Re: [pygame] Pixel alpha blends tutorial

[Shandy Brown <sjbrown@xxxxxxxxx>]

Thanks!

This couldn't have come at a better time, as I was just about to search
the mailing list for a way to fade in / out some PNG images that have an
alpha channel.

-sjbrown


On Sat, 25 Mar 2006, James Hofmann wrote:

> # A pixel-alpha sprite tutorial for pygame
> # by James Hofmann 2006
> # public-domain distributable
>
> # This tutorial was made for myself to see how I could
> use alpha-transparent
> # sprites for blending, e.g. between tiles on a map. I
> don't see similar tuts
> # focused on this use, so I cleaned it up a little and
> added comments.
> # There are probably many ways to improve on this.
>
> # To use this tutorial:
> # 1. Make sure pygame and Numeric are installed, and
> that pygame.Image
> # supports the "extended" formats. (if not, PIL can
> also be used to load images)
> # 2. Create an image called "edge.tga" with GIMP or
> Photoshop to these specs:
> # (I use GIMP)
> # a. Format: Resolution 256x16, 32-bit color.
> # b. Colored neither black nor white.
> # c. Some part of the image should use pixel
> alpha(this is easily done with
> # GIMP's eraser tool)
> # d. the .tga should be saved without compression.
>
> import pygame
> import Numeric
> from pygame.locals import *
>
> pygame.init()
> pygame.display.set_mode((320,200))
>
> ### Make the surfaces
>
> screen = pygame.display.get_surface()
>
> gradient = pygame.Surface((320,200)) # gradient is a
> little background to blit over
>
> for y in range(200):
>     pygame.draw.line(gradient, (y,0,0), (0,y),(320,y))
>
> orig = pygame.image.load("edge.tga")
> alphaarray = pygame.surfarray.array_alpha(orig) # try
> printing alphaarray
> newcol = pygame.Surface((256,16), SRCALPHA, 32) # you
> must force the surface to 32bit
> newcol.fill((255,255,255)) # we are giving the surface
> "new color" for the demo
>
> screen.blit(orig, (0,0))
>
> screen.blit(newcol, (0,16))
>
> newalpha = pygame.surfarray.pixels_alpha(newcol) #
> pixels_alpha gives us a ref
> for n in range(len(newalpha)):
>     newalpha[n] = alphaarray[n] # and so we simply
> replace each referenced pixel
> del newalpha # and then clean up (this unlocks the
> surface)
>
> # main issue with this implementation: it expects
> same-dimension surfaces.
> # best workaround is to always work with subsurfs the
> same size as the alpha.
>
> screen.blit(newcol, (0,32))
>
> pygame.display.update()
>
> ### Two demos, when running them press spacebar to
> continue:
> # demo 1 shows first the original edge.tga image,
> followed by the white-filled
> # "newcol" surface. Finally, it makes newcol use the
> alpha of edge.tga using
> # surfarray.
> #
> # demo 2 is a performance-sanity test: it recreates
> newcol many times over
> # a gradient background. Simply blitting newcol is
> very fast; it is the
> # blending that is the bottleneck.
>
> cont = 1
> clock = pygame.time.Clock()
>
> while cont>0:
>     for e in pygame.event.get():
>         if e.type==KEYDOWN:
>             if e.key==K_SPACE:
>                 cont = 0
>     clock.tick(30)
>
> cont = 1
>
> while cont>0:
>     cont+=1
>     for e in pygame.event.get():
>         if e.type==KEYDOWN:
>             if e.key==K_SPACE:
>                 cont = 0
>     screen.blit(gradient, (0,0))
>     for y in range(20):
>         newcol.fill((0,int((y/20.00)*255),0))
>         newalpha =
> pygame.surfarray.pixels_alpha(newcol)
>         for n in range(len(newalpha)):
>             newalpha[n] = alphaarray[n]
>         del newalpha
>         screen.blit(newcol, (cont,y*16))
>     pygame.display.update()
>     if cont>320:
>         cont=1
>     clock.tick(120)
>
>
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