Erase part of image on Tkinter canvas, exposing another image underneath

Question

(Python 2.7). I have a Tkinter canvas with two images that are both the height and width of the canvas, so they cover the whole window. One image is on top of the other. I want to, using the mouse, be able to erase part of the top image wherever I want, thus exposing the bottom image. Is this possible?

I'm curious in how to implement the Home.erase method below which is bound to a Tkinter motion event.

# -*- coding: utf-8 -*-

import io
from PIL import Image, ImageTk
import Tkinter as tk

#Image 2 is on top of image 1.
IMAGE1_DIR = "C:/path_to_image/image1.png"
IMAGE2_DIR = "C:/path_to_image/image2.png"

def create_image(filename, width=0, height=0):
    """
    Returns a PIL.Image object from filename - sized
    according to width and height parameters.

    filename: str.
    width: int, desired image width.
    height: int, desired image height.

    1) If neither width nor height is given, image will be returned as is.
    2) If both width and height are given, image will resized accordingly.
    3) If only width or only height is given, image will be scaled so specified
    parameter is satisfied while keeping image's original aspect ratio the same. 
    """
    with open(filename, "rb") as f:
        fh = io.BytesIO(f.read())

    #Create a PIL image from the data
    img = Image.open(fh, mode="r")

    #Resize if necessary.
    if not width and not height:
        return img
    elif width and height:
        return img.resize((int(width), int(height)), Image.ANTIALIAS)
    else:  #Keep aspect ratio.
        w, h = img.size
        scale = width/float(w) if width else height/float(h)
        return img.resize((int(w*scale), int(h*scale)), Image.ANTIALIAS)


class Home(object):
    """
    master: tk.Tk window.
    screen: tuple, (width, height).
    """
    def __init__(self, master, screen):
        self.screen = w, h = screen
        self.master = master

        self.frame = tk.Frame(self.master)
        self.frame.pack()
        self.can = tk.Canvas(self.frame, width=w, height=h)
        self.can.pack()

        #Photos will be as large as the screen.
        p1 = ImageTk.PhotoImage(image=create_image(IMAGE1_DIR, w, h))
        p2 = ImageTk.PhotoImage(image=create_image(IMAGE2_DIR, w, h))

        ## Place photos in center of screen.
        ## Create label to retain a reference to image so it doesn't dissapear.

        self.photo1 = self.can.create_image((w//2, h//2), image=p1)        
        label1 = tk.Label(image=p1)
        label1.image = p1

        #On top.
        self.photo2 = self.can.create_image((w//2, h//2), image=p2)
        label2 = tk.Label(image=p2)
        label2.image = p2

        #Key bindings.
        self.master.bind("<Return>", self.reset)
        self.master.bind("<Motion>", self.erase)

    #### Key Bindings ####
    def reset(self, event):
        """ Enter/Return key. """
        self.frame.destroy()
        self.__init__(self.master, self.screen)

    def erase(self, event):
        """
        Mouse motion binding.
        Erase part of top image (self.photo2) at location (event.x, event.y),
        consequently exposing part of the bottom image (self.photo1).
        """
        pass


def main(screen=(500, 500)):
    root = tk.Tk()
    root.resizable(0, 0)
    Home(root, screen)

    #Place window in center of screen.
    root.eval('tk::PlaceWindow %s center'%root.winfo_pathname(root.winfo_id()))

    root.mainloop()


if __name__ == '__main__':
    main()

Answer 1

Thanks to @martineau for the suggestions! Here is the working code.

from PIL import Image, ImageTk
import Tkinter as tk

#Image 2 is on top of image 1.
IMAGE1_DIR = "C:/path/image1.PNG"
IMAGE2_DIR = "C:/path/image2.PNG"

#Brush size in pixels.
BRUSH = 5
#Actual size is 2*BRUSH

def create_image(filename, width=0, height=0):
    """
    Returns a PIL.Image object from filename - sized
    according to width and height parameters.

    filename: str.
    width: int, desired image width.
    height: int, desired image height.

    1) If neither width nor height is given, image will be returned as is.
    2) If both width and height are given, image will resized accordingly.
    3) If only width or only height is given, image will be scaled so specified
    parameter is satisfied while keeping image's original aspect ratio the same. 
    """
    #Create a PIL image from the file.
    img = Image.open(filename, mode="r")

    #Resize if necessary.
    if not width and not height:
        return img
    elif width and height:
        return img.resize((int(width), int(height)), Image.ANTIALIAS)
    else:  #Keep aspect ratio.
        w, h = img.size
        scale = width/float(w) if width else height/float(h)
        return img.resize((int(w*scale), int(h*scale)), Image.ANTIALIAS)


class Home(object):
    """
    master: tk.Tk window.
    screen: tuple, (width, height).
    """
    def __init__(self, master, screen):
        self.screen = w, h = screen
        self.master = master

        self.frame = tk.Frame(self.master)
        self.frame.pack()
        self.can = tk.Canvas(self.frame, width=w, height=h)
        self.can.pack()

        self.image1 = create_image(IMAGE1_DIR, w, h)
        self.image2 = create_image(IMAGE2_DIR, w, h)        

        #Center of screen.
        self.center = w//2, h//2
        #Start with no photo on the screen.
        self.photo = False

        #Draw photo on screen.
        self.draw()

        #Key bindings.
        self.master.bind("<Return>", self.reset)
        self.master.bind("<Motion>", self.erase)

    def draw(self):
        """
        If there is a photo on the canvas, destroy it.
        Draw self.image2 on the canvas.
        """            
        if self.photo:
            self.can.delete(self.photo)
            self.label.destroy()

        p = ImageTk.PhotoImage(image=self.image2)
        self.photo = self.can.create_image(self.center, image=p)
        self.label = tk.Label(image=p)
        self.label.image = p

    #### Key Bindings ####
    def reset(self, event):
        """ Enter/Return key. """
        self.frame.destroy()
        self.__init__(self.master, self.screen)

    def erase(self, event):
        """
        Mouse motion binding.
        Erase part of top image (self.photo2) at location (event.x, event.y),
        consequently exposing part of the bottom image (self.photo1).
        """        
        for x in xrange(event.x-BRUSH, event.x+BRUSH+1):
            for y in xrange(event.y-BRUSH, event.y+BRUSH+1):
                try:
                    p = self.image1.getpixel((x, y))
                    self.image2.putpixel((x, y), p)
                except IndexError:
                    pass

        self.draw()



def main(screen=(500, 500)):
    root = tk.Tk()
    root.resizable(0, 0)
    Home(root, screen)

    #Place window in center of screen.
    root.eval('tk::PlaceWindow %s center'%root.winfo_pathname(root.winfo_id()))

    root.mainloop()


if __name__ == '__main__':
    main()