Re: [pygame] issue with pygame.rect.inflate_ip()

[Kevin Secretan <kevinmccloud@xxxxxxxxx>]

Hey David. Good catch, I was a bit too hasty in asserting a strict 
equality. Just goes to show the problem with truncating instead of 
rounding or using rationals. Scaling a line from 0->10 down to length
 5 results in chopping off 5 from the width, then adding 2.5 (which gets
 truncated to 2) to recenter. The new line then is from 2->7 with a 
center at (2+7)/2 = 4.5 = 4, but it should really be 2.5->7.5 with a 
center at (2.5+7.5)/2 = 5.

The original code is still correct in 
most respects, though perhaps the documentation should be changed. You 
can work around the issue by storing the original center and reassigning
 it after the inflation or by using a data structure with floats or by 
using your own inflate function that rounds.

The devs could also 
change the code such that the division by 2 will round properly but 
there are reasons why this might not be desirable. One relatively minor 
reason is that the change requires a branch for negative numbers to 
round properly. x/2 for x>=0 turns into (x+1)/2 == x + (1-x)/2 to 
avoid overflow. For x < 0, it turns into (x-1)/2 == x - (1+x)/2.

On Sun, Feb 5, 2012 at 2:54 PM, david BERTRAND <dvd.brtrnd@xxxxxxxxx> wrote:

Hi kevin, hi Lenard,

here is my try :

import pygame

pygame.init()

class Box(pygame.sprite.Sprite): Â ÂÂÂ Â Â Â

  def __init__(self):

    pygame.sprite.Sprite.__init__(self)

    self.image = pygame.Surface((10, 10))

    self.rect = self.image.get_rect()

b = Box() Â

print "initial b.rect : %s   b.rect.center : %s" % (b.rect, b.rect.center)

b.rect.inflate_ip(5, 5)

print "inflated b.rect : %s   b.rect.center : %s\n" % (b.rect, b.rect.center)

c = Box() Â

print "initial c.rect : %s   c.rect.center : %s" % (c.rect, c.rect.center)

c.rect.inflate_ip(-5, -5)

print "inflated c.rect : %s   c.rect.center : %s" % (c.rect, c.rect.center)

# outputs :

#

# initial b.rect : <rect(0, 0, 10, 10)> Â Â b.rect.center : (5, 5)

# inflated b.rect : <rect(-2, -2, 15, 15)> Â Â b.rect.center : (5, 5)

#

# initial c.rect : <rect(0, 0, 10, 10)> Â Â c.rect.center : (5, 5)

# inflated c.rect : <rect(2, 2, 5, 5)> Â Â c.rect.center : (4, 4)

I have just said it is different from what is espected ((-2, -2), (13, 13) and (5, 5) or (0, 0), (15, 15) and (7, 7)) for b.rect and b.rect center

And he results are worse with negatives.