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PoC: GUI-native crosshair cursor #30516

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PoC: GUI-native crosshair cursor #30516

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fe0fbd0
PoC: GUI-native crosshair cursor [skip ci]
timhoffm Sep 4, 2025
804b503
Limit crosshair to axes region
timhoffm Sep 5, 2025
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26 changes: 25 additions & 1 deletion lib/matplotlib/backends/backend_qt.py
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Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
from collections import namedtuple
import functools
import os
import sys
Expand Down Expand Up @@ -213,6 +214,9 @@ def _timer_stop(self):
self._timer.stop()


Crosshair = namedtuple('Crosshair', 'x, y, x0, x1, y0, y1')


class FigureCanvasQT(FigureCanvasBase, QtWidgets.QWidget):
required_interactive_framework = "qt"
_timer_cls = TimerQT
Expand All @@ -231,6 +235,7 @@ class FigureCanvasQT(FigureCanvasBase, QtWidgets.QWidget):
def __init__(self, figure=None):
_create_qApp()
super().__init__(figure=figure)
self._crosshair = None

self._draw_pending = False
self._is_drawing = False
Expand Down Expand Up @@ -305,6 +310,21 @@ def mouseEventCoords(self, pos=None):
y = self.figure.bbox.height / self.device_pixel_ratio - pos.y()
return x * self.device_pixel_ratio, y * self.device_pixel_ratio

def _update_crosshair(self, x, y):
previous_crosshair = self._crosshair
ax = self.inaxes((x, y))
if ax is None:
self._crosshair = None
else:
bbox = ax.get_position() # in figure coords
x0 = int(bbox.x0 * self.width())
x1 = int(bbox.x1 * self.width())
y0 = int((1 - bbox.y0) * self.height())
y1 = int((1 - bbox.y1) * self.height())
self._crosshair = Crosshair(x, y, x0, x1, y0, y1)
needs_repaint = previous_crosshair is not None or self._crosshair is not None
return needs_repaint

def enterEvent(self, event):
# Force querying of the modifiers, as the cached modifier state can
# have been invalidated while the window was out of focus.
Expand Down Expand Up @@ -344,8 +364,12 @@ def mouseDoubleClickEvent(self, event):
def mouseMoveEvent(self, event):
if self.figure is None:
return
mouse_xy = self.mouseEventCoords(event)
needs_repaint = self._update_crosshair(*mouse_xy)
if needs_repaint:
self.repaint()
MouseEvent("motion_notify_event", self,
*self.mouseEventCoords(event),
*mouse_xy,
buttons=self._mpl_buttons(event.buttons()),
modifiers=self._mpl_modifiers(),
guiEvent=event)._process()
Expand Down
11 changes: 11 additions & 0 deletions lib/matplotlib/backends/backend_qtagg.py
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Original file line number Diff line number Diff line change
Expand Up @@ -68,6 +68,17 @@ def paintEvent(self, event):
ctypes.c_long.from_address(id(buf)).value = 1

self._draw_rect_callback(painter)

if self._crosshair is not None:
x = self._crosshair.x
y = rect.height() - int(self._crosshair.y)
x0 = rect.left() + self._crosshair.x0
x1 = rect.left() + self._crosshair.x1
y0 = rect.top() + self._crosshair.y0
y1 = rect.top() + self._crosshair.y1
painter.setPen(QtGui.QPen(QtCore.Qt.GlobalColor.red))
painter.drawLine(x0, y, x1, y)
painter.drawLine(x, y0, x, y1)
finally:
painter.end()

Expand Down
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