" Platform constructor. Assumes constructed with user passing in
an array of 5 numbers like what's defined at the top of this
code. """
super().__init__()
self.image = pygame.Surface([width, height])
self.image.fill(GREEN)
self.rect = self.image.get_rect()
class Level(object):
""" This is a generic super-class used to define a level.
Create a child class for each level with level-specific
info. """
def __init__(self, player):
""" Constructor. Pass in a handle to player. Needed for when moving platforms
collide with the player. """
self.platform_list = pygame.sprite.Group()
self.enemy_list = pygame.sprite.Group()
self.player = player
# Background image
self.background = None
# Update everythign on this level
def update(self):
""" Update everything in this level."""
self.platform_list.update()
self.enemy_list.update()
def draw(self, screen):
""" Draw everything on this level. """
# Draw the background
screen.fill(BLUE)
# Draw all the sprite lists that we have
self.platform_list.draw(screen)
self.enemy_list.draw(screen)
# Create platforms for the level
class Level_01(Level):
""" Definition for level 1. """
def __init__(self, player):
""" Create level 1. """
# Call the parent constructor
Level.__init__(self, player)
# Array with width, height, x, and y of platform
level = [[210, 70, 500, 500],
[210, 70, 200, 400],
[210, 70, 600, 300],
]
# Go through the array above and add platforms
for platform in level:
block = Platform(platform[0], platform[1])
block.rect.x = platform[2]
block.rect.y = platform[3]
block.player = self.player
self.platform_list.add(block)
def main():
""" Main Program """
pygame.init()
# Set the height and width of the screen
size = [SCREEN_WIDTH, SCREEN_HEIGHT]
screen = pygame.display.set_mode(size)
pygame.display.set_caption("Platformer Jumper")
# Create the player
player = Player()
# Create all the levels
level_list = []
level_list.append(Level_01(player))
# Set the current level
current_level_no = 0
current_level = level_list[current_level_no]
active_sprite_list = pygame.sprite.Group()
player.level = current_level
player.rect.x = 340
player.rect.y = SCREEN_HEIGHT - player.rect.height
active_sprite_list.add(player)
accel_x = 0
max_speed = 6
# Loop until the user clicks the close button.
done = False
# Used to manage how fast the screen updates
clock = pygame.time.Clock()
# -------- Main Program Loop -----------
while not done:
player_running = False
for event in pygame.event.get():
if event.type == pygame.QUIT:
done = True
elif event.type == pygame.KEYDOWN:
if event.key == pygame.K_LEFT:
accel_x = -0.5
if event.key == pygame.K_RIGHT:
accel_x = 0.5
if event.key == pygame.K_SPACE:
player.jump()
elif event.type == pygame.KEYUP:
if event.key in (pygame.K_LEFT, pygame.K_RIGHT):
accel_x = 0
player.xVel += accel_x # Accelerate.
if abs(player.xVel) >= max_speed: # If max_speed is exceeded.
# Normalize the x_change and multiply it with the max_speed.
player.xVel = player.xVel / abs(player.xVel) * max_speed
# Decelerate if no key is pressed.
if accel_x == 0:
player.xVel *= 0.5
player.rect.x += player.xVel
# Update the player.
active_sprite_list.update()
# Update items in the level
current_level.update()
# If the player gets near the right side, shift the world left (-x)
if player.rect.right > SCREEN_WIDTH:
player.rect.right = SCREEN_WIDTH
# If the player gets near the left side, shift the world right (+x)
if player.rect.left < 0:
player.rect.left = 0
# ALL CODE TO DRAW SHOULD GO BELOW THIS COMMENT
current_level.draw(screen)
active_sprite_list.draw(screen)
# ALL CODE TO DRAW SHOULD GO ABOVE THIS COMMENT
# Limit to 60 frames per second
clock.tick(60)
# Go ahead and update the screen with what we've drawn.
pygame.display.flip()
# Be IDLE friendly. If you forget this line, the program will 'hang'
# on exit.
pygame.quit()
if __name__ == "__main__":
main()
Answers:
The isssue is caused, because the pygame.Rect operates with integral data:
The coordinates for Rect objects are all integers. […]
When you do
player.rect.x += player.xVel
it is the same as you would do:
player.rect.x = int(player.rect.x + player.xVel)
The fraction part of player.xVel gets lost. The result of the addition operation is truncated and the player tends to the coordinate with the lower value (left).
