Puzzled by my sprite's unequal +/ - velocity - pygame

Question

I have two sprites in my game. The zombie sprite works perfectly, moving in all directions at a velocity of 1.0. My player sprite however, despite moves more slowly in the positive x/y direction, despite all values in all directions being 2.25.
For the life of me I can't seem to see what is wrong here.
Full working code:
import pygame
import random
import sys
import itertools
import math
import time
from datetime import datetime
from librarymodified import *
from pygame.locals import *
# prints text using the supplied font
def print_text(font, x, y, text, color=(255,255,255)):
imgText = font.render(text, True, color)
DISPLAYSURF.blit(imgText, (x,y))
# MySprite class extends pygame.sprite.Sprite
class MySprite(pygame.sprite.Sprite):
def __init__(self, target):
pygame.sprite.Sprite.__init__(self) #extend the base Sprite class
self.master_image = None
self.frame = 0
self.old_frame = -1
self.frame_width = 1
self.frame_height = 1
self.first_frame = 0
self.last_frame = 0
self.columns = 1
self.last_time = 0
self.direction = 0
self.times_hit = 0
self.direction = 0
self.velocity = Point(0.0,0.0)
# times_hit property
def _get_times_hit(self): return self.times_hit
def _set_times_hit(self, hits): self.times_hit += hits
number_hits_taken = property(_get_times_hit, _set_times_hit)
#X property
def _getx(self): return self.rect.x
def _setx(self,value): self.rect.x = value
X = property(_getx,_setx)
#Y property
def _gety(self): return self.rect.y
def _sety(self,value): self.rect.y = value
Y = property(_gety,_sety)
#position property
def _getpos(self): return self.rect.topleft
def _setpos(self,pos): self.rect.topleft = pos
position = property(_getpos,_setpos)
def load(self, filename, width, height, columns):
self.master_image = pygame.image.load(filename).convert_alpha()
self.frame_width = width
self.frame_height = height
self.rect = Rect(0,0,width,height)
self.columns = columns
#try to auto-calculate total frames
rect = self.master_image.get_rect()
self.last_frame = (rect.width // width) * (rect.height // height) - 1
def update(self, current_time, rate=30):
#update animation frame number
if current_time > self.last_time + rate:
self.frame += 1
if self.frame > self.last_frame:
self.frame = self.first_frame
self.last_time = current_time
#build current frame only if it changed
if self.frame != self.old_frame:
frame_x = (self.frame % self.columns) * self.frame_width
frame_y = (self.frame // self.columns) * self.frame_height
rect = Rect(frame_x, frame_y, self.frame_width, self.frame_height)
self.image = self.master_image.subsurface(rect)
self.old_frame = self.frame
def __str__(self):
return str(self.frame) + "," + str(self.first_frame) + \
"," + str(self.last_frame) + "," + str(self.frame_width) + \
"," + str(self.frame_height) + "," + str(self.columns) + \
"," + str(self.rect)
#Point class
class Point(object):
def __init__(self, x, y):
self.__x = x
self.__y = y
#X property
def getx(self): return self.__x
def setx(self, x): self.__x = x
x = property(getx, setx)
#Y property
def gety(self): return self.__y
def sety(self, y): self.__y = y
y = property(gety, sety)
def __str__(self):
return "{X:" + "{:.0f}".format(self.__x) + \
",Y:" + "{:.0f}".format(self.__y) + "}"
def calc_velocity(direction, vel = 1.0):
velocity = Point(0, 0)
if direction == 0: # North
velocity.y = -vel
elif direction == 2: # East
velocity.x = vel
elif direction == 4: # south
velocity.y = vel
elif direction == 6: # west
velocity.x = -vel
return velocity
def reverse_direction(sprite):
if sprite.direction == 0:
sprite.direction = 4
elif sprite.direction == 2:
sprite.direction = 6
elif sprite.direction == 4:
sprite.direction = 0
elif sprite.direction == 6:
sprite.direction = 2
# main
pygame.init()
DISPLAYSURF = pygame.display.set_mode((800,600))
pygame.display.set_caption("Collision Detection")
font = pygame.font.SysFont(None, 36)
fpsclock = pygame.time.Clock()
fps = 30
# create sprite groups
zombie_group = pygame.sprite.Group()
player_group = pygame.sprite.Group()
health_group = pygame.sprite.Group()
# create player sprite
player = MySprite(DISPLAYSURF)
