Add collision check when set colors for labels layer (#6193)

# Description
This PR adds a check if two user-defined colors for labels above 2*23
collide in the same value after being cast to float32.

We need to cast values to this type because of the texture mechanism
currently used.

# References and relevant issues
extracted from #6182 requested in
#6182 (comment)

napari/layers/labels/_tests/test_labels.py

@@ -1,5 +1,6 @@
 import itertools
 import time
+import warnings
 from dataclasses import dataclass
 from tempfile import TemporaryDirectory
 from typing import List
@@ -1571,6 +1572,17 @@ def test_get_status_with_custom_index():
     )
 
 
+def test_collision_warning():
+    label = Labels(data=np.zeros((10, 10), dtype=np.uint8))
+    with pytest.warns(
+        RuntimeWarning, match="Because integer labels are cast to less-precise"
+    ):
+        label.color = {2**25 + 1: 'red', 2**25 + 2: 'blue'}
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        label.color = {1: 'red', 2: 'blue'}
+
+
 def test_labels_features_event():
     event_emitted = False
 

napari/layers/labels/labels.py

@@ -1,7 +1,16 @@
 import warnings
 from collections import deque
 from contextlib import contextmanager
-from typing import Callable, ClassVar, Dict, List, Optional, Tuple, Union
+from typing import (
+    Callable,
+    ClassVar,
+    Dict,
+    Iterable,
+    List,
+    Optional,
+    Tuple,
+    Union,
+)
 
 import numpy as np
 import numpy.typing as npt
@@ -550,13 +559,17 @@ def color(self, color):
 
         if self._background_label not in color:
             color[self._background_label] = 'transparent'
-        if None not in color:
-            color[None] = 'black'
+
+        none_color = color.pop(None, 'black')
+        self._validate_colors(color)
+
+        color[None] = none_color
 
         colors = {
             label: transform_color(color_str)[0]
             for label, color_str in color.items()
         }
+
         self._color = colors
         self._direct_colormap = direct_colormap(colors)
 
@@ -575,6 +588,45 @@ def color(self, color):
 
         self.color_mode = color_mode
 
+    @classmethod
+    def _validate_colors(cls, labels: Iterable[int]):
+        """Check whether any of the given labels will be aliased together.
+
+        See https://github.com/napari/napari/issues/6084 for details.
+        """
+        labels_int = np.fromiter(labels, dtype=int)
+        labels_unique = np.unique(cls._to_vispy_texture_dtype(labels_int))
+        if labels_unique.size == labels_int.size:
+            return
+
+        # recalculate here second time to provide best performance on colors that are not colliding
+        labels_unique, inverse, count = np.unique(
+            cls._to_vispy_texture_dtype(labels_int),
+            return_inverse=True,
+            return_counts=True,
+        )
+        collided_idx = np.where(count > 1)[0]
+        aliased_list = [
+            labels_int[np.where(inverse == idx)[0]] for idx in collided_idx
+        ]
+
+        alias_string = "\n".join(
+            trans._(
+                'Labels {col_li} will display as the same color as {col_la};',
+                col_li=",".join(str(i) for i in lst[:-1]),
+                col_la=str(lst[-1]),
+            )
+            for lst in aliased_list
+        )
+        warn_text = trans._(
+            "Because integer labels are cast to less-precise float for display, "
+            "the following label sets will render as the same color:\n"
+            "{alias_string}\n"
+            "See https://github.com/napari/napari/issues/6084 for details.",
+            alias_string=alias_string,
+        )
+        warnings.warn(warn_text, category=RuntimeWarning)
+
     def _is_default_colors(self, color):
         """Returns True if color contains only default colors, otherwise False.
 
@@ -802,7 +854,8 @@ def _on_editable_changed(self) -> None:
             self.mode = Mode.PAN_ZOOM
             self._reset_history()
 
-    def _to_vispy_texture_dtype(self, data):
+    @staticmethod
+    def _to_vispy_texture_dtype(data):
         """Convert data to a dtype that can be used as a VisPy texture.
 
         Labels layers allow all integer dtypes for data, but only a subset