[pygame] Pixel alpha blends tutorial

[James Hofmann <jwhinfinity@xxxxxxxxx>]

# 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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