feat: hover HUD with pixel coords and grey scale bar on sample camera

Bottom-right corner shows the hovered pixel coordinates with a grey
1-2-5 scale bar above them, derived from geom.pixel_in_mm and the
current zoom. Works in viewing mode (before the interaction gate),
clears on leave, and skips drawing outside the image.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/aare/gui/styles.py

@@ -460,6 +460,7 @@ PATH_END = "#d20f39"  # raster path gradient end + end circle (red)
 LEGEND_BG = "#eff1f5"  # base
 LEGEND_TEXT = "#4c4f69"  # text
 TOOLTIP_TEXT = "#4c4f69"  # camera coords tooltip pen — NOT the QToolTip popup
+SCALE_BAR_GREY = "#8c8fa1"  # hover HUD scale bar (Latte overlay1 grey)
 MARK_TOOLTIP_GOLD = "#df8e1d"  # yellow
 MARK_TOOLTIP_ORANGE = "#fe640b"  # peach
 MARK_TOOLTIP_RED = "#d20f39"  # red

src/aare/gui/widgets/camera_image.py

@@ -56,6 +56,7 @@ from aare.gui.styles import (
     MARKER_GREEN,
     PATH_END,
     PATH_START,
+    SCALE_BAR_GREY,
     SHADOW,
     TARGET_COLORS,
     THEME_SUNSET,
@@ -207,6 +208,9 @@ class SampleCameraImageLabel(QGraphicsView):
         self._detections = []  # list of dicts from publisher
         self._det_shape = None  # shape from payload [h,w] so we can scale
 
+        # Scene position under the cursor; drives the corner coords/scale HUD.
+        self._hover_pos: QPointF | None = None
+
     def _update_camera_interaction_feedback(self) -> None:
         if self._camera_available:
             self.viewport().setCursor(Qt.CursorShape.ArrowCursor)
@@ -482,6 +486,7 @@ class SampleCameraImageLabel(QGraphicsView):
         self._draw_detections(painter, rect)
         self._draw_target_point(painter)
         self._draw_overlay_legend(painter)
+        self._draw_hover_hud(painter)
         self._draw_help_overlay(painter)
 
     def resizeEvent(self, event):
@@ -577,9 +582,17 @@ class SampleCameraImageLabel(QGraphicsView):
         if self._session_badge_hovered:
             self._session_badge_hovered = False
             self.update()
+        if self._hover_pos is not None:
+            self._hover_pos = None
+            self.update()
         super().leaveEvent(event)
 
     def mouseMoveEvent(self, event):
+        # Hover HUD before the interaction gate: watching is free, so the
+        # coords/scale readout must work even without the session baton.
+        self._hover_pos = self.mapToScene(event.pos())
+        self.update()
+
         # Badge hover feedback must run BEFORE the interaction gate: the
         # badge is visible precisely when interaction is disabled.
         hovered = self._session_badge_rect is not None and self._session_badge_rect.contains(
@@ -1254,6 +1267,67 @@ class SampleCameraImageLabel(QGraphicsView):
 
         painter.restore()
 
+    @staticmethod
+    def _scale_bar(um_per_view_px: float) -> tuple[float, str]:
+        """Nice 1-2-5 scale-bar length (µm) for the current zoom, with label."""
+        target_um = um_per_view_px * 120.0  # aim for a ~120 px wide bar
+        exponent = math.floor(math.log10(target_um))
+        base = target_um / 10.0**exponent
+        nice = 5.0 if base >= 5.0 else 2.0 if base >= 2.0 else 1.0
+        bar_um = nice * 10.0**exponent
+        label = f"{bar_um / 1000.0:g} mm" if bar_um >= 1000.0 else f"{bar_um:g} µm"
+        return bar_um, label
+
+    def _draw_hover_hud(self, painter: QPainter):
+        # Bottom-right HUD: grey scale bar over the hovered pixel coordinates.
+        if self._hover_pos is None or self.pixmap_item is None:
+            return
+        if not self.pixmap_item.sceneBoundingRect().contains(self._hover_pos):
+            return
+
+        painter.save()
+        painter.resetTransform()
+        painter.setRenderHint(QPainter.RenderHint.Antialiasing, True)
+
+        font = QFont()
+        font.setPointSize(10)
+        font.setBold(True)
+        painter.setFont(font)
+        fm = QFontMetrics(font)
+
+        margin = 18
+        right = self.viewport().width() - margin
+        coords_text = f"{self._hover_pos.x():.0f}, {self._hover_pos.y():.0f} pxl"
+        coords_baseline = self.viewport().height() - margin - fm.descent()
+        grey = qcolor(SCALE_BAR_GREY)
+
+        def shadowed_text(x: float, baseline: float, text: str, color: QColor):
+            painter.setPen(QPen(qcolor(SHADOW, 200)))
+            painter.drawText(QPointF(x + 1, baseline + 1), text)
+            painter.setPen(QPen(color))
+            painter.drawText(QPointF(x, baseline), text)
+
+        shadowed_text(
+            right - fm.horizontalAdvance(coords_text), coords_baseline, coords_text, qcolor(WHITE)
+        )
+
+        # view-pixel scale: scene px -> viewport px via the current zoom.
+        zoom = self.transform().m11()
+        if zoom > 0:
+            um_per_view_px = self._geom.pixel_in_mm * 1000.0 / zoom
+            bar_um, label = self._scale_bar(um_per_view_px)
+            bar_px = bar_um / um_per_view_px
+            bar_y = coords_baseline - fm.ascent() - 12
+            painter.setPen(QPen(grey, 3))
+            painter.drawLine(QPointF(right - bar_px, bar_y), QPointF(right, bar_y))
+            painter.drawLine(QPointF(right - bar_px, bar_y - 4), QPointF(right - bar_px, bar_y + 4))
+            painter.drawLine(QPointF(right, bar_y - 4), QPointF(right, bar_y + 4))
+            shadowed_text(
+                right - (bar_px + fm.horizontalAdvance(label)) / 2, bar_y - 8, label, grey
+            )
+
+        painter.restore()
+
     @Slot(bool)
     def set_show_detections(self, show: bool):
         self._show_detections = show

tests/unit/gui/test_camera_image.py

@@ -229,6 +229,30 @@ def test_alt_wheel_axis_swap_still_changes_exposure(camera):
     assert len(sent) == n
 
 
+def test_hover_hud_coords_and_scale_bar(camera):
+    # Hover inside the image: bottom-right HUD paints coords + grey scale bar.
+    _mouse_move(camera, QPoint(400, 300))
+    assert camera._hover_pos is not None
+    camera.grab()
+
+    # Hover outside the image (negative scene coords): HUD skips drawing.
+    _mouse_move(camera, QPoint(-10, -10))
+    camera.grab()
+
+    # Leaving the view clears the readout entirely.
+    camera.leaveEvent(QEvent(QEvent.Type.Leave))
+    assert camera._hover_pos is None
+    camera.grab()
+
+
+def test_scale_bar_nice_numbers():
+    # 1-2-5 progression against the ~120 px target, and the mm label swap.
+    assert SampleCameraImageLabel._scale_bar(1.0) == (100.0, "100 µm")
+    assert SampleCameraImageLabel._scale_bar(2.0) == (200.0, "200 µm")
+    assert SampleCameraImageLabel._scale_bar(5.0) == (500.0, "500 µm")
+    assert SampleCameraImageLabel._scale_bar(10.0) == (1000.0, "1 mm")
+
+
 def test_autoscale_fits_from_the_first_frame(camera):
     from PySide6.QtGui import QPixmap