Spacing and leading

Once a run is shaped, the engine layers on the space adjustments it requests — Tracking between glyphs, Leading between baselines, and BaselineShift on the glyphs themselves — while kerning comes from the font, not from an IDML attribute.

In short: After a run's face is resolved and its glyphs are shaped, the engine applies the per-run spacing the CharacterStyleRange asks for. Tracking adds uniform letter spacing measured in 1/1000 em; Leading sets the baseline-to-baseline step in points, defaulting to Auto (the largest point size on the line × 1.2) when absent; and BaselineShift nudges glyphs off the baseline without changing the line step. Kerning is not an IDML amount — it comes from the font during shaping, with a KerningMethod switch choosing which kerning to apply and a Ligatures switch for standard ligatures.

Once a run's face is resolved and its glyphs are shaped, the engine layers on the per-run space adjustments the CharacterStyleRange requests: horizontal spacing between glyphs (Tracking), the vertical step between baselines (Leading), and a vertical nudge of the glyphs themselves (BaselineShift). Kerning is in this picture too, but it arrives differently — it comes from the font, not from an IDML attribute.

Tracking — uniform letter spacing

Tracking adds the same amount of space to every glyph's advance in the run. Its unit is 1/1000 em — InDesign's convention — so the space added per glyph scales with the type size. The engine converts it to points as tracking ÷ 1000 × point_size and adds that to each glyph's horizontal advance. At 12 pt, a Tracking of 100 adds 100 ÷ 1000 × 12 = 1.2 pt to every advance; a negative value tightens the run.

Tracking is a post-shape adjustment: it changes advances after shaping, so it does not disturb the shaping decisions themselves — kerning pairs and ligatures are already chosen by the time tracking is applied. The composer measures runs with tracking included, so column fit and line-breaking see the tracked widths.

Leading — the baseline step

Leading is the vertical distance from one line's baseline to the next, in points. It is carried as a typed child of the run's Properties element (like AppliedFont), and the parser reads it from there.

When Leading is absent, the line uses Auto leading, which the engine computes as point size × 1.2. When a line mixes runs of different sizes, the auto value is driven by the largest point size on the line, so the tallest run sets the line's vertical rhythm. (InDesign exposes the 1.2 factor as a document-level AutoLeading percentage — 120% by default; the engine uses the 120% default and does not yet read a per-document override of it.)

Baseline shift

BaselineShift moves the run's glyphs off the baseline without changing the line step: positive lifts them up, negative drops them down, measured in points. The engine applies it per glyph at emit time, so it stacks cleanly on top of shaping and tracking. It is the mechanism behind manually raised footnote markers, small adjustments to inline symbols, and the like.

Kerning comes from the font

There is no IDML attribute that carries a kerning amount between glyph pairs. Kerning is a property of the font, applied during shaping: the engine lets the shaper apply the face's OpenType kern data by default. What IDML does expose is a KerningMethod that selects which kerning to use, plus a Ligatures switch:

• KerningMethod="Metrics" (the default) uses the font's built-in pair kerning.
• KerningMethod="Optical" asks InDesign to compute kerning from glyph outlines instead. The engine does not have an optical-kerning pass yet, so it falls back to Metrics — the two are tracked as distinct so an optical pass can land later without disturbing anything else.
• KerningMethod="None" disables kerning for the run entirely.
• Ligatures="false" turns off standard ligatures (liga / clig); they are on by default.

Parsed, not yet renderedKerningMethod="Optical" — parsed and distinguished from Metrics, but no optical-kerning pass exists yet; it renders as Metrics. core/crates/paged-text/src/shape.rs

A note on what is not here: IDML carries a long tail of OpenType and CJK spacing controls — discretionary ligatures, contextual alternates beyond the defaults, fractions, and the mojikumi/aki spacing tables for Japanese — that this engine does not expose as switches. Standard ligatures and metric kerning are the two font-driven features it applies; the rest of the OpenType feature surface is not in scope for the current shaper.

Frequently asked questions

What unit is Tracking measured in, and how does it scale? Tracking is in 1/1000 em, InDesign's convention, so the space added scales with the type size. The engine converts it to points as tracking ÷ 1000 × point_size and adds that to each glyph's advance — at 12 pt, a Tracking of 100 adds 0.1 × 12 = 1.2 pt per glyph; a negative value tightens the run.

What is Auto leading, and how does the engine compute it? When Leading is absent, the line uses Auto leading, which the engine computes as point size × 1.2 (InDesign's 120% default). When a line mixes sizes, the auto value is driven by the largest point size on the line. The engine uses the 120% default and does not yet read a per-document AutoLeading override.

Where does kerning come from if there is no IDML kerning attribute? Kerning is a property of the font, applied during shaping. The engine lets the shaper apply the face's OpenType kern data by default. IDML's KerningMethod selects which kerning to use: Metrics (the default) uses the font's pair kerning, None disables it, and Optical is parsed but currently falls back to Metrics because no optical-kerning pass exists yet.

Does the engine apply OpenType features like discretionary ligatures or fractions? No. Standard ligatures (liga/clig, on by default and toggled by Ligatures) and metric kerning are the two font-driven features it applies. The longer tail — discretionary ligatures, contextual alternates beyond the defaults, fractions, and the Japanese mojikumi/aki spacing tables — is not in scope for the current shaper.