Clean up outline_color integration and fix boundary rendering

- Simplify outline_color type for labels to ColorLike | None (no tuple)
- Store as Color | None in LabelsRenderParams (consistent with shapes)
- Add proper type annotation to _draw_labels
- Simplify _map_color_seg boundary rendering: build clean RGBA image
  with outline_color on the eroded ring, transparent elsewhere
- Remove duplicate matplotlib.colors import (mcolors → colors)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/basic.py

@@ -641,7 +641,7 @@ def render_labels(
         na_color: ColorLike | None = "default",
         outline_alpha: float | int = 0.0,
         fill_alpha: float | int | None = None,
-        outline_color: ColorLike | tuple[ColorLike] | None = None,
+        outline_color: ColorLike | None = None,
         scale: str | None = None,
         colorbar: bool | str | None = "auto",
         colorbar_params: dict[str, object] | None = None,
@@ -696,7 +696,7 @@ def render_labels(
         outline_color : ColorLike | None
             Color of the outline of the labels. Can either be a named color ("red"), a hex representation
             ("#000000") or a list of floats that represent RGB/RGBA values (1.0, 0.0, 0.0, 1.0). If None, the outline
-            color is set to "black".
+            color defaults to "black".
         scale :  str | None
             Influences the resolution of the rendering. Possibilities for setting this parameter:
                 1) None (default). The image is rasterized to fit the canvas size. For multiscale images, the best scale

src/spatialdata_plot/pl/render.py

@@ -1349,7 +1349,10 @@ def _render_labels(
             assert color_source_vector is None
 
     def _draw_labels(
-        seg_erosionpx: int | None, seg_boundaries: bool, alpha: float, outline_color=None
+        seg_erosionpx: int | None,
+        seg_boundaries: bool,
+        alpha: float,
+        outline_color: Color | None = None,
     ) -> matplotlib.image.AxesImage:
         labels = _map_color_seg(
             seg=label.values,

src/spatialdata_plot/pl/render_params.py

@@ -285,7 +285,7 @@ class LabelsRenderParams:
     outline: bool = False
     palette: ListedColormap | list[str] | None = None
     outline_alpha: float = 1.0
-    outline_color: ColorLike | tuple[ColorLike] | None = None
+    outline_color: Color | None = None
     fill_alpha: float = 0.4
     transfunc: Callable[[float], float] | None = None
     scale: str | None = None

src/spatialdata_plot/pl/utils.py

@@ -13,7 +13,6 @@
 import dask
 import datashader as ds
 import matplotlib
-import matplotlib.colors as mcolors
 import matplotlib.patches as mpatches
 import matplotlib.path as mpath
 import matplotlib.pyplot as plt
@@ -1210,35 +1209,14 @@ def _map_color_seg(
     )
 
     if seg_boundaries:
-        if seg.shape[0] == 1:
-            seg = np.squeeze(seg, axis=0)
-
-        # Binary boundary mask
-        boundary_mask = seg.astype(bool)
-
-        # Ensure seg_im is float in 0-1 and has 3 channels
-        seg_float = seg_im.astype(float)
-        if seg_float.ndim == 2:
-            seg_float = np.stack([seg_float] * 3, axis=-1)  # H x W x 3
-
-        # Add alpha channel from val_im (preserve original mask)
-        alpha_channel = (val_im > 0).astype(float)
-        seg_float = np.dstack((seg_float, alpha_channel))  # H x W x 4
-
-        # Convert outline_color to RGBA
-        if outline_color is None:
-            outline_rgba = (0, 0, 0, 1.0)  # default black
-        elif isinstance(outline_color, str):
-            outline_rgba = mcolors.to_rgba(outline_color)  # named color or hex string
-        else:
-            # assume it's your Color object
-            outline_rgba = mcolors.to_rgba(outline_color.get_hex_with_alpha())
-
-        # Apply outline color to boundary pixels, but keep original alpha from val_im
-        seg_float[boundary_mask, :3] = outline_rgba[:3]  # RGB
-        seg_float[boundary_mask, 3] = alpha_channel[boundary_mask] * outline_rgba[3]  # scale alpha
+        outline_rgba = colors.to_rgba(outline_color.get_hex_with_alpha() if outline_color is not None else "black")
 
-        return seg_float  # H x W x 4, valid RGBA
+        # Build RGBA image: outline_color on the eroded ring, transparent elsewhere
+        outline_mask = val_im > 0
+        rgba = np.zeros((*val_im.shape, 4), dtype=float)
+        rgba[outline_mask, :3] = outline_rgba[:3]
+        rgba[outline_mask, 3] = outline_rgba[3]
+        return rgba
 
     if len(val_im.shape) != len(seg_im.shape):
         val_im = np.expand_dims((val_im > 0).astype(int), axis=-1)
@@ -2463,7 +2441,7 @@ def _validate_label_render_params(
     na_color: ColorLike | None,
     norm: Normalize | None,
     outline_alpha: float | int,
-    outline_color: ColorLike | tuple[ColorLike] | None,
+    outline_color: ColorLike | None,
     scale: str | None,
     table_name: str | None,
     table_layer: str | None,