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Apply brightness/contrast in perceptual space on GIMP 3 - #58
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Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
WalkthroughThe brightness and contrast adjustment now builds a perceptual-space LUT in Python and applies it to the VALUE channel through ChangesBrightness and contrast adjustment
Priority: ⬇️ Low Estimated code review effort: 2 (Simple) | ~10 minutes Change: Bug fix Merge Risk: 🟡 Moderate · up to Brightness and contrast requests can fail instead of applying the requested adjustment until the API call is corrected. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In `@gimp-mcp-plugin.py`:
- Around line 1881-1902: Update the drawable.curves_explicit call in the
brightness/contrast adjustment to pass the LUT point count between the VALUE
channel and lut, using len(lut), while preserving the existing failure handling
and undo-group behavior.
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Problem
adjust_brightness_contrastcallsgimp-drawable-brightness-contrast, which on GIMP 3 applies the formula in linear light. Even small values badly distort the image. On GIMP 3.2.6, with a 0–255 gray gradient at brightness +8 / contrast +12:The same happens on a 16-bit linear image.
Fix
Apply GIMP's own brightness/contrast formula (
valueshifted bybrightness / 2, then(value - 0.5) * tan((contrast + 1) * π/4) + 0.5) as a 256-entry LUT throughGimp.Drawable.curves_explicit()on the VALUE channel.curves_explicitworks in perceptual space. I checked this with a threshold LUT on both an 8-bit non-linear and a 16-bit linear image: the output flips at input 128 in both cases, where linear would flip near 188.The public tool signature and the −127..127 ranges are unchanged. A failed
curves_explicitcall now raises instead of passing silently.Verification
Tested on GIMP 3.2.6 (Windows 11) through the MCP tool, using throwaway 256×1 gradients (0–255), comparing every output pixel to the formula computed in perceptual space:
The ±1 at the brightness extremes comes from LUT interpolation.
Also checked:
python -m py_compilepassesNot run:
ruff checkandrun_tests.py.🤖 Generated with Claude Code
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