player.load("farmer walk.png", 96, 96, 8)
player.position = (80,80)
player.direction = 4
player_group.add(player)
# create zombie sprite
zombie_image = pygame.image.load("zombie walk.png").convert_alpha()
for i in range(1):
zombie = MySprite(DISPLAYSURF)
zombie.load("zombie walk.png", 96, 96, 8)
zombie.position = (random.randint(0, 700), random.randint(0, 500))
zombie.direction = random.randint(0,3) * 2
zombie_group.add(zombie)
# create health sprite
health = MySprite(DISPLAYSURF)
health.load("health.png", 32, 32, 1)
health.position = (400, 300)
health_group.add(health)
game_over = False
player_moving = False
player_health = 100
# colors
BLACK = (0, 0, 0)
WHITE = (255, 255, 255)
RED = (255, 0, 0)
GREEN = (0, 255, 0)
BLUE = (0, 0, 0)
YELLOW = (255, 255, 0)
##DISPLAYSURF.fill(BLACK)
##pygame.mouse.set_visible(True)
# event loop
while True:
ticks = pygame.time.get_ticks() # ms since pygame.init() called
for event in pygame.event.get():
if event.type == QUIT:
pygame.quit()
sys.exit()
if event.type == MOUSEMOTION:
mousex, mousey = event.pos
# keyboard polling
keys = pygame.key.get_pressed()
if keys[K_ESCAPE]:
pygame.quit()
sys.exit()
elif keys[K_UP] or keys[K_w]:
player.direction = 0
player_moving = True
elif keys[K_RIGHT] or keys[K_d]:
player.direction = 2
player_moving = True
elif keys[K_LEFT] or keys[K_a]:
player.direction = 6
player_moving = True
elif keys[K_DOWN] or keys[K_s]:
player.direction = 4
player_moving = True
else:
player_moving = False
# these things should not happen if game is over
if not game_over:
# update player sprite
player_group.update(ticks, 50)
# use player direction to calculate frame range
player.first_frame = player.direction * player.columns
player.last_frame = player.first_frame + player.columns-1
if player.frame < player.first_frame:
player.frame = player.first_frame
if not player_moving:
# stop animating when player is not moving
player.frame = player.first_frame = player.last_frame
else:
# move player in that direction
player.velocity = calc_velocity(player.direction, 1.5)
player.velocity.x *= 1.5
player.velocity.y *= 1.5
# manually move player
if player_moving:
player.X += player.velocity.x
player.Y += player.velocity.y
if player.X <0: player.X = 0
elif player.X > 700: player.X = 700
if player.Y <0: player.Y = 0
elif player.Y > 500: player.Y = 500
# update zombie sprites
zombie_group.update(ticks, 50)
# manually update zombies
for z in zombie_group:
# set zombie animation range
z.first_frame = z.direction * z.columns
z.last_frame = z.first_frame + z.columns-1
if z.frame < z.first_frame:
z.frame = z.first_frame
z.velocity = calc_velocity(z.direction)
# keep zombie on screen
z.X += z.velocity.x
z.Y += z.velocity.y
if z.X < 0 or z.X > 700 or z.Y < 0 or z.Y > 500:
reverse_direction(z)
# check for sprite collision
attacker = 0
attacker = pygame.sprite.spritecollideany(player, zombie_group)
if attacker != None:
# more precise check
if pygame.sprite.collide_rect_ratio(0.5)(player, attacker):
player_health -= 10
if attacker.X < player.X: attacker.X -= 10
elif attacker.X > player.X: attacker.X += 10
else:
attacker = None
# update health drop
health_group.update(ticks, 50)
# check for collision with health
if pygame.sprite.collide_rect_ratio(0.5)(player, health):
player_health += 30
if player_health >100: player_health = 100
health.X = random.randint(0, 700)
health.Y = random.randint(0, 500)
# is player dead?
if player_health <= 0:
game_over = True
# clear screen
DISPLAYSURF.fill((50,50,100))
# draw sprites
player_group.draw(DISPLAYSURF)
zombie_group.draw(DISPLAYSURF)
health_group.draw(DISPLAYSURF)
# draw energy bar
pygame.draw.rect(DISPLAYSURF, WHITE, (299, 555, 203, 31), 2)
pygame.draw.rect(DISPLAYSURF, GREEN, (301, 557, player_health * 2, 28))
# print zombie and player velocities for purpose of testing
print_text(font, 350, 460, "Zombie X vel: " +\
str(zombie.velocity.x) + "\nY vel: " +\
str(zombie.velocity.y))
print_text(font, 350, 500, "Player X vel: " +\
str(player.velocity.x) + "\nY vel: " +\
str(player.velocity.y))
if game_over:
print_text(font, 300, 200, "G A M E O V E R")
pygame.display.update()
fpsclock.tick(fps)