Add a floating point x coordinate (self.px) to the class Player and use it to calculate the position of the player. Use round to set the integral rectangle position from self.px:
class Player(pygame.sprite.Sprite):
def __init__(self):
# [...]
# Set a referance to the image rect.
self.rect = self.image.get_rect()
self.px = self.rect.x
# [...]
def update(self):
# [...]
block_hit_list = pygame.sprite.spritecollide(self, self.level.platform_list, False)
block_hit_list = pygame.sprite.spritecollide(self, self.level.platform_list, False)
for block in block_hit_list:
# If we are moving right,
# set our right side to the left side of the item we hit
if self.xVel > 0:
self.rect.right = block.rect.left
elif self.xVel < 0:
# Otherwise if we are moving left, do the opposite.
self.rect.left = block.rect.right
self.px = self.rect.x
def main():
# [...]
player.rect.x = 340
player.px = player.rect.x
# [...]
while not done:
# [...]
player.px += player.xVel
player.rect.x = round(player.px)
# [...]
When i move right using the right key, i accelerate to a max speed. When i release it, i do decelerate to a stop so that is fine. However, when moving left using the left key, and after releasing it, i continue moving at a fixed speed and then come to an abrupt stop after a short while. Any idea what could be wrong with my code?
The original code is from http://programarcadegames.com/python_examples/show_file.php?file=platform_jumper.py
import pygame
# Global constants
# Colors
BLACK = (0, 0, 0)
WHITE = (255, 255, 255)
GREEN = (0, 255, 0)
RED = (255, 0, 0)
BLUE = (0, 0, 255)
# Screen dimensions
SCREEN_WIDTH = 800
SCREEN_HEIGHT = 600
class Player(pygame.sprite.Sprite):
""" This class represents the bar at the bottom that the player
controls. """
# -- Methods
def __init__(self):
""" Constructor function """
# Call the parent's constructor
super().__init__()
# Create an image of the block, and fill it with a color.
# This could also be an image loaded from the disk.
width = 40
height = 60
self.image = pygame.Surface([width, height])
self.image.fill(RED)
# Set a referance to the image rect.
self.rect = self.image.get_rect()
# Set speed vector of player
self.xVel = 0
self.yVel = 0
# List of sprites we can bump against
self.level = None
def update(self):
""" Move the player. """
# Gravity
self.calc_grav()
# Move left/right
# See if we hit anything
block_hit_list = pygame.sprite.spritecollide(self, self.level.platform_list, False)
for block in block_hit_list:
# If we are moving right,
# set our right side to the left side of the item we hit
if self.xVel > 0:
self.rect.right = block.rect.left
elif self.xVel < 0:
# Otherwise if we are moving left, do the opposite.
self.rect.left = block.rect.right
# Move up/down
self.rect.y += self.yVel
# Check and see if we hit anything
block_hit_list = pygame.sprite.spritecollide(self, self.level.platform_list, False)
for block in block_hit_list:
# Reset our position based on the top/bottom of the object.
if self.yVel > 0:
self.rect.bottom = block.rect.top
elif self.yVel < 0:
self.rect.top = block.rect.bottom
# Stop our vertical movement
self.yVel = 0
def calc_grav(self):
""" Calculate effect of gravity. """
if self.yVel == 0:
self.yVel = 1
else:
self.yVel += .35
# See if we are on the ground.
if self.rect.y >= SCREEN_HEIGHT - self.rect.height and self.yVel >= 0:
self.yVel = 0
self.rect.y = SCREEN_HEIGHT - self.rect.height
def jump(self):
""" Called when user hits 'jump' button. """
# move down a bit and see if there is a platform below us.
# Move down 2 pixels because it doesn't work well if we only move down
# 1 when working with a platform moving down.
self.rect.y += 2
platform_hit_list = pygame.sprite.spritecollide(self, self.level.platform_list, False)
self.rect.y -= 2
# If it is ok to jump, set our speed upwards
if len(platform_hit_list) > 0 or self.rect.bottom >= SCREEN_HEIGHT:
self.yVel = -10
class Platform(pygame.sprite.Sprite):
""" Platform the user can jump on """
def __init__(self, width, height):
""" Platform constructor. Assumes constructed with user passing in
an array of 5 numbers like what's defined at the top of this
code. """
super().__init__()
self.image = pygame.Surface([width, height])
self.image.fill(GREEN)
self.rect = self.image.get_rect()
class Level(object):
""" This is a generic super-class used to define a level.
