# p18_12.py
import pygame
import sys
from pygame.locals import *

pygame.init()

size = width, height = 640, 480
bg = (0, 0, 0)

clock = pygame.time.Clock()
screen = pygame.display.set_mode(size)
pygame.display.set_caption("FishC Demo")

turtle = pygame.image.load("turtle.png").convert_alpha()
background  = pygame.image.load("background.jpg").convert()
position = turtle.get_rect()
position.center = width // 2, height // 2

def blit_alpha(target, source, location, opacity):
    x = location[0]
    y = location[1]
    temp = pygame.Surface((source.get_width(), source.get_height())).convert()
    temp.blit(target, (-x, -y ))
    temp.blit(source, (0, 0))
    temp.set_alpha(opacity)        
    target.blit(temp, location)

while True:
    for event in pygame.event.get():
        if event.type == QUIT:
            sys.exit()

    screen.blit(background, (0, 0))
    blit_alpha(screen, turtle, position, 200)
    
    pygame.display.flip()
    
    clock.tick(30)
