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0014 — Text is shaped once, in the core, for measuring and drawing alike

Status: Accepted

Context

Layout measures text long before anything is drawn, and draws it once the page is settled. With Skia, measuring and drawing went through the same SKFont, so the two agreed by construction — but Skia chose the fallback faces, applied no kerning, and could not tell the PDF writer which glyph it had used for which character.

The managed writer (ADR 0001) needs exactly that: glyph indices to subset and to show, the characters behind them for a ToUnicode map, and advances that match what layout reserved. Any second opinion about which face sets a character, or how wide it is, puts a glyph in one place and its space in another.

Decision

Text is shaped in one place. A TypeShaper resolves a type style to a face — the registered or installed face nearest its weight and slant, else a common substitute for its kind, else any registered face — and walks text as glyphs, choosing a fallback face for each character the face lacks. The GlyphWalk it returns is the only way text is measured, fitted or drawn:

  • the measurer sums the walk's advances, pair kerning and tracking;
  • fitting stops the same sum early, so a prefix it accepts measures within the width;
  • the PDF surface shows the walk's glyphs, positioned by the font's widths, with kerning as text-array adjustments and tracking as character spacing.

Kerning from the fonts' GPOS and kern tables is on. Faces are embedded as Type 0 fonts over CID fonts: TrueType subset to the glyphs used, numbered in order of first use; CFF whole. Every glyph remembers the character it was first used for, and the ToUnicode map records it.

Typefaces are registered with a TypefaceLibrary. A registered typeface shadows an installed one of the same name, so a document whose typefaces are registered comes out the same everywhere; Fallbacks names the typefaces tried first for a missing character. A typeface nobody has is substituted, as a desktop publisher would, rather than refusing the document; only when no face at all is available does export fail.

Consequences

  • Measuring and drawing cannot disagree, and kerned text now matches what print tools set.
  • Every future backend — page images through Skia, or anything else — draws from the same walk and inherits the same layout, instead of shaping text its own way.
  • The walk is where the text engine grows (phase 3): ligatures and other GSUB features, mark positioning, bidirectional reordering and complex scripts change what the walk yields, not its callers.
  • Export streams pages out as they finish, so a failed export to a caller's stream leaves part of a document there; export to a path writes beside the target and moves into place.