Move and zoom a tkinter canvas with mouse

Question

Here a description of what I would like : Draw a collection of geometric objects (here, rectangles) in a tkinter canvas, and beeing abble to explore this canvas using mouse. Click and drag move the canvas, scrolling zooms in and zooms out.

Using this topic, I found the click and drag part : Move a tkinter canvas with Mouse with-mouse

I managed to write something for scrolling zoom. Both moving and zooming work well separatly.

The problem : If I move and then zoom in, the focus of the zoom is not anymore the position of the cursor.

Any suggestion ?

Here a piece of code to test

[edit : should now works for linux and windows]

import Tkinter as tk
import random

class Example(tk.Frame):
    def __init__(self, root):
        tk.Frame.__init__(self, root)
        self.canvas = tk.Canvas(self, width=400, height=400, background="bisque")
        self.xsb = tk.Scrollbar(self, orient="horizontal", command=self.canvas.xview)
        self.ysb = tk.Scrollbar(self, orient="vertical", command=self.canvas.yview)
        self.canvas.configure(yscrollcommand=self.ysb.set, xscrollcommand=self.xsb.set)
        self.canvas.configure(scrollregion=(0,0,1000,1000))

        self.xsb.grid(row=1, column=0, sticky="ew")
        self.ysb.grid(row=0, column=1, sticky="ns")
        self.canvas.grid(row=0, column=0, sticky="nsew")
        self.grid_rowconfigure(0, weight=1)
        self.grid_columnconfigure(0, weight=1)

        #Plot some rectangles
        for n in range(50):
            x0 = random.randint(0, 900)
            y0 = random.randint(50, 900)
            x1 = x0 + random.randint(50, 100)
            y1 = y0 + random.randint(50,100)
            color = ("red", "orange", "yellow", "green", "blue")[random.randint(0,4)]
            self.canvas.create_rectangle(x0,y0,x1,y1, outline="black", fill=color, activefill="black", tags=n)
        self.canvas.create_text(50,10, anchor="nw", text="Click and drag to move the canvas\nScroll to zoom.")

        # This is what enables using the mouse:
        self.canvas.bind("<ButtonPress-1>", self.move_start)
        self.canvas.bind("<B1-Motion>", self.move_move)
        #linux scroll
        self.canvas.bind("<Button-4>", self.zoomerP)
        self.canvas.bind("<Button-5>", self.zoomerM)
        #windows scroll
        self.canvas.bind("<MouseWheel>",self.zoomer)

    #move
    def move_start(self, event):
        self.canvas.scan_mark(event.x, event.y)
    def move_move(self, event):
        self.canvas.scan_dragto(event.x, event.y, gain=1)

    #windows zoom
    def zoomer(self,event):
        if (event.delta > 0):
            self.canvas.scale("all", event.x, event.y, 1.1, 1.1)
        elif (event.delta < 0):
            self.canvas.scale("all", event.x, event.y, 0.9, 0.9)
        self.canvas.configure(scrollregion = self.canvas.bbox("all"))

    #linux zoom
    def zoomerP(self,event):
        self.canvas.scale("all", event.x, event.y, 1.1, 1.1)
        self.canvas.configure(scrollregion = self.canvas.bbox("all"))
    def zoomerM(self,event):
        self.canvas.scale("all", event.x, event.y, 0.9, 0.9)
        self.canvas.configure(scrollregion = self.canvas.bbox("all"))

if __name__ == "__main__":
    root = tk.Tk()
    Example(root).pack(fill="both", expand=True)
    root.mainloop()

Answer 1

This is a simplified zoom example. You should use more advanced techniques to not cram the memory with a huge resized image for the large zooms.

Do not forget to place a path to your image at the end of the script.

P.S. For the advanced zoom example look here.

# -*- coding: utf-8 -*-
# WARNING: This is a simplified zoom example.
# You should use more advanced techniques to not cram the memory
# with a huge resized image for the large zooms.
import random
import tkinter as tk
from tkinter import ttk
from PIL import Image, ImageTk

class AutoScrollbar(ttk.Scrollbar):
    ''' A scrollbar that hides itself if it's not needed.
        Works only if you use the grid geometry manager '''
    def set(self, lo, hi):
        if float(lo) <= 0.0 and float(hi) >= 1.0:
            self.grid_remove()
        else:
            self.grid()
        ttk.Scrollbar.set(self, lo, hi)

    def pack(self, **kw):
        raise tk.TclError('Cannot use pack with this widget')

    def place(self, **kw):
        raise tk.TclError('Cannot use place with this widget')

