FontForge Overlapping Paths During Font Generation

When generating a font in FontForge, leaving overlapping black paths unresolved can lead to unexpected rendering bugs, inverted transparent spaces, and rasterization errors in target applications. Font renderers rely on mathematical fill rules and contour directions to determine which areas are solid black and which are empty space. Leaving overlapping contours unmerged forces the operating system or browser to interpret the intersections, often producing visual glitches or failing font validation tests.

Fill Rules and the "White Hole" Effect

Font rendering engines primarily rely on two algorithms: the Non-Zero Winding Rule and the Even-Odd Rule.

  • Non-Zero Winding Rule: The renderer determines whether a space is filled by checking the direction (clockwise or counter-clockwise) of the bounding path. If two overlapping shapes have opposing directions, the overlapping region cancels out, turning into an accidental transparent hole.
  • Even-Odd Rule: The renderer draws a ray from a point to infinity and counts how many path segments it crosses. An odd number fills the area; an even number leaves it empty. Under this rule, two overlapping black shapes will invert the intersection area into an unintended white knockout, regardless of path direction.

Differences Between TrueType and OpenType (CFF)

The consequences of overlapping geometry depend heavily on the export format:

  • OpenType-CFF (PostScript): PostScript outlines historically prohibit self-intersecting paths and overlapping contours within a glyph. While modern PostScript engines handle overlaps better than legacy RIPs (Raster Image Processors), exporting overlaps in CFF fonts frequently triggers rasterization artifacts, miscalculated anti-aliasing on screens, or rejection by professional print engines.
  • TrueType (TTF): The TrueType specification permits overlapping contours, and modern variable fonts even rely on overlapping components to keep point counts consistent across design axes. However, in static fonts, overlapping paths can still interfere with automatic hinting, lead to inconsistent outlines at small point sizes, and produce inconsistent rendering across different operating systems.

FontForge Export Warnings

When you generate a font (File > Generate Fonts) with unresolved overlaps, FontForge typically runs a pre-flight validation check. If overlapping paths exist, the program will flag an alert regarding:

  • Intersecting paths
  • Self-intersecting contours
  • Missing or incorrect path directions

Ignoring these warnings generates the file with the raw, separate paths preserved, leaving the interpretation entirely to the end-user’s rasterizer.

How to Resolve Overlaps in FontForge

To ensure predictable rendering across all platforms, static font glyphs should be flattened into single, unified outlines prior to generation:

  1. Select the Contours: Open the glyph in the outline view and select all overlapping elements (Ctrl + A).
  2. Apply Remove Overlap: Navigate to Element > Overlap > Remove Overlap (or press Ctrl + Shift + O). FontForge will calculate the boolean union of the paths, merging intersecting shapes into a single boundary and eliminating interior nodes.
  3. Correct Direction: Navigate to Element > Correct Direction (or press Ctrl + Shift + D). This ensures that external paths run in the standard direction and internal cutouts (such as the counter in an "O" or "P") run in the opposite direction.
  4. Simplify Points: Merging overlapping paths can create redundant anchor points at intersections. Run Element > Simplify > Simplify (Ctrl + Shift + M) to remove unnecessary points and clean up the path geometry.