Fix datashader fill_alpha applied twice (#552)

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/_datashader.py

@@ -268,16 +268,20 @@ def _render_ds_image(
     shaded: Any,
     factor: float,
     zorder: int,
-    alpha: float,
     extent: list[float] | None,
     nan_result: Any | None = None,
 ) -> Any:
-    """Render a shaded datashader image onto matplotlib axes, with optional NaN overlay."""
+    """Render a shaded datashader image onto matplotlib axes, with optional NaN overlay.
+
+    Alpha is NOT passed to ``ax.imshow`` because it is already encoded in
+    the RGBA channels produced by ``ds.tf.shade(min_alpha=...)``.  Passing
+    it again would apply transparency twice (see #367).
+    """
     if nan_result is not None:
         rgba_nan, trans_nan = _create_image_from_datashader_result(nan_result, factor, ax)
-        _ax_show_and_transform(rgba_nan, trans_nan, ax, zorder=zorder, alpha=alpha, extent=extent)
+        _ax_show_and_transform(rgba_nan, trans_nan, ax, zorder=zorder, extent=extent)
     rgba_image, trans_data = _create_image_from_datashader_result(shaded, factor, ax)
-    return _ax_show_and_transform(rgba_image, trans_data, ax, zorder=zorder, alpha=alpha, extent=extent)
+    return _ax_show_and_transform(rgba_image, trans_data, ax, zorder=zorder, extent=extent)
 
 
 def _render_ds_outlines(
@@ -315,7 +319,7 @@ def _render_ds_outlines(
                 how="linear",
             )
             rgba, trans = _create_image_from_datashader_result(shaded, factor, ax)
-            _ax_show_and_transform(rgba, trans, ax, zorder=render_params.zorder, alpha=alpha, extent=extent)
+            _ax_show_and_transform(rgba, trans, ax, zorder=render_params.zorder, extent=extent)
 
 
 def _build_ds_colorbar(

src/spatialdata_plot/pl/render.py

@@ -486,7 +486,6 @@ def _render_shapes(
             shaded,
             factor,
             render_params.zorder,
-            render_params.fill_alpha,
             x_ext + y_ext,
             nan_result=nan_shaded,
         )
@@ -886,7 +885,6 @@ def _render_points(
             shaded,
             factor,
             render_params.zorder,
-            render_params.alpha,
             x_ext + y_ext,
             nan_result=nan_shaded,
         )

tests/_images/Points_datashader_continuous_color.png

@@ -701,3 +701,22 @@ def test_datashader_points_visible_with_nonuniform_scale(sdata_blobs: SpatialDat
     """
     _set_transformations(sdata_blobs["blobs_points"], {"global": Scale([1, 5], axes=("x", "y"))})
     sdata_blobs.pl.render_points("blobs_points", method="datashader", color="black").pl.show()
+
+
+def test_datashader_alpha_not_applied_twice(sdata_blobs: SpatialData):
+    """Datashader alpha must not be applied twice (once in shade, once in imshow).
+
+    Regression test for https://github.com/scverse/spatialdata-plot/issues/367.
+    Before the fix, alpha was passed both to ds.tf.shade(min_alpha=...) and to
+    ax.imshow(alpha=...), resulting in effective transparency of alpha**2.
+    """
+    fig, ax = plt.subplots()
+    sdata_blobs.pl.render_points(method="datashader", alpha=0.5, color="red").pl.show(ax=ax)
+
+    axes_images = [c for c in ax.get_children() if isinstance(c, matplotlib.image.AxesImage)]
+    for img in axes_images:
+        assert img.get_alpha() is None, (
+            f"Datashader AxesImage has alpha={img.get_alpha()}, which would be applied "
+            "on top of the alpha already in the RGBA channels — causing double transparency."
+        )
+    plt.close(fig)

tests/_images/Points_mpl_and_datashader_point_sizes_agree_after_altered_dpi.png

@@ -1093,3 +1093,28 @@ def test_datashader_outline_width_uses_points_units(sdata_blobs: SpatialData):
     sdata_blobs.pl.render_shapes(
         element="blobs_polygons", method="datashader", outline_alpha=1.0, outline_width=(8.0, 3.0)
     ).pl.show()
+
+
+def test_datashader_alpha_not_applied_twice(sdata_blobs: SpatialData):
+    """Datashader fill_alpha and outline_alpha must not be applied twice.
+
+    Regression test for https://github.com/scverse/spatialdata-plot/issues/367.
+    Before the fix, alpha was passed both to ds.tf.shade(min_alpha=...) and to
+    ax.imshow(alpha=...), resulting in effective transparency of alpha**2.
+    """
+    fig, ax = plt.subplots()
+    sdata_blobs.pl.render_shapes(
+        method="datashader",
+        fill_alpha=0.5,
+        color="red",
+        outline_alpha=0.5,
+        outline_color="blue",
+    ).pl.show(ax=ax)
+
+    axes_images = [c for c in ax.get_children() if isinstance(c, matplotlib.image.AxesImage)]
+    for img in axes_images:
+        assert img.get_alpha() is None, (
+            f"Datashader AxesImage has alpha={img.get_alpha()}, which would be applied "
+            "on top of the alpha already in the RGBA channels — causing double transparency."
+        )
+    plt.close(fig)