Fix outline linewidth mismatch between matplotlib and datashader (#543)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/render.py

@@ -342,18 +342,20 @@ def _render_shapes(
             agg = cvs.polygons(transformed_element, geometry="geometry", agg=ds.count())
 
         # render outlines if needed
+        # outline_linewidth is in points (1pt = 1/72 inch); datashader line_width is in canvas pixels
+        ds_lw_factor = fig_params.fig.dpi / 72
         assert len(render_params.outline_alpha) == 2  # shut up mypy
         if render_params.outline_alpha[0] > 0:
             agg_outlines = cvs.line(
                 transformed_element,
                 geometry="geometry",
-                line_width=render_params.outline_params.outer_outline_linewidth,
+                line_width=render_params.outline_params.outer_outline_linewidth * ds_lw_factor,
             )
         if render_params.outline_alpha[1] > 0:
             agg_inner_outlines = cvs.line(
                 transformed_element,
                 geometry="geometry",
-                line_width=render_params.outline_params.inner_outline_linewidth,
+                line_width=render_params.outline_params.inner_outline_linewidth * ds_lw_factor,
             )
 
         ds_span = None

tests/_images/Shapes_datashader_can_render_shapes_with_double_outline.png

@@ -999,3 +999,17 @@ def test_plot_datashader_single_category(sdata_blobs: SpatialData):
     sdata_blobs["table"] = table
 
     sdata_blobs.pl.render_shapes(element="blobs_polygons", color="category", method="datashader").pl.show()
+
+
+def test_datashader_outline_width_uses_points_units(sdata_blobs: SpatialData):
+    """Datashader outlines should scale with DPI like matplotlib (points, not raw pixels).
+
+    Regression test for https://github.com/scverse/spatialdata-plot/issues/493.
+    """
+    sdata_blobs.pl.render_shapes(
+        element="blobs_polygons", method="datashader", outline_alpha=1.0, outline_width=10.0
+    ).pl.show()
+
+    sdata_blobs.pl.render_shapes(
+        element="blobs_polygons", method="datashader", outline_alpha=1.0, outline_width=(8.0, 3.0)
+    ).pl.show()