Key-Press Widget Placement

Demonstrates the on_key callback API: press a key while the plot is focused to add an overlay widget centred on the current cursor position, or press Backspace / Delete to remove the last widget you clicked.

Key bindings

Key	Action
q	Add a rectangle
w	Add a circle
e	Add an annulus
Backspace (macOS ⌫) Delete (Windows / Linux)	Remove last-clicked

Built-in 2-D shortcuts (not overridden in this example):

Key	Action
r	Reset zoom / pan
c	Toggle colorbar
l	Toggle log scale
s	Toggle symlog scale

The cursor coordinates reported in the event (event.img_x, event.img_y) are in image-pixel space, so widgets are centred exactly where the cursor was when the key was pressed.

Note

Move the mouse over the image first so the plot panel receives focus, then press a key. On macOS the backspace key (⌫) is used for deletion; on Windows / Linux use the Delete key.

import numpy as np
import anyplotlib as vw

# ── Synthetic test image ──────────────────────────────────────────────────────
rng = np.random.default_rng(0)
N = 256
x = np.linspace(0, 4 * np.pi, N)
XX, YY = np.meshgrid(x, x)
data = np.sin(XX) * np.cos(YY) + 0.15 * rng.standard_normal((N, N))

# ── Figure ────────────────────────────────────────────────────────────────────
fig, ax = vw.subplots(figsize=(520, 520))
plot = ax.imshow(data)

# ── Key handlers ─────────────────────────────────────────────────────────────

@plot.on_key('q')
def add_rectangle(event):
    """Press 'q' — add a rectangle centred on the cursor."""
    cx, cy = event.img_x, event.img_y
    half_w, half_h = N * 0.08, N * 0.08
    plot.add_widget(
        "rectangle",
        x=cx - half_w, y=cy - half_h,
        w=half_w * 2,  h=half_h * 2,
        color="#ffd54f",
    )


@plot.on_key('w')
def add_circle(event):
    """Press 'w' — add a circle centred on the cursor."""
    plot.add_widget(
        "circle",
        cx=event.img_x, cy=event.img_y,
        r=N * 0.07,
        color="#80cbc4",
    )


@plot.on_key('e')
def add_annulus(event):
    """Press 'e' — add an annulus centred on the cursor."""
    plot.add_widget(
        "annular",
        cx=event.img_x, cy=event.img_y,
        r_outer=N * 0.12,
        r_inner=N * 0.06,
        color="#ce93d8",
    )


# macOS sends 'Backspace' for the ⌫ key; Windows/Linux send 'Delete'.
# Register both so the example works cross-platform.
@plot.on_key('Backspace')
@plot.on_key('Delete')
def delete_last(event):
    """Press Backspace/Delete — remove the last widget that was clicked."""
    wid = event.last_widget_id
    if wid and wid in {w.id for w in plot.list_widgets()}:
        plot.remove_widget(wid)


# ── Catch-all handler (optional) — log every registered key press ─────────────

@plot.on_key
def log_key(event):
    img_x = getattr(event, 'img_x', None)
    img_y = getattr(event, 'img_y', None)
    pos = f"({img_x:.1f}, {img_y:.1f})" if img_x is not None else "n/a"
    print(f"[on_key] key={event.key!r}  img={pos}"
          f"  last_widget={event.last_widget_id!r}")

fig