Fix datashader fill_alpha applied twice#552
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Alpha was passed both to ds.tf.shade(min_alpha=...) and to ax.imshow(alpha=...), resulting in effective transparency of fill_alpha² instead of fill_alpha. Remove the second application since the RGBA channels from datashader already encode transparency. Affects shapes (fill_alpha), points (alpha), and shape outlines (outline_alpha). Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Regenerated from CI after fixing the double-alpha bug (#367). Only 2 tests exceeded the RMS tolerance; the rest were within bounds. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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- Add regression test for outline_alpha double-application - Make points test docstring consistent with shapes test - Replace :issue:`367` with plain #367 (no Sphinx role configured) - Remove redundant inline comment in shapes test Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Exercises both code paths (_render_ds_image and _render_ds_outlines) in a single render call, halving the test cost. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #552 +/- ##
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Coverage 73.66% 73.66%
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Files 10 10
Lines 2738 2738
Branches 637 637
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Hits 2017 2017
Misses 447 447
Partials 274 274
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Summary
fill_alpha(andalphafor points,outline_alphafor outlines) was applied twice — once inds.tf.shade(min_alpha=...)baking it into the RGBA image, then again inax.imshow(alpha=...)multiplying the alpha channel a second timefill_alpha²instead offill_alpha(e.g., 0.25 instead of 0.5), making datashader elements appear lighter than their matplotlib equivalents_render_ds_image()and_render_ds_outlines(), since the RGBA channels from datashader already encode transparency correctly