Why FontForge Shows Red Spline Point Warnings

FontForge displays red warning highlights on spline points to alert designers to geometry, coordinate, or validation issues that can cause rendering defects or font compilation errors. This article explains the primary causes of these red indicators—such as non-integral coordinates, missing extrema, and control handle anomalies—and provides straightforward methods to resolve them for clean, production-ready outlines.

Non-Integral Coordinates

The most frequent cause of red highlights on spline points is fractional (non-integer) positioning. Font formats such as TrueType and PostScript standard outlines rely on integer coordinates on the design grid. When a point or control handle lands on a decimal value (for example, X: 120.45 instead of 120), FontForge flags the point with a red warning.

To fix this:

  • Select the affected points or press Ctrl + A to select the entire glyph.
  • Navigate to Element > Round to Int. FontForge will snap all points and off-curve handles to the nearest whole integer on the em-square grid.

Missing or Misplaced Extrema

Proper font rendering engines require that all curve extremes (the highest, lowest, leftmost, and rightmost points of a curved spline) have an on-curve point located exactly at the apex. If a curve bulges past its bounding points without an on-curve anchor at the maximum or minimum, FontForge highlights the region in red.

To fix this:

  • Select the curve segment.
  • Navigate to Element > Add Extrema (or press Ctrl + Shift + X). This automatically places on-curve points at the required locations without altering the visual shape of the contour.

Control Handle and Direction Vector Issues

Red highlights also indicate invalid Bezier handle configurations that violate font geometry standards. Common handle defects include:

  • Overlapping Points: An off-curve control point occupying the exact same coordinate as an on-curve point, effectively creating a zero-length handle.
  • Opposing Handles: Direction handles that cross over themselves, causing microscopic loops or self-intersections within the spline.
  • Collinear Inconsistencies: Handles intended to form smooth curve transitions that are misaligned by tiny fractions, breaking the tangent between adjacent splines.

You can inspect and correct these issues manually by dragging the handles with the Pointer tool or by selecting the point and checking Point > Get Info to inspect the numerical handle offsets.

Automated Glyph Validation

FontForge includes a real-time validation engine that continuously checks glyphs against standard OpenType specifications. If real-time validation is enabled, any geometry that fails a built-in check—such as wrong contour direction (clockwise vs. counter-clockwise) or open paths—will be visually flagged in red.

To run a diagnostic on all red points:

  • Go to Element > Validation > Find Problems.
  • Check the criteria you wish to audit (such as "Missing Extrema", "Self Intersecting", and "Check Open Paths").
  • Click OK to let FontForge guide you step-by-step through every flagged spline point with recommendations for automated correction.