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Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) #699

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Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper)#699
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housekeeping 🧹

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@timtreis
timtreis
opened on Jun 6, 2026

Summary

Color / cmap / norm resolution is scattered and re-implemented across render.py and utils.py, with genuine inconsistencies (not just duplication) — including a colorbar-vs-pixels mismatch and a private-matplotlib-API poke. Unifying it removes a recurring bug class and shrinks every render function.

1. CmapParams is a dumb data bag; callers re-implement its safe handling

render_params.pyCmapParams carries cmap, norm, na_color, cmap_is_default but no behavior, so every consumer re-does the same operations:

  • "copy the stateful Normalize before use" is hand-written 5× (render.py:705, 1231, 1765, 1862). Forgetting it risks cross-channel/cross-call mutation.
  • cmap-alpha is applied by poking matplotlib's private _lut (render.py:1803-1804) — fragile across matplotlib versions and against the "no private APIs of other packages" guideline.
  • na_color → hex conversions and isinstance(na_color, Color) guards are scattered ~10× (utils.py:1411/1468/1898/..., render.py:630/1318).

Fix: give CmapParams methods — fresh_norm(), cmap_with_alpha(alpha), is_user_cmap, na_hex. Behavior-preserving; spot-check the 1-channel baseline for the cmap-alpha change.

2. Continuous→RGBA normalization reimplemented 4–6× with drift

The "NaN-safe normalize then cmap(norm(.))" logic exists in _map_color_seg (utils.py:1510-1514, 1567-1571), _get_collection_shape Case B (render.py:650-669) and again at 683-691. They have already drifted: _map_color_seg uses ~np.isnan, _get_collection_shape uses np.isfinite (differ on ±inf); the vmin==vmax guard exists in one but not the other. The colorbar span is derived independently from the pixel span — so the colorbar can disagree with the rendered pixels (the #687/#688 churn class).

Fix: a single resolve_continuous_norm(values, cmap_params) -> Normalize that feeds both the pixels and the colorbar ScalarMappable, plus _continuous_to_rgba(values, cmap_params).

3. The four _render_images blend branches collapse to one helper

Paths 2A/2B/2C/2D (render.py:~1877-2015) are six copies of stack → reduce → (clip) with three different reductions. Collapse to one _composite_channels(layers, channel_cmaps, alpha). (The averaging-vs-additive inconsistency in this same code is tracked separately in findings/multichannel-compositing-bugs.md; unifying the helper is the structural enabler.)

Sequencing / risk

Land the safe foundations first — CmapParams methods (#1) — then resolve_continuous_norm (#2), then the composite helper (#3). Effort: medium. Risk: low for the CmapParams methods (no baseline impact); medium for the norm/composite unification (CI baseline regen). #2 also fixes the colorbar-vs-pixels mismatch, so it's correctness, not just cleanup.

Part of a maintainability/refactor audit of main.