class Zoom(ttk.Frame):
    ''' Simple zoom with mouse wheel '''
    def __init__(self, mainframe, path):
        ''' Initialize the main Frame '''
        ttk.Frame.__init__(self, master=mainframe)
        self.master.title('Simple zoom with mouse wheel')
        # Vertical and horizontal scrollbars for canvas
        vbar = AutoScrollbar(self.master, orient='vertical')
        hbar = AutoScrollbar(self.master, orient='horizontal')
        vbar.grid(row=0, column=1, sticky='ns')
        hbar.grid(row=1, column=0, sticky='we')
        # Open image
        self.image = Image.open(path)
        # Create canvas and put image on it
        self.canvas = tk.Canvas(self.master, highlightthickness=0,
                                xscrollcommand=hbar.set, yscrollcommand=vbar.set)
        self.canvas.grid(row=0, column=0, sticky='nswe')
        vbar.configure(command=self.canvas.yview)  # bind scrollbars to the canvas
        hbar.configure(command=self.canvas.xview)
        # Make the canvas expandable
        self.master.rowconfigure(0, weight=1)
        self.master.columnconfigure(0, weight=1)
        # Bind events to the Canvas
        self.canvas.bind('<ButtonPress-1>', self.move_from)
        self.canvas.bind('<B1-Motion>',     self.move_to)
        self.canvas.bind('<MouseWheel>', self.wheel)  # with Windows and MacOS, but not Linux
        self.canvas.bind('<Button-5>',   self.wheel)  # only with Linux, wheel scroll down
        self.canvas.bind('<Button-4>',   self.wheel)  # only with Linux, wheel scroll up
        # Show image and plot some random test rectangles on the canvas
        self.imscale = 1.0
        self.imageid = None
        self.delta = 0.75
        width, height = self.image.size
        minsize, maxsize = 5, 20
        for n in range(10):
            x0 = random.randint(0, width - maxsize)
            y0 = random.randint(0, height - maxsize)
            x1 = x0 + random.randint(minsize, maxsize)
            y1 = y0 + random.randint(minsize, maxsize)
            color = ('red', 'orange', 'yellow', 'green', 'blue')[random.randint(0, 4)]
            self.canvas.create_rectangle(x0, y0, x1, y1, outline='black', fill=color,
                                         activefill='black', tags=n)
        # Text is used to set proper coordinates to the image. You can make it invisible.
        self.text = self.canvas.create_text(0, 0, anchor='nw', text='Scroll to zoom')
        self.show_image()
        self.canvas.configure(scrollregion=self.canvas.bbox('all'))

    def move_from(self, event):
        ''' Remember previous coordinates for scrolling with the mouse '''
        self.canvas.scan_mark(event.x, event.y)

    def move_to(self, event):
        ''' Drag (move) canvas to the new position '''
        self.canvas.scan_dragto(event.x, event.y, gain=1)

    def wheel(self, event):
        ''' Zoom with mouse wheel '''
        scale = 1.0
        # Respond to Linux (event.num) or Windows (event.delta) wheel event
        if event.num == 5 or event.delta == -120:
            scale        *= self.delta
            self.imscale *= self.delta
        if event.num == 4 or event.delta == 120:
            scale        /= self.delta
            self.imscale /= self.delta
        # Rescale all canvas objects
        x = self.canvas.canvasx(event.x)
        y = self.canvas.canvasy(event.y)
        self.canvas.scale('all', x, y, scale, scale)
        self.show_image()
        self.canvas.configure(scrollregion=self.canvas.bbox('all'))

    def show_image(self):
        ''' Show image on the Canvas '''
        if self.imageid:
            self.canvas.delete(self.imageid)
            self.imageid = None
            self.canvas.imagetk = None  # delete previous image from the canvas
        width, height = self.image.size
        new_size = int(self.imscale * width), int(self.imscale * height)
        imagetk = ImageTk.PhotoImage(self.image.resize(new_size))
        # Use self.text object to set proper coordinates
        self.imageid = self.canvas.create_image(self.canvas.coords(self.text),
                                                anchor='nw', image=imagetk)
        self.canvas.lower(self.imageid)  # set it into background
        self.canvas.imagetk = imagetk  # keep an extra reference to prevent garbage-collection

path = 'doge2.jpg'  # place path to your image here
root = tk.Tk()
app = Zoom(root, path=path)
root.mainloop()

Answer 2

Mouse events are reported in 'screen coordinates'. When you have a scrolled canvas, you often need to convert those numbers to 'canvas (ie. scrollregion) coordinates'.

eg. for your zoom focus:

true_x = canvas.canvasx(event.x)
true_y = canvas.canvasy(event.y)