extract.py

from PIL import Image
import os

white = (255, 255, 255, 255)

# folder with the original image
img_folder = "img/"
# folder where the alpha is saved
imgAlpha_folder = "img/alpha/"


# Create target Directory if don't exist for the alpha folder
if not os.path.exists(imgAlpha_folder):
    os.mkdir(imgAlpha_folder)
    print("Directory " , imgAlpha_folder ,  " Created ")
else:
    print("Directory " , imgAlpha_folder ,  " already exists")


for element in os.listdir(img_folder):
    if element.endswith('.png'):
        # print("'%s' is a png" % element)

        # Open the image and convert it to RGBA, just in case it was indexed
        image = Image.open(img_folder + element).convert('RGBA')

        # print(image.getbands)

        # Extract just the alpha channel
        alpha = image.split()[-1]

        # Unfortunately the alpha channel is still treated as such and can't be dumped
        # as-is

        # Create a new image with an opaque black background
        bg = Image.new("RGBA", image.size, (0,0,0,255))

        # Copy the alpha channel to the new image using itself as the mask
        bg.paste(alpha, mask=alpha)

        # Get the number of colors in the alpha channel
        colors = bg.getcolors()

        # If there is only one color, and it's white ( opaque) then do nothing
        if ( colors[0][1] == white and len(colors) == 1 ):
            print('no alpha channel')
        else: # else save the alpha channel as a new image with the same name + "_alpha"

            # Since the bg image started as RGBA, we can save some space by converting it
            # to grayscale ('L') Optionally, we can convert the image to be indexed which
            # saves some more space ('P') In my experience, converting directly to 'P'
            # produces both the Gray channel and an Alpha channel when viewed in GIMP,
            # althogh the file sizes is about the same

            mask_path = imgAlpha_folder + element.replace('.png', '_alpha.png')

            bg.convert('L').convert('P', palette=Image.ADAPTIVE, colors=8).save(mask_path, optimize=True)