napari · jni · Aug 12, 2024 · Aug 7, 2024 · Aug 7, 2024 · Aug 7, 2024
@@ -91,6 +91,10 @@
 size = np.array([10, 20, 20])
 viewer.add_points(points, size=size)
 
+# Add scale bar of a defined length to the exported figure
+viewer.scale_bar.visible = True
+viewer.scale_bar.length = 250
+
 # Export figure and change theme before and after exporting to show that the background canvas margins
 # are not in the exported figure.
 viewer.theme = "light"

@@ -5,4 +5,7 @@
 def test_scale_bar_instantiation(make_napari_viewer):
     viewer = make_napari_viewer()
     model = ScaleBarOverlay()
-    VispyScaleBarOverlay(overlay=model, viewer=viewer)
+    vispy_scale_bar = VispyScaleBarOverlay(overlay=model, viewer=viewer)
+    assert vispy_scale_bar.overlay.length is None
+    model.length = 50
+    assert vispy_scale_bar.overlay.length == 50
@@ -16,7 +16,7 @@ class VispyScaleBarOverlay(ViewerOverlayMixin, VispyCanvasOverlay):
     """Scale bar in world coordinates."""
 
     def __init__(self, *, viewer, overlay, parent=None) -> None:
-        self._target_length = 150
+        self._target_length = 150.0
         self._scale = 1
         self._unit: pint.Unit
 
@@ -35,6 +35,7 @@ def __init__(self, *, viewer, overlay, parent=None) -> None:
         self.overlay.events.font_size.connect(self._on_text_change)
         self.overlay.events.ticks.connect(self._on_data_change)
         self.overlay.events.unit.connect(self._on_unit_ch
10000
ange)
+        self.overlay.events.length.connect(self._on_length_change)
 
         self.viewer.events.theme.connect(self._on_data_change)
         self.viewer.camera.events.zoom.connect(self._on_zoom_change)
@@ -45,6 +46,9 @@ def _on_unit_change(self):
         self._unit = get_unit_registry()(self.overlay.unit)
         self._on_zoom_change(force=True)
 
+    def _on_length_change(self):
+        self._on_zoom_change(force=True)
+
     def _calculate_best_length(
         self, desired_length: float
     ) -> tuple[float, pint.Quantity]:
@@ -112,18 +116,24 @@ def _on_zoom_change(self, *, force: bool = False):
         # convert desired length to world size
         target_world_pixels = scale_canvas2world * target_canvas_pixels
 
-        # calculate the desired length as well as update the value and units
-        target_world_pixels_rounded, new_dim = self._calculate_best_length(
-            target_world_pixels
-        )
-        target_canvas_pixels_rounded = (
-            target_world_pixels_rounded / scale_canvas2world
-        )
-        scale = target_canvas_pixels_rounded
+        # If length is set, use that value to calculate the scale bar length
+        if self.overlay.length is not None:
+            target_canvas_pixels = self.overlay.length / scale_canvas2world
+            new_dim = self.overlay.length * self._unit.units
+        else:
+            # calculate the desired length as well as update the value and units
+            target_world_pixels_rounded, new_dim = self._calculate_best_length(
+                target_world_pixels
+            )
+            target_canvas_pixels = (
+                target_world_pixels_rounded / scale_canvas2world
+            )
+
+        scale = target_canvas_pixels
 
         # Update scalebar and text
         self.node.transform.scale = [scale, 1, 1, 1]
-        self.node.text.text = f'{new_dim:~}'
+        self.node.text.text = f'{new_dim:g~#P}'
         self.x_size = scale  # needed to offset properly
         self._on_position_change()
 
@@ -171,3 +181,4 @@ def reset(self):
         self._on_data_change()
         self._on_box_change()
         self._on_text_change()
+        self._on_length_change()
@@ -5,3 +5,9 @@ def test_scale_bar():
     """Test creating scale bar object"""
     scale_bar = ScaleBarOverlay()
     assert scale_bar is not None
+
+
+def test_scale_bar_fixed_length():
+    """Test creating scale bar object with fixed length"""
+    scale_bar = ScaleBarOverlay(length=50)
+    assert scale_bar.length == 50
@@ -35,6 +35,9 @@ class ScaleBarOverlay(CanvasOverlay):
     unit : Optional[str]
         Unit to be used by the scale bar. The value can be set
         to `None` to display no units.
+    length : Optional[float]
+        Fixed length of the scale bar in physical units. If set to `None`,
+        it is determined automatically based on zoom level.
     position : CanvasPosition
         The position of the overlay in the canvas.
     visible : bool
@@ -54,3 +57,4 @@ class ScaleBarOverlay(CanvasOverlay):
         default_factory=lambda: ColorValue([0, 0, 0, 0.6])
     )
     unit: Optional[str] = None
+    length: Optional[float] = None