Purpose of the Knife Tool in FontForge

The Knife tool in FontForge is a specialized vector-editing utility designed to slice contours and separate glyph paths. This article explains the primary function of the Knife tool, how it alters vector outlines, common typographic use cases such as creating stencil lettering, and the practical mechanics of using it within the glyph editing window.

Primary Function of the Knife Tool

At its core, the Knife tool cuts through existing vector paths. When you drag the Knife tool across an outline in the Glyph Outline View, FontForge calculates the exact intersection points between your cut line and the glyph's contours. It then inserts new on-curve points at those intersections and breaks the continuous path, dividing a single closed or open contour into separate segments.

Common Use Cases

  • Creating Stencil and Segmented Fonts: The most frequent design use for the Knife tool is slicing solid glyphs to create bridges or gaps, which is essential for stencil typography, inline effects, or fragmented letterforms.
  • Separating Overlapping Components: When importing vector artwork or auto-tracing scanned drawings, shapes often merge undesirably. The Knife tool allows you to manually bisect interconnected paths so they can be repositioned, deleted, or assigned to different layers.
  • Converting Closed Paths to Open Paths: Standard drawing tools in FontForge naturally produce closed contours for font rendering. Slicing a contour with the Knife tool opens the path, allowing you to manipulate stroke endpoints individually.
  • Cleaning Up Problematic Intersections: Complex path intersections can cause rendering errors or validation warnings during font generation. Slicing complex geometry allows you to discard unwanted segments manually before joining clean endpoints.

How to Use the Knife Tool

  1. Open a glyph in the Outline View.
  2. Select the Knife tool from the Tools palette (represented by a knife icon).
  3. Click and drag a line across the path or paths you want to cut.
  4. Release the mouse button. FontForge immediately places new points at the intersection boundaries and severs the path.
  5. Switch back to the Pointer tool to select, move, or delete the newly separated path segments.