Create a child class for each level with level-specific
info. """
def __init__(self, player):
""" Constructor. Pass in a handle to player. Needed for when moving platforms
collide with the player. """
self.platform_list = pygame.sprite.Group()
self.enemy_list = pygame.sprite.Group()
self.player = player
# Background image
self.background = None
# Update everythign on this level
def update(self):
""" Update everything in this level."""
self.platform_list.update()
self.enemy_list.update()
def draw(self, screen):
""" Draw everything on this level. """
# Draw the background
screen.fill(BLUE)
# Draw all the sprite lists that we have
self.platform_list.draw(screen)
self.enemy_list.draw(screen)
# Create platforms for the level
class Level_01(Level):
""" Definition for level 1. """
def __init__(self, player):
""" Create level 1. """
# Call the parent constructor
Level.__init__(self, player)
# Array with width, height, x, and y of platform
level = [[210, 70, 500, 500],
[210, 70, 200, 400],
[210, 70, 600, 300],
]
# Go through the array above and add platforms
for platform in level:
block = Platform(platform[0], platform[1])
block.rect.x = platform[2]
block.rect.y = platform[3]
block.player = self.player
self.platform_list.add(block)
def main():
""" Main Program """
pygame.init()
# Set the height and width of the screen
size = [SCREEN_WIDTH, SCREEN_HEIGHT]
screen = pygame.display.set_mode(size)
pygame.display.set_caption("Platformer Jumper")
# Create the player
player = Player()
# Create all the levels
level_list = []
level_list.append(Level_01(player))
# Set the current level
current_level_no = 0
current_level = level_list[current_level_no]
active_sprite_list = pygame.sprite.Group()
player.level = current_level
player.rect.x = 340
player.rect.y = SCREEN_HEIGHT - player.rect.height
active_sprite_list.add(player)
accel_x = 0
max_speed = 6
# Loop until the user clicks the close button.
done = False
# Used to manage how fast the screen updates
clock = pygame.time.Clock()
# -------- Main Program Loop -----------
while not done:
player_running = False
for event in pygame.event.get():
if event.type == pygame.QUIT:
done = True
elif event.type == pygame.KEYDOWN:
if event.key == pygame.K_LEFT:
accel_x = -0.5
if event.key == pygame.K_RIGHT:
accel_x = 0.5
if event.key == pygame.K_SPACE:
player.jump()
elif event.type == pygame.KEYUP:
if event.key in (pygame.K_LEFT, pygame.K_RIGHT):
accel_x = 0
player.xVel += accel_x # Accelerate.
if abs(player.xVel) >= max_speed: # If max_speed is exceeded.
# Normalize the x_change and multiply it with the max_speed.
player.xVel = player.xVel / abs(player.xVel) * max_speed
# Decelerate if no key is pressed.
if accel_x == 0:
player.xVel *= 0.5
player.rect.x += player.xVel
# Update the player.
active_sprite_list.update()
# Update items in the level
current_level.update()
# If the player gets near the right side, shift the world left (-x)
if player.rect.right > SCREEN_WIDTH:
player.rect.right = SCREEN_WIDTH
# If the player gets near the left side, shift the world right (+x)
if player.rect.left < 0:
player.rect.left = 0
# ALL CODE TO DRAW SHOULD GO BELOW THIS COMMENT
current_level.draw(screen)
active_sprite_list.draw(screen)
# ALL CODE TO DRAW SHOULD GO ABOVE THIS COMMENT
# Limit to 60 frames per second
clock.tick(60)
# Go ahead and update the screen with what we've drawn.
pygame.display.flip()
# Be IDLE friendly. If you forget this line, the program will 'hang'
# on exit.
pygame.quit()
if __name__ == "__main__":
main()
The isssue is caused, because the pygame.Rect operates with integral data:
The coordinates for Rect objects are all integers. […]
When you do
player.rect.x += player.xVel
it is the same as you would do:
player.rect.x = int(player.rect.x + player.xVel)
The fraction part of player.xVel gets lost. The result of the addition operation is truncated and the player tends to the coordinate with the lower value (left).
Add a floating point x coordinate (self.px) to the class Player and use it to calculate the position of the player. Use round to set the integral rectangle position from self.px:
class Player(pygame.sprite.Sprite):
def __init__(self):
# [...]
# Set a referance to the image rect.
self.rect = self.image.get_rect()
self.px = self.rect.x
# [...]
def update(self):
# [...]
block_hit_list = pygame.sprite.spritecollide(self, self.level.platform_list, False)
block_hit_list = pygame.sprite.spritecollide(self, self.level.platform_list, False)
for block in block_hit_list:
# If we are moving right,
# set our right side to the left side of the item we hit
if self.xVel > 0:
self.rect.right = block.rect.left
elif self.xVel < 0:
# Otherwise if we are moving left, do the opposite.
self.rect.left = block.rect.right
self.px = self.rect.x
def main():
# [...]
player.rect.x = 340
player.px = player.rect.x
# [...]
while not done:
# [...]
player.px += player.xVel
player.rect.x = round(player.px)
# [...]