Module: Fontisan::Ufo::Compile::Filters::RemoveOverlaps

Defined in:
lib/fontisan/ufo/compile/filters/remove_overlaps.rb

Overview

Boolean union of overlapping contours within each glyph. Two contours that overlap visually are merged into one, removing the seam. Required for some hinting programs and for SVG/COLR rendering correctness.

Why bounding-box approximation, not polygon clipping

Full polygon clipping (Greiner-Hormann or Vatti) operates on straight-line polygons. UFO contours are Bezier curves. Applying polygon clipping would require:

1. Flatten Bezier to polygon edges (tessellation).
2. Run polygon clipping (union).
3. Re-fit Bezier curves on the result (curve fitting).

Step 3 is the killer: curve fitting is lossy. The output curves won't match the input curves' control points, changing the glyph's rendering at small sizes. For a font pipeline, preserving curve fidelity matters more than mathematical union correctness.

The bounding-box approximation handles the common cases (overlapping duplicates, fully-contained sub-contours) without losing any curve data. It does NOT handle partial overlaps — those remain as separate contours. For fonts that need partial-overlap removal, preprocess in an editor (FontLab, Glyphs) or use the clipper gem as a pre-compile step with explicit curve-flattening tolerances.

Class Method Summary collapse

Class Method Details

.run(glyphs, **_opts) ⇒ Array<Fontisan::Ufo::Glyph>

Returns the same array, mutated in place.

Parameters:

Returns:



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# File 'lib/fontisan/ufo/compile/filters/remove_overlaps.rb', line 38

def self.run(glyphs, **_opts)
  glyphs.each { |g| remove_overlaps_in_glyph(g) }
  glyphs
end