How to get coordinates on scrollable canvas in Tkinter?

The canvas widget in Tkinter has two coordinate systems: window coordinates and canvas coordinates. Window coordinates always start from (0,0) at the top-left corner of the visible window, while canvas coordinates represent the actual position of items within the entire canvas area, including scrolled regions.

When working with scrollable canvases, mouse events return window coordinates, but you often need canvas coordinates to place or interact with canvas items correctly.

Converting Window to Canvas Coordinates

Tkinter provides two methods to convert window coordinates to canvas coordinates:

canvas.canvasx(event.x)  # Convert x coordinate
canvas.canvasy(event.y)  # Convert y coordinate

These methods are essential when your canvas is scrollable, as they account for the current scroll position.

Example: Getting Mouse Coordinates

This example demonstrates how to capture and convert mouse coordinates on a canvas:

import tkinter as tk

# Create the main window
root = tk.Tk()
root.title("Canvas Coordinates Example")
root.geometry("600x400")

# Create a canvas with scrollbars
canvas = tk.Canvas(root, bg="white", scrollregion=(0, 0, 1000, 1000))
canvas.pack(fill=tk.BOTH, expand=True)

# Add some visual elements to the canvas
for i in range(10):
    for j in range(10):
        x = i * 100 + 50
        y = j * 100 + 50
        canvas.create_rectangle(x-20, y-20, x+20, y+20, 
                              fill="lightblue", outline="blue")
        canvas.create_text(x, y, text=f"({x},{y})")

def on_click(event):
    # Get window coordinates
    window_x = event.x
    window_y = event.y
    
    # Convert to canvas coordinates
    canvas_x = canvas.canvasx(window_x)
    canvas_y = canvas.canvasy(window_y)
    
    print(f"Window coords: ({window_x}, {window_y})")
    print(f"Canvas coords: ({canvas_x}, {canvas_y})")
    print("-" * 30)

def on_drag(event):
    canvas_x = canvas.canvasx(event.x)
    canvas_y = canvas.canvasy(event.y)
    print(f"Dragging at canvas: ({canvas_x:.1f}, {canvas_y:.1f})")

# Bind mouse events
canvas.bind("<Button-1>", on_click)
canvas.bind("<B1-Motion>", on_drag)

# Add instruction label
label = tk.Label(root, text="Click and drag on the canvas to see coordinates")
label.pack()

root.mainloop()

Window coords: (150, 100)
Canvas coords: (150.0, 100.0)
------------------------------
Dragging at canvas: (175.0, 120.0)
Dragging at canvas: (200.0, 140.0)

Scrollable Canvas Example

Here's a more comprehensive example with actual scrollbars to demonstrate the coordinate conversion:

import tkinter as tk

root = tk.Tk()
root.title("Scrollable Canvas Coordinates")
root.geometry("400x300")

# Create frame for canvas and scrollbars
frame = tk.Frame(root)
frame.pack(fill=tk.BOTH, expand=True)

# Create canvas
canvas = tk.Canvas(frame, bg="lightyellow", scrollregion=(0, 0, 800, 600))

# Create scrollbars
h_scrollbar = tk.Scrollbar(frame, orient=tk.HORIZONTAL, command=canvas.xview)
v_scrollbar = tk.Scrollbar(frame, orient=tk.VERTICAL, command=canvas.yview)

# Configure canvas scrolling
canvas.configure(xscrollcommand=h_scrollbar.set, yscrollcommand=v_scrollbar.set)

# Pack scrollbars and canvas
h_scrollbar.pack(side=tk.BOTTOM, fill=tk.X)
v_scrollbar.pack(side=tk.RIGHT, fill=tk.Y)
canvas.pack(side=tk.LEFT, fill=tk.BOTH, expand=True)

# Draw a grid pattern
for i in range(0, 800, 50):
    canvas.create_line(i, 0, i, 600, fill="gray", width=1)
for i in range(0, 600, 50):
    canvas.create_line(0, i, 800, i, fill="gray", width=1)

# Add coordinate labels at grid intersections
for x in range(0, 800, 100):
    for y in range(0, 600, 100):
        canvas.create_text(x+5, y+5, text=f"({x},{y})", anchor="nw", fill="red")

def show_coordinates(event):
    canvas_x = canvas.canvasx(event.x)
    canvas_y = canvas.canvasy(event.y)
    
    # Create or update coordinate display
    canvas.delete("coord_marker")
    canvas.create_oval(canvas_x-3, canvas_y-3, canvas_x+3, canvas_y+3, 
                      fill="red", tags="coord_marker")
    
    print(f"Canvas coordinates: ({canvas_x:.1f}, {canvas_y:.1f})")

canvas.bind("<Button-1>", show_coordinates)

# Instructions
tk.Label(root, text="Scroll around and click to see canvas coordinates").pack()

root.mainloop()

Key Points

• canvasx() and canvasy() account for scroll position automatically
• Window coordinates change as you scroll, but canvas coordinates remain consistent
• These methods return float values for precise positioning
• Essential for accurate item placement on scrollable canvases

Conclusion

Use canvasx() and canvasy() to convert mouse coordinates from window space to canvas space. This ensures accurate positioning when working with scrollable canvases, allowing you to place items at the correct locations regardless of scroll position.