What Happens with Open Paths in FontForge Export

When you export a font with open paths from FontForge, the resulting font files will typically suffer from rendering errors, unexpected visual artifacts, or validation failures. Standard outline font formats such as TrueType (TTF) and OpenType (OTF) require completely closed contours to determine which areas of a glyph should be filled with ink and which should remain transparent. Leaving a contour open breaks the mathematical rules used by font renderers, leading to automatic path closing, distorted glyph shapes, or dropped characters across different operating systems and applications.

FontForge Export Warnings

During the font generation process (File > Generate Fonts), FontForge runs an automated validation check. If open contours exist in any glyph, the program will trigger a warning dialog stating that the font contains errors, specifically listing "Open contour" issues. While FontForge allows you to ignore these warnings and force the export, doing so leaves the structural defects intact within the binary font file.

Automatic Closure and Visual Artifacts

Digital outline fonts do not support native "strokes" with open ends; they only support filled shapes defined by winding rules (Non-Zero Winding or Even-Odd). When an open path is exported:

  • Direct Line Closure: FontForge or the target rasterizer will automatically draw an invisible straight line from the path's end point back to its start point to force the shape closed.
  • Unintended Fills: Any area caught between the open endpoints and the rest of the contour will be filled with solid color, creating unwanted triangular wedges, spikes, or blocked-out cavities where negative space was intended.
  • Inconsistent Direction: Open contours often fail direction checks (clockwise vs. counter-clockwise), which can invert the fill logic and turn solid elements transparent or vice versa.

Inconsistent Platform Rasterization

Different operating systems and graphics engines handle malformed fonts unpredictably:

  • FreeType (Linux, Android): May attempt to close the contour automatically, leading to the visual artifacts described above.
  • DirectWrite (Windows): Can produce severe rendering glitches, drop the affected glyph entirely, or substitute it with a fallback font.
  • Core Text (macOS, iOS): Strict validation in Apple environments frequently results in the glyph failing to draw on the screen, showing up as a blank space or a missing-glyph box ("tofu").

Rejection by Web and Print Tools

Open paths cause technical failures in modern font deployment pipelines:

  • Web Font Compilers: Tools such as woff2_compress or Google’s font sanitation libraries (OTS / OpenType Sanitizer) analyze font tables before loading. If an open contour creates invalid geometry, the browser will classify the web font as corrupt and refuse to load it.
  • Print Processors: Raster Image Processors (RIPs) used in commercial printing and PDF conversion utilities often fail to interpret unclosed vector paths, which can halt print jobs or introduce severe vector distortions on the final printed page.

How to Resolve Open Paths in FontForge

To prevent these issues, all open paths must be addressed before generating the final font files:

  1. Locate Errors: Run Element > Find Problems, check the Open paths option under the Paths tab, and click Find to cycle through affected glyphs.
  2. Close Outlines: If the contour is meant to be a solid shape, select the path and choose Element > Paths > Close Path.
  3. Expand Single Strokes: If the design relies on single-line strokes (such as an open line for the stem of a letter), select the stroke and choose Element > Expand Stroke to convert the line into a closed, double-sided vector outline suitable for export.