Charstring

Charstring is a term used in digital typography to denote the program or string of instructions that describes the outline of a glyph in many PostScript-derived font formats. In this context, a charstring is a compact sequence of numbers and drawing operators that the font rasterizer executes to render the glyph.

In Type 1 fonts, the charstring is a stack-based language that uses operators like moveto, lineto, curveto, and closepath, along with relative variants such as rmoveto and rlineto. The operand numbers specify coordinates, radii, and control points; an endchar operator marks the end of the glyph description.

Compact Font Format (CFF) uses Type 2 charstrings, which retain the same geometric primitives but employ a different operator set and subroutine mechanism to compress data. Type 2 charstrings support subroutines and local/global hints; they can be nested. They include endchar and a variety of curve and line operators, and may incorporate specialized operators such as flex for efficient representation of curves.

Rendering and usage: When a font is loaded, each glyph's charstring is interpreted by the font engine to construct the outline. Charstrings can be optimized by font editors and can be converted between Type 1 and CFF in OpenType fonts. Modern fonts commonly store outlines in CFF within OpenType fonts on many platforms, while Type 1 still appears in some older font families. The charstring model remains central to how scalable outlines are defined and rendered across different font technologies.