How to make a tkinter canvas polygon transparent?

Question

Is there a better way to make the fill of a polygon transparent, rather than using stipple? Heres an example:

import tkinter as tk


class GUI:
    def __init__(self, master, x, y):
        self.master = master
        self.canvas = tk.Canvas(master, width=x, height=y)
        self.canvas.pack()
        self.canvas.create_polygon(10, 10, 10, 20, 200, 300, 250, 150, 10, 10,
        outline="green", fill="blue")
        self.canvas.create_polygon(100, 10, 10, 40, 50, 300, 250, 400, 100, 10,
        outline="green", fill="red", stipple="gray50")


x, y = 500, 500
root = tk.Tk()
gui = GUI(root, x, y)
root.mainloop()

I would like to make the transparency of the red polygon realistic, like any software given the alpha parameter.

Answer 1

My solution is inspired by this answer on a similar question but for rectangles, not polygons.

Unfortunately, Tkinter doesn't support RGBA, so it's impossible just to pass fill args fill="#ff000055". Instead, we can create an image which contains the rectangle and has RGBA channel using PIL.

Here is an example:

from tkinter import *
from PIL import Image, ImageDraw, ImageTk


def create_polygon(*args, **kwargs):
    if "alpha" in kwargs:         
        if "fill" in kwargs:
            # Get and process the input data
            fill = root.winfo_rgb(kwargs.pop("fill"))\
                   + (int(kwargs.pop("alpha") * 255),)
            outline = kwargs.pop("outline") if "outline" in kwargs else None

            # We need to find a rectangle the polygon is inscribed in
            # (max(args[::2]), max(args[1::2])) are x and y of the bottom right point of this rectangle
            # and they also are the width and height of it respectively (the image will be inserted into
            # (0, 0) coords for simplicity)
            image = Image.new("RGBA", (max(args[::2]), max(args[1::2])))
            ImageDraw.Draw(image).polygon(args, fill=fill, outline=outline)

            images.append(ImageTk.PhotoImage(image))  # prevent the Image from being garbage-collected
            return canvas.create_image(0, 0, image=images[-1], anchor="nw")  # insert the Image to the 0, 0 coords
        raise ValueError("fill color must be specified!")
    return canvas.create_polygon(*args, **kwargs)

images = []  # to hold the newly created image(s)        

root = Tk()

canvas = Canvas(width=260, height=310)
canvas.pack()

create_polygon(10, 10, 10, 20, 200, 300, 250, 150, 10, 10, fill="blue", alpha=0.5)
create_polygon(150, 100, 200, 120, 240, 180, 210, 200, 150, 150, 100, 200, fill="blue", alpha=0.2)

root.mainloop()