Answer 1

Problem
The sprite group is drawing your sprite using the sprite's rect attribute. A pygame Rect object can only hold integers, so it'll truncate all floating point numbers.
Let's say you have a x = 5.
If you add 1.1: x += 1.1 <=> x = x + 1.1 <=> x = 5 + 1.1 <=> x = 6.1 which will be truncated to x = 6. It have increased by 1.
If you subtract 1.1: x -= 1.1 <=> x = x - 1.1 <=> x = 5 - 1.1 <=> x = 3.9 which will be truncated to x = 3. It have decreased by 2.
In other words: You'll move faster in the left direction than the right (the same principle applies to negative numbers). Here's an example demonstrating it:
import pygame
pygame.init()
class Player(pygame.sprite.Sprite):
def __init__(self, group):
super(Player, self).__init__(group)
self.image = pygame.Surface((32, 32))
self.rect = self.image.get_rect()
screen = pygame.display.set_mode((100, 100))
group = pygame.sprite.Group()
player = Player(group)
clock = pygame.time.Clock()
while True:
clock.tick(10)
for event in pygame.event.get():
if event.type == pygame.QUIT:
quit()
elif event.type == pygame.KEYDOWN:
if event.key == pygame.K_RIGHT:
x = player.rect.x + 1.1
print("Actual x:", x)
player.rect.x = player.rect.x + 1.1
print("Truncated x:", player.rect.x)
elif event.key == pygame.K_LEFT:
x = player.rect.x - 1.1
print("Actual x:", x)
player.rect.x = player.rect.x - 1.1
print("Truncated x:", player.rect.x)
screen.fill((255, 255, 255))
group.draw(screen)
pygame.display.update()
Solution
Using floating point numbers for position is great; it makes it possible to move a sprite less than a pixel every frame (if your game updates 120 times per second and you want your sprite to move only 30 pixels per second).
However, you have to compensate for the fact that the rect objects cannot hold them. The most straightforward solution is to have an attribute position which keep track of the position of the sprite in floating point precision. Then at every update change the rect to the position of the attribute. Like this:
import pygame
pygame.init()
class Player(pygame.sprite.Sprite):
def __init__(self, group):
super(Player, self).__init__(group)
self.image = pygame.Surface((32, 32))
self.rect = self.image.get_rect()
self.position = self.rect.x # Or whatever point of the rect you want the position to be.
screen = pygame.display.set_mode((100, 100))
group = pygame.sprite.Group()
player = Player(group)
clock = pygame.time.Clock()
while True:
clock.tick(10)
for event in pygame.event.get():
if event.type == pygame.QUIT:
quit()
if event.type == pygame.KEYDOWN:
if event.key == pygame.K_RIGHT:
player.position += 1.1
player.rect.x = player.position
elif event.key == pygame.K_LEFT:
player.position -= 1.1
player.rect.x = player.position
screen.fill((255, 255, 255))
group.draw(screen)
pygame.display.update()
I've only showed how this movement works in the x-axis, but it's exactly the same on the y-axis.

Answer 2

Ok. I think the problem is that the number of pixels that the sprite is moved each update is rounded down, so that 2.25 becomes 2 pixels and -2.25 becomes -3 pixels. Moving by a fractional number of pixels doesn't make sense I think.
If you change lines 229 - 233 to
else:
# move player in that direction
player.velocity = calc_velocity(player.direction, 2.0)
player.velocity.x *= 2.0
player.velocity.y *= 2.0
The velocity is now an integer and there would be no rounding problems. It is faster though. Is there some reason why you don't just have the velocity as an integer instead of a float squared?