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Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) · Issue #699 · scverse/spatialdata-plot · GitHub
      Skip to content
      Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) #699
      Closed
      Closed
      Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper)#699
      Labels
      housekeeping 🧹
      Description
      @timtreis
      timtreis
      opened on Jun 6, 2026
      Summary
      Color / cmap / norm resolution is scattered and re-implemented across render.py and utils.py, with genuine inconsistencies (not just duplication) — including a colorbar-vs-pixels mismatch and a private-matplotlib-API poke. Unifying it removes a recurring bug class and shrinks every render function.
      1. CmapParams is a dumb data bag; callers re-implement its safe handling
      render_params.pyCmapParams carries cmap, norm, na_color, cmap_is_default but no behavior, so every consumer re-does the same operations:
      • "copy the stateful Normalize before use" is hand-written 5× (render.py:705, 1231, 1765, 1862). Forgetting it risks cross-channel/cross-call mutation.
      • cmap-alpha is applied by poking matplotlib's private _lut (render.py:1803-1804) — fragile across matplotlib versions and against the "no private APIs of other packages" guideline.
      • na_color → hex conversions and isinstance(na_color, Color) guards are scattered ~10× (utils.py:1411/1468/1898/..., render.py:630/1318).
      Fix: give CmapParams methods — fresh_norm(), cmap_with_alpha(alpha), is_user_cmap, na_hex. Behavior-preserving; spot-check the 1-channel baseline for the cmap-alpha change.
      2. Continuous→RGBA normalization reimplemented 4–6× with drift
      The "NaN-safe normalize then cmap(norm(.))" logic exists in _map_color_seg (utils.py:1510-1514, 1567-1571), _get_collection_shape Case B (render.py:650-669) and again at 683-691. They have already drifted: _map_color_seg uses ~np.isnan, _get_collection_shape uses np.isfinite (differ on ±inf); the vmin==vmax guard exists in one but not the other. The colorbar span is derived independently from the pixel span — so the colorbar can disagree with the rendered pixels (the #687/#688 churn class).
      Fix: a single resolve_continuous_norm(values, cmap_params) -> Normalize that feeds both the pixels and the colorbar ScalarMappable, plus _continuous_to_rgba(values, cmap_params).
      3. The four _render_images blend branches collapse to one helper
      Paths 2A/2B/2C/2D (render.py:~1877-2015) are six copies of stack → reduce → (clip) with three different reductions. Collapse to one _composite_channels(layers, channel_cmaps, alpha). (The averaging-vs-additive inconsistency in this same code is tracked separately in findings/multichannel-compositing-bugs.md; unifying the helper is the structural enabler.)
      Sequencing / risk
      Land the safe foundations first — CmapParams methods (#1) — then resolve_continuous_norm (#2), then the composite helper (#3). Effort: medium. Risk: low for the CmapParams methods (no baseline impact); medium for the norm/composite unification (CI baseline regen). #2 also fixes the colorbar-vs-pixels mismatch, so it's correctness, not just cleanup.
      Part of a maintainability/refactor audit of main.
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Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) · Issue #699 · scverse/spatialdata-plot · GitHub
          Skip to content
          Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) #699
          Closed
          Closed
          Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper)#699
          Labels
          housekeeping 🧹
          Description
          @timtreis
          timtreis
          opened on Jun 6, 2026
          Summary
          Color / cmap / norm resolution is scattered and re-implemented across render.py and utils.py, with genuine inconsistencies (not just duplication) — including a colorbar-vs-pixels mismatch and a private-matplotlib-API poke. Unifying it removes a recurring bug class and shrinks every render function.
          1. CmapParams is a dumb data bag; callers re-implement its safe handling
          render_params.pyCmapParams carries cmap, norm, na_color, cmap_is_default but no behavior, so every consumer re-does the same operations:
          • "copy the stateful Normalize before use" is hand-written 5× (render.py:705, 1231, 1765, 1862). Forgetting it risks cross-channel/cross-call mutation.
          • cmap-alpha is applied by poking matplotlib's private _lut (render.py:1803-1804) — fragile across matplotlib versions and against the "no private APIs of other packages" guideline.
          • na_color → hex conversions and isinstance(na_color, Color) guards are scattered ~10× (utils.py:1411/1468/1898/..., render.py:630/1318).
          Fix: give CmapParams methods — fresh_norm(), cmap_with_alpha(alpha), is_user_cmap, na_hex. Behavior-preserving; spot-check the 1-channel baseline for the cmap-alpha change.
          2. Continuous→RGBA normalization reimplemented 4–6× with drift
