Fix set_zero_in_cmap_to_transparent with datashader rendering#577
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Fix set_zero_in_cmap_to_transparent with datashader rendering#577
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Datashader's tf.shade() strips the alpha channel from matplotlib colormaps, so entries with alpha=0 (from set_zero_in_cmap_to_transparent) rendered as opaque white instead of transparent. Fix by masking aggregate values that map to transparent cmap entries to NaN before shading — NaN is datashader's native transparency mechanism. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Codecov Report❌ Patch coverage is
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## main #577 +/- ##
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- Coverage 75.36% 75.25% -0.12%
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Files 10 10
Lines 2935 2966 +31
Branches 684 693 +9
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+ Hits 2212 2232 +20
- Misses 441 446 +5
- Partials 282 288 +6
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Add test_plot_transparent_cmap_shapes_matplotlib and test_plot_transparent_cmap_shapes_datashader to verify that shapes with value=0 are rendered transparently over an image when using set_zero_in_cmap_to_transparent, for both rendering backends. Placeholder baselines included — CI will generate the real ones. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Each visual test now shows a side-by-side comparison with the standard viridis cmap on the left and the transparent cmap on the right, making the transparency effect clearly visible. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
When span=None, datashader auto-scales from non-NaN values. After masking zeros to NaN, the remaining minimum value (e.g. 2.0) would map to cmap(0.0) — the transparent-white entry — and min_alpha would override the alpha, producing opaque white. Fix: _mask_transparent_cmap_entries now returns the pre-masking span so ds.tf.shade uses the original data range, preventing re-scaling into the transparent bin. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The NaN-masking approach was fundamentally flawed: masking values to NaN caused datashader to auto-scale the remaining values, shifting other shapes' colors and potentially making them white. New approach: let datashader shade normally (preserving all colors and spans), then post-process the RGBA output to apply the cmap's alpha channel. This: - Does not modify aggregate values or spans - Preserves exact colors for all non-transparent shapes - Works for any cmap with transparent entries at any position - Handles span=None correctly (no auto-scaling issues) Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Cover additional datashader code paths: - Points with transparent cmap (separate from shapes) - Shapes with clip=False norm (3-part under/in/over shading) Placeholder baselines — CI will generate real ones. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Summary
set_zero_in_cmap_to_transparentproduced opaque white shapes instead of transparent ones when rendering with datashaderds.tf.shade()strips the alpha channel from matplotlib colormaps entirely — entries with alpha=0 were rendered as opaque white_get_collection_shape)Known limitation: when all aggregate values are identical and map to the transparent cmap entry, the upstream single-value path in
_ds_shade_continuousconverts the cmap to a hex string before reaching the masking logic. This is a pre-existing edge case.