How to Autotrace Bitmaps in FontForge

FontForge allows you to automatically convert bitmap images into vector outlines, but it does not contain a fully native, standalone tracing engine. Instead, it provides a built-in interface that relies on external tracing utilities like Potrace or AutoTrace installed on your system. This guide covers how this integration works, what tools you need to install, and the step-by-step process to trace bitmaps into font glyphs automatically.

The Built-in Autotrace Mechanism

FontForge includes an Autotrace command under its menus, but it acts as a bridge rather than an internal algorithm. When you trigger the command, FontForge passes the bitmap data to an external program installed on your computer, waits for the vector output, and places the resulting Bézier paths into your font layer.

To use this feature, you must have one of two external programs installed and accessible in your system's PATH:

  1. Potrace: Ideal for high-contrast, black-and-white bitmap tracing. It is fast and widely supported.
  2. AutoTrace: Capable of tracing both black-and-white and color images with distinct outline options.

If neither tool is installed, FontForge will show an error indicating that the autotracing program cannot be found.

Step-by-Step Guide to Tracing Bitmaps

1. Configure the Tracing Program

Before tracing, ensure FontForge knows which engine to use:

  • Go to File > Preferences > Apps.
  • Verify that the path to Autotrace or Potrace is set correctly, or confirm that the executables are located in your system environment variables.

2. Import the Bitmap into a Glyph

  • Double-click an empty glyph slot in the Font View to open the Glyph Window.
  • Switch to the Back (Background) layer using the Layers palette.
  • Go to File > Import.
  • Set the file format filter to an image format (such as PNG, BMP, or TIFF) and select your source image.

3. Run the Autotrace Command

  • With the image positioned and scaled appropriately in the background, go to the top menu.
  • Select Element > Autotrace (or press the corresponding shortcut, typically Shift + Ctrl + T).
  • FontForge will process the image through the external utility and generate vector outlines directly in the Fore (Foreground) layer.

Tips for Clean Traces

  • Use Pure Black and White: Convert your images to high-contrast monochrome before importing. Grayscale or anti-aliased edges can produce unwanted artifacts or double outlines.
  • Simplify Paths: Traced paths often generate excessive control points. After tracing, select the outlines in the Foreground layer and go to Element > Simplify > Simplify to optimize point placement.
  • Check Direction: Font formats require specific path directions for proper rendering. Use Element > Correct Direction to ensure counter-spaces (holes in letters) display correctly.