Fix select_by_color silently no-opping on GIMP 3.2

[AnthonyHerman]

Summary

select_by_color looks up the PDB procedure gimp-by-color-select, which does not exist on GIMP 3.2 — it was renamed to gimp-image-select-color. The current code guards the call with if proc: and silently falls through to return {"status": "success", ...} when the lookup returns None, so no selection is ever created, but the caller is told the operation succeeded.

This is dangerous because downstream tools assume a selection exists. In particular, fill_selection with fill_type='transparent' calls Gimp.Drawable.edit_clear(drawable), which clears the entire layer when no selection is active. So a typical "remove black background" workflow

add_alpha → select_by_color('#000000') → fill_selection(transparent)

silently wipes the whole image instead of just the BG.

Fix

• Use the GIMP 3.2 procedure name gimp-image-select-color.
• Raise RuntimeError when the lookup returns None instead of silently no-opping, so future renames are caught immediately.
• The new procedure config exposes only image / drawable / color / operation; threshold, antialias, feather, and sample-merged are read from the GIMP context. Set them via Gimp.context_* inside a context_push() / context_pop() pair so we don't leak settings into the rest of the session.

How I hit this

Running an MCP-driven "remove black background, export transparent PNG for a black t-shirt print" pipeline on a JPEG. After select_by_color('#000000', threshold=30) followed by fill_selection(fill_type='transparent'), every pixel ended up with alpha=0 (RGB preserved). Confirmed with pdb.procedure_exists('gimp-by-color-select') → False and pdb.procedure_exists('gimp-image-select-color') → True on GIMP 3.2.2.

Note on existing test coverage

run_tests.py:92 exercises select_by_color, but only checks that the response status is success. Since the buggy path returns success without doing anything, the test passes against a broken plugin. Worth a follow-up to assert the selection is non-empty (e.g. via Gimp.Selection.is_empty()), but I left test changes out of this PR to keep the diff minimal.

Test plan

• On GIMP 3.2.2, run select_by_color({'color': '#000000', 'threshold': 30}) on a black-background image and verify the selection bounds are non-empty (Gimp.Selection.bounds(image) returns non_empty=True).
• Run fill_selection({'fill_type': 'transparent'}) after the select and confirm only the BG pixels go to alpha=0; foreground pixels keep alpha=1.0.
• ruff check gimp-mcp-plugin.py — clean.
• Maintainer sanity-check on a non-snap / different-distro GIMP 3.2 build.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Refactor
  • Improved the "Select by Color" tool's internal implementation for better stability and consistency.
  • Enhanced error handling to explicitly fail when the required procedure is unavailable.
  • Refined selection parameter handling with optimized defaults (antialias enabled, feather disabled, integer-based threshold).

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

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Run ID: e95e52bc-98e7-4df3-acb6-839294957c51

📥 Commits

Reviewing files that changed from the base of the PR and between 1e8ef90 and c14ec61.

📒 Files selected for processing (1)

• gimp-mcp-plugin.py

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Walkthrough

The _select_by_color function is refactored to use the GIMP PDB procedure gimp-image-select-color instead of gimp-by-color-select, with selection parameters moved from config properties to context settings managed via Gimp.context_push()/pop().

Changes

Color Selection API Update

Layer / File(s)	Summary
Procedure Call & Error Handling gimp-mcp-plugin.py (lines 2379–2398)	_select_by_color switches from gimp-by-color-select to gimp-image-select-color with explicit error if the procedure is unavailable.
Context Management gimp-mcp-plugin.py (lines 2379–2398)	Call is wrapped in Gimp.context_push()/pop() to isolate selection sampling settings (antialias enabled, feather disabled, integer threshold, merged/transparent sampling disabled).
Config Payload gimp-mcp-plugin.py (lines 2379–2398)	Procedure config is simplified to pass only image, drawable, color, and operation; prior properties (threshold, antialias, feather, feather-radius, sample-merged, sample-transparent) are removed.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main fix: addressing the silent no-op behavior of select_by_color on GIMP 3.2 by switching to the correct PDB procedure name.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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