Use Python+PIL to add multiple shapes transparency to a given PNG with antialiasing

pil-png-transparency.py

# -*- encoding: UTF-8 -*-

import os
import math

from PIL import Image, ImageDraw

def crop(source, out, top_angle, bottom_angle, crop_height=None):
    im = Image.open(source)
    im = im.convert("RGBA")
    width, height = im.size

    if crop_height and crop_height < height:
        offset = (height - crop_height) / 2
        im = im.crop((0, offset, width, height-offset))

    width, height = im.size

    def get_y_from_angle(angle):
        ''' return y position from give angle '''
        y = width / math.tan((90-angle)*math.pi/180)
        return y

    def get_positions_from_angle(angle, position):
        ''' compute shape coordinates based on angle+vertical position '''
        y = get_y_from_angle(angle)
        if position == 'top':
            y_left = y if y>0 else 0
            y_right = abs(y) if y<0 else 0
            positions = [
                (0, 0),
                (width, 0),
                (width, y_right),
                (0, y_left),
            ]
        else:
            y_left = height - (abs(y) if y<0 else 0)
            y_right = height - (y if y>0 else 0)
            positions = [
                (width, height),
                (0, height),
                (0, y_left),
                (width, y_right),
            ]
        return positions

    def removeShapes(image, shapes):
        ''' remove a list of shapes from the image and give them transparency '''
        alpha_mask = Image.new('L', (width*2, height*2), 255)
        alpha_mask_draw = ImageDraw.Draw(alpha_mask)
        for shape in shapes:
            shape = [(x*2, y*2) for x, y in shape]
            alpha_mask_draw.polygon(shape, fill=0, outline=0)
        alpha_mask = alpha_mask.resize(image.size, Image.ANTIALIAS)
        image.putalpha(alpha_mask)
        return image

    output = removeShapes(im, shapes= [
        get_positions_from_angle(top_angle, 'top'),
        get_positions_from_angle(bottom_angle, 'bottom')
    ])
    output.save(out, 'png')

    print '[+] created {0}'.format(out)


if __name__=='__main__':
    ROOT_PATH = os.path.abspath(os.path.dirname(__file__))
    source = os.path.join(ROOT_PATH, 'PAGE-JEU.png')
    out = os.path.join(ROOT_PATH, 'out.png')
    crop(source, out, 2, -2, crop_height=1000)