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Fix outline linewidth mismatch between matplotlib and datashader #543

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Merged
timtreis merged 2 commits into from
Mar 5, 2026
Merged

Fix outline linewidth mismatch between matplotlib and datashader #543

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2baaccf
Convert outline linewidth from points to canvas pixels for datashader
timtreis Mar 5, 2026
d666ae9
Update reference image for double outline with DPI-scaled linewidth
timtreis Mar 5, 2026
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6 changes: 4 additions & 2 deletions src/spatialdata_plot/pl/render.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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
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Binary file modified tests/_images/Shapes_datashader_can_render_shapes_with_double_outline.png
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14 changes: 14 additions & 0 deletions tests/pl/test_render_shapes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -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()