RE: [pygame] Determining per-pixel alpha

["Lee Harr" <missive@hotmail.com>]

> How can I tell if a surface has per-pixel alpha?
>
> I wrote this function to scale an arbitrary surface, but it fails if
> the surface has per-pixel alpha (the background shows up
> TRANSPARENT instead of transparent) with this output:

Hm... I changed my TRANSPARENT color to have an alpha channel
and now it works the way I want... That's good. I am still a bit fuzzy
on the distinctions between colorkey, alpha, and per-pixel alpha
though, if someone feels like explaining something  :o)


import pygame
TRANSPARENT = (0, 51, 0, 0)
def scale_image(img, width, height, keepAspectRatio=1):
   """return a scaled copy of a L{pygame.Surface}

   @param img: Original L{pygame.Surface}.
   @param width: width of returned surface.
   @param height: height of returned surface.
   @param keepAspectRatio: If True, the returned surface will be padded 
with
       transparent borders.

   """

   if keepAspectRatio:
       flags = img.get_flags()
       new_img = pygame.Surface((width, height), flags, img)

       img_w, img_h = img.get_size()
       ratio = float(img_w) / img_h
       new_ratio = float(width) / height

       colorkey = img.get_colorkey()
       if colorkey is None:
           colorkey = TRANSPARENT
       new_img.fill(colorkey)
       new_img.set_colorkey(colorkey)

       alpha = img.get_alpha()
       if alpha is not None:
           new_img.set_alpha(alpha)

       if ratio >= new_ratio:
           new_w = int(width)
           new_h = int(float(new_w) / ratio)
           x = 0
           y = (height - new_h) / 2
       else:
           new_h = int(height)
           new_w = int(float(new_h) * ratio)
           x = (width - new_w) / 2
           y = 0

       scaled_img = pygame.transform.scale(img, (new_w, new_h))
       new_img.blit(scaled_img, (x, y))

   else:
       new_img = pygame.transform.scale(img, (width, height))

   return new_img

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