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Merlion

Text measurement

Layout needs the size of every label before it can place anything. Merlion measures labels from committed font metric tables, never from a DOM, so output is identical on every target.

Font tables

Section titled “Font tables”
  • Default font: Inter (SIL Open Font License 1.1, no Reserved Font Name). The tables cover the Regular and SemiBold weights.
  • A table holds each glyph’s advance width in font units (keyed by Unicode code point), the pair kerning values, unitsPerEm, ascender, descender and line gap.
  • Coverage: Latin, Latin Extended-A/B, Greek, Cyrillic, general punctuation, arrows and box drawing. A code point outside the table is measured at 1 em if it falls in the CJK, Hangul or fullwidth ranges, and at the table’s average advance width otherwise. Each fallback adds an Info diagnostic I010 UnmeasuredGlyph once per diagram.
  • tools/fontgen generates the tables into const Rust arrays in the core. The generated source is committed, together with the input font’s SHA-256 and the OFL text. The generator is the only place a font-parsing crate is used, and it is never published (supply-chain.md).

Measuring

Section titled “Measuring”
  • Width = sum of advance widths + kerning adjustments, scaled to the font size (default 14 px).
  • Height = (ascender − descender + line gap), scaled, per line. A label’s height is the sum of its line heights and its width is its widest line.
  • Title + detail node labels (svg-output.md) measure the title line at the font size and every detail line with the same tables at 0.8 × the font size; a detail line’s height and baseline offset scale by the same factor. The last title line is 2 px taller at 14 px (1/7 of the font size), which leaves a gap between the tiers. Every other label measures all lines at the font size.
  • Labels wrap at a maximum width (default 200 px) on whitespace, with a hard break inside any word longer than the maximum. Explicit <br> in a label forces a break.
  • Stylesheets never set font tokens (svg-output.md), so measurement depends only on the source and the render options; font size is RenderOptions.font_size.
  • Padding comes from the node shape (see layout.md), never from the text measure.
  • Measurement applies pair kerning and no other OpenType feature. The browser draws the same way because the SVG’s embedded style disables contextual alternates and ligatures (Inter’s calt substitutes glyphs in sequences such as ->) and resets letter-spacing, word-spacing, text-transform and the font weight and stretch that the host page would otherwise pass down (svg-output.md). tools/fontgen reads advances with the same features off.

Serving the font

Section titled “Serving the font”

The SVG must be drawn in the font that was measured, or labels overflow their boxes. The renderer outputs one of these, chosen by the font option:

font Output Use
"link" (default) Nothing inline. font-family: var(--merlion-font, Inter, …); the host page loads Inter Sites that already serve Inter, or that include merlion-font.css from @fractalbox/merlion-themes: two @font-face rules for the committed subsets under the family Merlion Inter (so a site’s own Inter is never replaced) and --merlion-font set to it on :root. @fractalbox/merlion-astro adds that stylesheet by default and passes fontCss: true; the rehype plugin warns once per build when font: "link" is used and its fontCss option is not set, because a page without Inter draws in the fallback font and labels overflow
"embed" The committed WOFF2 subset (covering exactly the table’s coverage ranges) as a data: URI inside the SVG’s <style> Standalone SVG files
"system" Metrics for a system-font stack (system-ui) at a declared tolerance Accepted drift of up to ±6% in label width; boxes get matching extra padding

The core does not subset fonts at render time; that would need font parsing and a Brotli encoder in the runtime path. tools/fontgen produces the WOFF2 subset once, at development time, and commits it next to the metric tables. The file’s size is unmeasured; TODO(owner): record it once generated, and add per-diagram subsetting only if the size proves to matter.

The OFL counts a subset as a Modified Version. That is permitted because Inter declares no Reserved Font Name, so the subset keeps the name “Inter”. The OFL notice travels with the subset file and, in "embed" mode, inside the SVG as an XML comment (licensing.md).