Fix Cross-Platform Font Rendering in FontForge

Fonts exported from FontForge often render inconsistently across Windows, macOS, and Linux due to divergent font rendering engines, mismatched vertical metrics, and conflicting outline standards. This guide details how to resolve these cross-platform discrepancies directly within FontForge by standardizing vertical metrics, cleaning up glyph geometry, configuring proper hinting, and aligning OpenType and TrueType table settings.

1. Synchronize Vertical Metrics

Windows and macOS read different vertical metric tables, causing fonts to display with different line heights or unwanted clipping across platforms. To ensure consistent baseline alignment and line spacing:

  1. Navigate to Element > Font Info > OS/2 > Metrics.
  2. Set the three primary metric sets to match harmoniously:
    • OS/2 Typo Metrics: Set Typo Ascent and Typo Descent to match your intended design grid. The sum of TypoAscent + abs(TypoDescent) + TypoLineGap should equal your target line height.
    • Windows Metrics: Set Win Ascent and Win Descent to represent the absolute highest and lowest points of all glyphs (including diacritics) to prevent clipping in Windows applications.
    • Mac Metrics (HHead): Set HHead Ascent and HHead Descent to equal the Win Ascent and Win Descent (or Typo Ascent and Typo Descent, depending on whether you follow modern web metrics standards).
  3. Check the Use Typo Metrics box (sets bit 7 in fsSelection). This forces modern Windows applications to respect the Typo metrics instead of relying solely on the Win metrics.

2. Correct Path Directions and Overlaps

Different operating systems and formats expect specific path directions. TrueType outlines require clockwise exterior contours, while PostScript (OpenType-CFF) outlines require counter-clockwise contours. Incorrect contour directions cause glyphs to render hollowed out or with missing fills.

  1. Select all glyphs (Edit > Select > Select All).
  2. Go to Element > Correct Direction to let FontForge orient paths automatically based on the target format.
  3. Remove internal overlaps by choosing Element > Overlap > Remove. Merged outlines prevent rendering artifacts in rasterizers that handle non-zero winding rules poorly.
  4. Round coordinates to integers via Element > Round > To Int. Fractional coordinates frequently cause fuzzy rendering or anti-aliasing artifacts on low-DPI displays.

3. Handle Hinting Correctly

Windows uses ClearType (a sub-pixel hinting engine), Linux uses FreeType, and macOS relies almost entirely on CoreText (which largely ignores hinting in favor of outline fidelity). Bad hints will severely distort outlines on Windows while appearing fine on macOS.

  • For TrueType (TTF): Avoid using FontForge’s native TrueType autohinter, as it produces inconsistent results across modern Windows renderers. Instead, export the unhinted TTF from FontForge and process it through a specialized tool like ttfautohint.
  • For OpenType-CFF (OTF): Use FontForge’s PostScript hinting by selecting all glyphs and clicking Hints > Autohint. OpenType PostScript hints are simpler and typically render more consistently across platforms.

4. Resolve Name Table Inconsistencies

Operating systems parse the name table differently, causing fonts to show up under incorrect weights, styles, or families.

  1. Open Element > Font Info > TTF Names.
  2. Ensure the Family, Subfamily, Full Name, and PostScript Name fields contain no special characters or trailing spaces.
  3. Maintain the standard 4-style model (Regular, Italic, Bold, Bold Italic) in the primary name entries, and use the Preferred Family (Typographic Family) and Preferred Styles fields for extended weights (e.g., Light, Medium, Black).

5. Validate the Font Before Export

Before generating the final font files, run the built-in validator to catch structural errors that different operating systems handle unpredictably:

  1. Select all glyphs and go to Element > Validation > Find Problems.
  2. Check for missing points at extrema, self-intersecting paths, and flipped references.
  3. Fix all detected errors, then export your binaries via File > Generate Fonts, choosing either OpenType (CFF) or TrueType depending on your target distribution.