Re: [pygame] Surface blitting not effective

[Lenard Lindstrom <len-l@xxxxxxxxx>]

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