          The "NaN-safe normalize then cmap(norm(.))" logic exists in _map_color_seg (utils.py:1510-1514, 1567-1571), _get_collection_shape Case B (render.py:650-669) and again at 683-691. They have already drifted: _map_color_seg uses ~np.isnan, _get_collection_shape uses np.isfinite (differ on ±inf); the vmin==vmax guard exists in one but not the other. The colorbar span is derived independently from the pixel span — so the colorbar can disagree with the rendered pixels (the #687/#688 churn class).
          Fix: a single resolve_continuous_norm(values, cmap_params) -> Normalize that feeds both the pixels and the colorbar ScalarMappable, plus _continuous_to_rgba(values, cmap_params).
          3. The four _render_images blend branches collapse to one helper
          Paths 2A/2B/2C/2D (render.py:~1877-2015) are six copies of stack → reduce → (clip) with three different reductions. Collapse to one _composite_channels(layers, channel_cmaps, alpha). (The averaging-vs-additive inconsistency in this same code is tracked separately in findings/multichannel-compositing-bugs.md; unifying the helper is the structural enabler.)
          Sequencing / risk
          Land the safe foundations first — CmapParams methods (#1) — then resolve_continuous_norm (#2), then the composite helper (#3). Effort: medium. Risk: low for the CmapParams methods (no baseline impact); medium for the norm/composite unification (CI baseline regen). #2 also fixes the colorbar-vs-pixels mismatch, so it's correctness, not just cleanup.
          Part of a maintainability/refactor audit of main.
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(function(){
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var __m = "*";
var __re = new RegExp('^' + ".*" + '
Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) · Issue #699 · scverse/spatialdata-plot · GitHub
              Skip to content
              Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) #699
              Closed
              Closed
              Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper)#699
              Labels
              housekeeping 🧹
              Description
              @timtreis
              timtreis
              opened on Jun 6, 2026
              Summary
              Color / cmap / norm resolution is scattered and re-implemented across render.py and utils.py, with genuine inconsistencies (not just duplication) — including a colorbar-vs-pixels mismatch and a private-matplotlib-API poke. Unifying it removes a recurring bug class and shrinks every render function.
              1. CmapParams is a dumb data bag; callers re-implement its safe handling
              render_params.pyCmapParams carries cmap, norm, na_color, cmap_is_default but no behavior, so every consumer re-does the same operations:
              • "copy the stateful Normalize before use" is hand-written 5× (render.py:705, 1231, 1765, 1862). Forgetting it risks cross-channel/cross-call mutation.
              • cmap-alpha is applied by poking matplotlib's private _lut (render.py:1803-1804) — fragile across matplotlib versions and against the "no private APIs of other packages" guideline.
              • na_color → hex conversions and isinstance(na_color, Color) guards are scattered ~10× (utils.py:1411/1468/1898/..., render.py:630/1318).
              Fix: give CmapParams methods — fresh_norm(), cmap_with_alpha(alpha), is_user_cmap, na_hex. Behavior-preserving; spot-check the 1-channel baseline for the cmap-alpha change.
              2. Continuous→RGBA normalization reimplemented 4–6× with drift
              The "NaN-safe normalize then cmap(norm(.))" logic exists in _map_color_seg (utils.py:1510-1514, 1567-1571), _get_collection_shape Case B (render.py:650-669) and again at 683-691. They have already drifted: _map_color_seg uses ~np.isnan, _get_collection_shape uses np.isfinite (differ on ±inf); the vmin==vmax guard exists in one but not the other. The colorbar span is derived independently from the pixel span — so the colorbar can disagree with the rendered pixels (the #687/#688 churn class).
              Fix: a single resolve_continuous_norm(values, cmap_params) -> Normalize that feeds both the pixels and the colorbar ScalarMappable, plus _continuous_to_rgba(values, cmap_params).
              3. The four _render_images blend branches collapse to one helper
              Paths 2A/2B/2C/2D (render.py:~1877-2015) are six copies of stack → reduce → (clip) with three different reductions. Collapse to one _composite_channels(layers, channel_cmaps, alpha). (The averaging-vs-additive inconsistency in this same code is tracked separately in findings/multichannel-compositing-bugs.md; unifying the helper is the structural enabler.)
              Sequencing / risk
              Land the safe foundations first — CmapParams methods (#1) — then resolve_continuous_norm (#2), then the composite helper (#3). Effort: medium. Risk: low for the CmapParams methods (no baseline impact); medium for the norm/composite unification (CI baseline regen). #2 also fixes the colorbar-vs-pixels mismatch, so it's correctness, not just cleanup.
