import glob
from PIL import Image

#Find all gifs which have an underscore in the middle and save the prefix names
prefixes = set()
for filename in glob.glob('*.gif'):
    name, _ = filename.split('.')
    if '_' in name:
        prefix, _ = name.split('_')
        prefixes.add(prefix)


#The first element of the tuple is the position suffix
#The second one is another tuple containing the column 
#and the row to which each position be pasted.
#Example: fr1 (front 1) will be pasted into first row, first column
#lf2 (left 2) will be pasted to column 1 row 3 (the fourth row)
positions = [("fr1", (0, 0)), 
    ("fr2", (1, 0)) , 
    ("rt1", (0, 1)), 
    ("rt2", (1, 1)), 
    ("bk1", (0, 2)), 
    ("bk2", (1, 2)), 
    ("lf1", (0, 3)),
    ("lf2", (1, 3))]

#For each prefix, create a new image with alpha channel
for prefix in prefixes:
    sprite = Image.new('RGBA', (64, 128))
    for position in positions:
        suffix, coord = position
        src_img = prefix + "_" + suffix + ".gif"
        img = Image.open(src_img)
        sprite.paste(img, (coord[0] * 32, coord[1] * 32))

        #Change alpha value of white pixels to 0
        #Snippet found at:
        #https://stackoverflow.com/questions/765736/how-to-use-pil-to-make-all-white-pixels-transparent
        pixdata = sprite.load()
        width, height = sprite.size
        for y in range(height):
            for x in range(width):
                if pixdata[x, y] == (255, 255, 255, 255):
                    pixdata[x, y] = (255, 255, 255, 0)
    #save sprite
    sprite.save(prefix + ".png", "PNG")