Well, wouldn't you know it? By changing:
bgsurface = pygame.display.get_surface()
to:
bgsurface = pygame.display.get_surface().copy()
i.e. blitting a copy of the window instead of the window to the window, alpha
transparency works perfectly. I probably should have known, but I didn't
realize
that the window was blitting itself to itself.
But now I have a question: What exactly happens when you blit a surface to
itself? It seems pretty clear that it caused alpha transparency for something
else being blitted to it to not work in this case, but I still don't quite
understand why. Sorry, but I just like to understand why things happen. :)
________________________________
From: Lenard Lindstrom <le...@telus.net>
To: pygame-users@seul.org
Sent: Sun, October 10, 2010 1:46:36 PM
Subject: Re: [pygame] Surface blitting not effective
There are a couple of things happening in this program which make it hard to
know what is happening with the alpha blit. First, the display surface is
blitted to itself:
class Game(object):
....
def __init__(self):
....
self.window = pygame.display.set_mode(self.rect.size)
....
def menu(self, options, cursor, anim_wait, color=pygame.Color(0,0,0,155),
border=16, sep=8):
....
#Create background surface
bgsurface = pygame.display.get_surface()
....
#Begin menu loop
while 1:
....
#Draw
self.window.blit(bgsurface, self.rect)
....
Second, the menu surface is of an unknown format:
#Create menu surface
dlgsurface = pygame.Surface((xsize,ysize)).convert()
dlgsurface.fill(color)
dlgsurface.set_alpha(color.a)
Surface.convert() makes dlgsurface have the same properties as the display. If
the display has per-pixel-alpha (Is that possible?) then so will dlgsurface.
Blanket alpha, as set by Surface.set_alpha(), is ignored in this case. Better
to
create the menu surface with a known format:
dlgsurface = pygame.Surface((xsize, ysize), 0, 24) # no per-pixel alpha
Lenard Lindstrom
On 09/10/10 08:31 PM, B W wrote:
> Your blits were copying the source to positions outside the target surface. I
>always suspect this if I can't see my graphics. You can insert statements like
>"dlgsurface.get_rect().collidepoint(choicerect.topleft)" or "print
>dlgsurface.get_rect().colliderect(choicerect)" to tell whether the source rect
>intersects the target rect.
>
> I has a sneaking suspicion about the alpha problem and it paid off. Someone
> who
>understands the internals of color alphas and surface alphas would have to
>explain the why. :)
>
> #Create menu surface
> dlgsurface = pygame.Surface((xsize,ysize)).convert()
> ##change
> # dlgsurface.fill(color)
> dlgsurface.fill(*color[3:])
> dlgsurface.set_alpha(color.a)
> dlgrect = dlgsurface.get_rect(center=self.rect.center)
> ##new
> drawrect = dlgsurface.get_rect()
>
> cursor_rect = []
> ypos = border
>
> for option in options:
> ##change
> # choicerect = option.get_rect(centerx=dlgrect.centerx, \
> # top=dlgrect.top + ypos)
> choicerect = option.get_rect(centerx=drawrect.centerx, \
> top=drawrect.top + ypos)
> dlgsurface.blit(option, choicerect)
> ##change
> #
> cursor_rect.append(cursor[0].get_rect(centery=choicerect.centery,
>\
> # left = dlgrect.left+border))
> cursor_rect.append(cursor[0].get_rect(
> centery=dlgrect.top+choicerect.centery,
> left=dlgrect.left+border))
> ypos += option.get_height() + sep
>