              Part of a maintainability/refactor audit of main.
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try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '
Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) · Issue #699 · scverse/spatialdata-plot · GitHub
                  Skip to content
                  Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) #699
                  Closed
                  Closed
                  Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper)#699
                  Labels
                  housekeeping 🧹
                  Description
                  @timtreis
                  timtreis
                  opened on Jun 6, 2026
                  Summary
                  Color / cmap / norm resolution is scattered and re-implemented across render.py and utils.py, with genuine inconsistencies (not just duplication) — including a colorbar-vs-pixels mismatch and a private-matplotlib-API poke. Unifying it removes a recurring bug class and shrinks every render function.
                  1. CmapParams is a dumb data bag; callers re-implement its safe handling
                  render_params.pyCmapParams carries cmap, norm, na_color, cmap_is_default but no behavior, so every consumer re-does the same operations:
                  • "copy the stateful Normalize before use" is hand-written 5× (render.py:705, 1231, 1765, 1862). Forgetting it risks cross-channel/cross-call mutation.
                  • cmap-alpha is applied by poking matplotlib's private _lut (render.py:1803-1804) — fragile across matplotlib versions and against the "no private APIs of other packages" guideline.
                  • na_color → hex conversions and isinstance(na_color, Color) guards are scattered ~10× (utils.py:1411/1468/1898/..., render.py:630/1318).
                  Fix: give CmapParams methods — fresh_norm(), cmap_with_alpha(alpha), is_user_cmap, na_hex. Behavior-preserving; spot-check the 1-channel baseline for the cmap-alpha change.
                  2. Continuous→RGBA normalization reimplemented 4–6× with drift
                  The "NaN-safe normalize then cmap(norm(.))" logic exists in _map_color_seg (utils.py:1510-1514, 1567-1571), _get_collection_shape Case B (render.py:650-669) and again at 683-691. They have already drifted: _map_color_seg uses ~np.isnan, _get_collection_shape uses np.isfinite (differ on ±inf); the vmin==vmax guard exists in one but not the other. The colorbar span is derived independently from the pixel span — so the colorbar can disagree with the rendered pixels (the #687/#688 churn class).
                  Fix: a single resolve_continuous_norm(values, cmap_params) -> Normalize that feeds both the pixels and the colorbar ScalarMappable, plus _continuous_to_rgba(values, cmap_params).
                  3. The four _render_images blend branches collapse to one helper
                  Paths 2A/2B/2C/2D (render.py:~1877-2015) are six copies of stack → reduce → (clip) with three different reductions. Collapse to one _composite_channels(layers, channel_cmaps, alpha). (The averaging-vs-additive inconsistency in this same code is tracked separately in findings/multichannel-compositing-bugs.md; unifying the helper is the structural enabler.)
                  Sequencing / risk
                  Land the safe foundations first — CmapParams methods (#1) — then resolve_continuous_norm (#2), then the composite helper (#3). Effort: medium. Risk: low for the CmapParams methods (no baseline impact); medium for the norm/composite unification (CI baseline regen). #2 also fixes the colorbar-vs-pixels mismatch, so it's correctness, not just cleanup.
                  Part of a maintainability/refactor audit of main.
                  Metadata
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var __m = "*";
var __re = new RegExp('^' + ".*" + '
Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) · Issue #699 · scverse/spatialdata-plot · GitHub
                      Skip to content
                      Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) #699
                      Closed
                      Closed
                      Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper)#699
                      Labels
                      housekeeping 🧹
                      Description
                      @timtreis
                      timtreis
                      opened on Jun 6, 2026
                      Summary
                      Color / cmap / norm resolution is scattered and re-implemented across render.py and utils.py, with genuine inconsistencies (not just duplication) — including a colorbar-vs-pixels mismatch and a private-matplotlib-API poke. Unifying it removes a recurring bug class and shrinks every render function.
                      1. CmapParams is a dumb data bag; callers re-implement its safe handling
                      render_params.pyCmapParams carries cmap, norm, na_color, cmap_is_default but no behavior, so every consumer re-does the same operations:
                      • "copy the stateful Normalize before use" is hand-written 5× (render.py:705, 1231, 1765, 1862). Forgetting it risks cross-channel/cross-call mutation.
