Concepts

A font is the compiled file a system installs: a .ttf or .otf. It is a build artefact. You do not edit it any more than you edit a compiled program.

A source is what you draw. Runebender’s sources are UFO directories and a designspace file that names them. A UFO is a directory of small text files, so a source belongs in version control. A day’s drawing reads as a reviewable diff, and a mistake is one git checkout away.

Everything in a font is measured in a grid called the em, and the grid’s resolution is units per em, or UPM. Runebender’s new fonts use 1000; many use 1024 or 2048.

Coordinates are units, not pixels. A stem 80 units wide is 80/1000 of the em, so it stays the same weight whether the text is set at 9pt or 90pt. That is the whole point of the unit: the drawing is resolution independent, and the size is applied later.

Change UPM only at the start of a project. It does not rescale anything already drawn.

A glyph is made of contours: closed paths of points.

Point types

On-curve	The outline passes through it.
Off-curve	A control point, or handle, that bends the segment beside it without being on it.

Two curve mathematics are in use. Cubic curves have two handles per segment and are what UFO and PostScript outlines use. Quadratic curves have one, and are what TrueType uses. A compiler converts between them; Runebender can too.

Path direction matters. The outer contour and the counter inside it must run opposite ways, or the fill rule cannot tell the hole from the shape.

A variable font holds a range rather than a single drawing. You draw the ends, called masters, and every weight between them is calculated.

The masters must be compatible for that to work. Same number of contours, same number of points in each, same order, same start point. The computer pairs point one with point one and moves it partway. If the counts differ, there is no pairing and no interpolation.

This is the single fact that governs how variable fonts are drawn. It is why you draw the heavy weight by modifying a copy of the light one rather than starting fresh, and why Checking a Font exists.

The pieces of a designspace

Axis	A dimension of variation: weight, width, optical size. It has a range and a default.
Master	A drawing at a fixed position. The ends of the range, usually.
Instance	A named position, such as SemiBold. It is a label on a coordinate, not a drawing.
Intermediate	A correction for one glyph at one position, where the straight interpolation is wrong.

An axis has two coordinate systems. Design space is where your masters sit, and user space is what a document asks for, such as weight 700. An avar mapping bends one onto the other, which is how the midpoint of a slider can be made to look like the middle rather than measuring like it.

Typing produces characters. A font produces glyphs. Shaping is the step between: an engine reads the characters, applies the font’s own rules, and returns positioned glyphs.

For Latin, that is often close to one character per glyph. For Arabic, Devanagari and other complex scripts it is not. A letter changes form with its neighbours, marks are positioned against anchors, and sequences merge into ligatures. Those rules are feature code, and the editor runs them live so you see the result while drawing rather than after compiling.

Most accented glyphs are not drawn. They are composites: a reference to a base letter plus a reference to a mark, held together by anchors. Change the base and every glyph built on it follows.

A font can reach a few hundred glyphs of Latin coverage from a few dozen drawings this way. See Components and Anchors.