fix: inherit color from source artist

src/plopp/backends/pythreejs/scatter3d.py

@@ -2,7 +2,7 @@
 # Copyright (c) 2023 Scipp contributors (https://github.com/scipp)
 
 import uuid
-from typing import Literal
+from typing import Literal, cast
 
 import numpy as np
 import pythreejs as p3
@@ -75,8 +75,12 @@ def __init__(
         self._x = x
         self._y = y
         self._z = z
-        self._color = color
-        self._artist_number = artist_number
+        self._color = None
+        if colormapper is None:
+            self._color = np.array(
+                to_rgb(f'C{artist_number}' if color is None else color),
+                dtype='float32',
+            )
 
         # TODO: remove pixel_size in the next release
         self._size = size if pixel_size is None else pixel_size
@@ -124,18 +128,8 @@ def _make_positions(self) -> np.ndarray:
     def _make_colors(self) -> np.ndarray:
         if self._colormapper is not None:
             return self._colormapper.rgba(self._data)[..., :3].astype('float32')
-        else:
-            return np.broadcast_to(
-                np.array(
-                    to_rgb(
-                        f'C{self._artist_number}'
-                        if self._color is None
-                        else self._color
-                    ),
-                    dtype='float32',
-                ),
-                (self._data.coords[self._x].shape[0], 3),
-            )
+        color = cast(np.ndarray, self._color)
+        return np.broadcast_to(color, (self._data.coords[self._x].shape[0], 3))
 
     def _make_geometry(
         self, positions: np.ndarray, colors: np.ndarray
@@ -212,6 +206,10 @@ def color(self) -> np.ndarray:
     def color(self, val: np.ndarray):
         self.geometry.attributes['color'].array = val
 
+    def _copy_color_from(self, other: 'Scatter3d') -> None:
+        self._color = other.color[0].copy()
+        self.color = self._make_colors()
+
     @property
     def geometry(self) -> p3.BufferGeometry:
         """

src/plopp/widgets/clip3d.py

@@ -503,6 +503,11 @@ def update_controls(self):
             self._nodes.clear()
 
         self.update_state()
+        if self._view.colormapper is None:
+            for source_id, node in self._nodes.items():
+                self._view.artists[node.id]._copy_color_from(
+                    self._view.artists[source_id]
+                )
 
     def _set_opacity(self, change: dict[str, Any]):
         """

tests/backends/pythreejs/pythreejs_scatter3d_test.py

@@ -4,6 +4,7 @@
 import numpy as np
 import pytest
 import scipp as sc
+from matplotlib import cycler, rc_context
 
 from plopp.backends.pythreejs.canvas import Canvas
 from plopp.backends.pythreejs.scatter3d import Scatter3d
@@ -30,6 +31,17 @@ def test_update():
     assert sc.identical(scat._data, da * 2.5)
 
 
+def test_update_with_different_number_of_points_preserves_default_color():
+    with rc_context({'axes.prop_cycle': cycler(color=['#102030'])}):
+        scat = Scatter3d(canvas=Canvas(), data=scatter(20), x='x', y='y', z='z')
+    color = scat.color[0].copy()
+
+    with rc_context({'axes.prop_cycle': cycler(color=['#708090'])}):
+        scat.update(scatter(30))
+
+    np.testing.assert_array_equal(scat.color, np.broadcast_to(color, (30, 3)))
+
+
 def test_bounding_box():
     da = scatter()
     pix = 0.5

tests/widgets/clip3d_test.py

@@ -1,14 +1,48 @@
 # SPDX-License-Identifier: BSD-3-Clause
 # Copyright (c) 2023 Scipp contributors (https://github.com/scipp)
+import numpy as np
 import pytest
 import scipp as sc
+from matplotlib import cycler, rc_context
 
 from plopp import Node
 from plopp.data.testing import data_array, scatter
 from plopp.graphics import scatter3dfigure
 from plopp.widgets import ClippingManager
 
 
+def test_cut_colors_match_original_artists():
+    a = scatter(seed=1)
+    b = scatter(seed=2)
+    b.coords['x'] += sc.scalar(60, unit='m')
+    nodes = [Node(a), Node(b)]
+    with rc_context({'axes.prop_cycle': cycler(color=['#102030', '#405060'])}):
+        fig = scatter3dfigure(*nodes, x='x', y='y', z='z', cbar=False)
+    clip = ClippingManager(fig)
+
+    with rc_context({'axes.prop_cycle': cycler(color=['#708090', '#a0b0c0'])}):
+        clip.add_y_cut.click()
+
+    for source in nodes:
+        cut = clip._nodes[source.id]
+        assert np.array_equal(
+            fig.artists[cut.id].color[0], fig.artists[source.id].color[0]
+        )
+
+    npoints = [fig.artists[node.id].data.shape[0] for node in clip._nodes.values()]
+    ycut = clip.cuts[-1]
+    ycut.slider.value = [ycut.slider.min, ycut.slider.value[1]]
+    clip.update_state()
+    assert [
+        fig.artists[node.id].data.shape[0] for node in clip._nodes.values()
+    ] != npoints
+    for source in nodes:
+        cut = clip._nodes[source.id]
+        assert np.array_equal(
+            fig.artists[cut.id].color[0], fig.artists[source.id].color[0]
+        )
+
+
 @pytest.mark.parametrize('multiple_nodes', [False, True])
 def test_add_remove_cuts(multiple_nodes):
     a = scatter()