                      • cmap-alpha is applied by poking matplotlib's private _lut (render.py:1803-1804) — fragile across matplotlib versions and against the "no private APIs of other packages" guideline.
                      • na_color → hex conversions and isinstance(na_color, Color) guards are scattered ~10× (utils.py:1411/1468/1898/..., render.py:630/1318).
                      Fix: give CmapParams methods — fresh_norm(), cmap_with_alpha(alpha), is_user_cmap, na_hex. Behavior-preserving; spot-check the 1-channel baseline for the cmap-alpha change.
                      2. Continuous→RGBA normalization reimplemented 4–6× with drift
                      The "NaN-safe normalize then cmap(norm(.))" logic exists in _map_color_seg (utils.py:1510-1514, 1567-1571), _get_collection_shape Case B (render.py:650-669) and again at 683-691. They have already drifted: _map_color_seg uses ~np.isnan, _get_collection_shape uses np.isfinite (differ on ±inf); the vmin==vmax guard exists in one but not the other. The colorbar span is derived independently from the pixel span — so the colorbar can disagree with the rendered pixels (the #687/#688 churn class).
                      Fix: a single resolve_continuous_norm(values, cmap_params) -> Normalize that feeds both the pixels and the colorbar ScalarMappable, plus _continuous_to_rgba(values, cmap_params).
                      3. The four _render_images blend branches collapse to one helper
                      Paths 2A/2B/2C/2D (render.py:~1877-2015) are six copies of stack → reduce → (clip) with three different reductions. Collapse to one _composite_channels(layers, channel_cmaps, alpha). (The averaging-vs-additive inconsistency in this same code is tracked separately in findings/multichannel-compositing-bugs.md; unifying the helper is the structural enabler.)
                      Sequencing / risk
                      Land the safe foundations first — CmapParams methods (#1) — then resolve_continuous_norm (#2), then the composite helper (#3). Effort: medium. Risk: low for the CmapParams methods (no baseline impact); medium for the norm/composite unification (CI baseline regen). #2 also fixes the colorbar-vs-pixels mismatch, so it's correctness, not just cleanup.
                      Part of a maintainability/refactor audit of main.
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Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) · Issue #699 · scverse/spatialdata-plot · GitHub
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                          Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) #699
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                          Closed
                          Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper)#699
                          Labels
                          housekeeping 🧹
                          Description
                          @timtreis
                          timtreis
                          opened on Jun 6, 2026
                          Summary
                          Color / cmap / norm resolution is scattered and re-implemented across render.py and utils.py, with genuine inconsistencies (not just duplication) — including a colorbar-vs-pixels mismatch and a private-matplotlib-API poke. Unifying it removes a recurring bug class and shrinks every render function.
                          1. CmapParams is a dumb data bag; callers re-implement its safe handling
                          render_params.pyCmapParams carries cmap, norm, na_color, cmap_is_default but no behavior, so every consumer re-does the same operations:
                          • "copy the stateful Normalize before use" is hand-written 5× (render.py:705, 1231, 1765, 1862). Forgetting it risks cross-channel/cross-call mutation.
                          • cmap-alpha is applied by poking matplotlib's private _lut (render.py:1803-1804) — fragile across matplotlib versions and against the "no private APIs of other packages" guideline.
                          • na_color → hex conversions and isinstance(na_color, Color) guards are scattered ~10× (utils.py:1411/1468/1898/..., render.py:630/1318).
                          Fix: give CmapParams methods — fresh_norm(), cmap_with_alpha(alpha), is_user_cmap, na_hex. Behavior-preserving; spot-check the 1-channel baseline for the cmap-alpha change.
                          2. Continuous→RGBA normalization reimplemented 4–6× with drift
                          The "NaN-safe normalize then cmap(norm(.))" logic exists in _map_color_seg (utils.py:1510-1514, 1567-1571), _get_collection_shape Case B (render.py:650-669) and again at 683-691. They have already drifted: _map_color_seg uses ~np.isnan, _get_collection_shape uses np.isfinite (differ on ±inf); the vmin==vmax guard exists in one but not the other. The colorbar span is derived independently from the pixel span — so the colorbar can disagree with the rendered pixels (the #687/#688 churn class).
                          Fix: a single resolve_continuous_norm(values, cmap_params) -> Normalize that feeds both the pixels and the colorbar ScalarMappable, plus _continuous_to_rgba(values, cmap_params).
                          3. The four _render_images blend branches collapse to one helper
                          Paths 2A/2B/2C/2D (render.py:~1877-2015) are six copies of stack → reduce → (clip) with three different reductions. Collapse to one _composite_channels(layers, channel_cmaps, alpha). (The averaging-vs-additive inconsistency in this same code is tracked separately in findings/multichannel-compositing-bugs.md; unifying the helper is the structural enabler.)
                          Sequencing / risk
                          Land the safe foundations first — CmapParams methods (#1) — then resolve_continuous_norm (#2), then the composite helper (#3). Effort: medium. Risk: low for the CmapParams methods (no baseline impact); medium for the norm/composite unification (CI baseline regen). #2 also fixes the colorbar-vs-pixels mismatch, so it's correctness, not just cleanup.
                          Part of a maintainability/refactor audit of main.
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Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) · Issue #699 · scverse/spatialdata-plot · GitHub
                              Skip to content
                              Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper) #699
                              Closed
                              Closed
                              Refactor: unify color/cmap/norm pipeline (CmapParams methods, single continuous-norm, one image-composite helper)#699
                              Labels
                              housekeeping 🧹
                              Description
                              @timtreis
                              timtreis
                              opened on Jun 6, 2026
                              Summary
                              Color / cmap / norm resolution is scattered and re-implemented across render.py and utils.py, with genuine inconsistencies (not just duplication) — including a colorbar-vs-pixels mismatch and a private-matplotlib-API poke. Unifying it removes a recurring bug class and shrinks every render function.
                              1. CmapParams is a dumb data bag; callers re-implement its safe handling
                              render_params.pyCmapParams carries cmap, norm, na_color, cmap_is_default but no behavior, so every consumer re-does the same operations:
                              • "copy the stateful Normalize before use" is hand-written 5× (render.py:705, 1231, 1765, 1862). Forgetting it risks cross-channel/cross-call mutation.
                              • cmap-alpha is applied by poking matplotlib's private _lut (render.py:1803-1804) — fragile across matplotlib versions and against the "no private APIs of other packages" guideline.
                              • na_color → hex conversions and isinstance(na_color, Color) guards are scattered ~10× (utils.py:1411/1468/1898/..., render.py:630/1318).
                              Fix: give CmapParams methods — fresh_norm(), cmap_with_alpha(alpha), is_user_cmap, na_hex. Behavior-preserving; spot-check the 1-channel baseline for the cmap-alpha change.
                              2. Continuous→RGBA normalization reimplemented 4–6× with drift
                              The "NaN-safe normalize then cmap(norm(.))" logic exists in _map_color_seg (utils.py:1510-1514, 1567-1571), _get_collection_shape Case B (render.py:650-669) and again at 683-691. They have already drifted: _map_color_seg uses ~np.isnan, _get_collection_shape uses np.isfinite (differ on ±inf); the vmin==vmax guard exists in one but not the other. The colorbar span is derived independently from the pixel span — so the colorbar can disagree with the rendered pixels (the #687/#688 churn class).
                              Fix: a single resolve_continuous_norm(values, cmap_params) -> Normalize that feeds both the pixels and the colorbar ScalarMappable, plus _continuous_to_rgba(values, cmap_params).
                              3. The four _render_images blend branches collapse to one helper
                              Paths 2A/2B/2C/2D (render.py:~1877-2015) are six copies of stack → reduce → (clip) with three different reductions. Collapse to one _composite_channels(layers, channel_cmaps, alpha). (The averaging-vs-additive inconsistency in this same code is tracked separately in findings/multichannel-compositing-bugs.md; unifying the helper is the structural enabler.)
                              Sequencing / risk
                              Land the safe foundations first — CmapParams methods (#1) — then resolve_continuous_norm (#2), then the composite helper (#3). Effort: medium. Risk: low for the CmapParams methods (no baseline impact); medium for the norm/composite unification (CI baseline regen). #2 also fixes the colorbar-vs-pixels mismatch, so it's correctness, not just cleanup.
                              Part of a maintainability/refactor audit of main.
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