View Source Pdf.Reader.Font.Widths (ExPDF v1.0.1)

Per-font glyph-width lookup for text advance computation.

Builds closures of type (binary() -> [non_neg_integer()]) that return, for a binary of raw font bytes, a list of glyph-space advance widths (one per glyph code encoded in the binary).

Simple fonts (Type1, TrueType)

Width lookup uses /Widths, /FirstChar, /LastChar from the font dict. Out-of-range codes fall back to /MissingWidth from /FontDescriptor, or 0 if absent. (§ 9.6.2.1, § 9.6.4)

Standard-14 fallback

When /Widths is entirely absent — typical of Standard 14 Type 1 fonts (Helvetica, Times-Roman, Courier, Symbol, ZapfDingbats and their styled variants) — the spec (§ 9.6.2.2) requires the reader to use bundled AFM metrics. We do not bundle AFMs (documented gap), so every glyph is approximated as 500 units (~0.5 em), the rough average across the Adobe Standard 14 AFM tables. This restores usable text positioning for Standard-14-only PDFs (most government forms, RFCs, simple reports); exact column alignment still requires AFM bundling.

Source: Adobe Font Metrics for the Core 14 — public domain via https://github.com/adobe-type-tools/Adobe-Core-14-Font-AFM-Files (avg glyph width across the 14 fonts ≈ 500 units).

CIDFonts (Type0 → DescendantFonts[0])

Width lookup uses /W (Form A and Form B entries) and /DW (default: 1000). (§ 9.7.4.3)

Cache

Widths closures for fonts referenced via {:ref, n, g} are cached in Document.cache under key {:font_widths, {n, g}}, mirroring the decoder cache strategy of Pdf.Reader.Font.

Spec references

PDF 1.7 § 9.4.4 — Text advance formula (tx per glyph)
PDF 1.7 § 9.6.2.1 — Simple font /Widths, /FirstChar, /LastChar
PDF 1.7 § 9.6.4 — Font descriptor /MissingWidth
PDF 1.7 § 9.7.4.3 — CIDFont /W and /DW arrays

Summary

Types

widths_fn()

Functions

build_widths_fn(font_ref, doc)

Build a widths closure for a font dict or indirect reference.

build_widths_for_resources(resources, doc)

Build widths closures for all fonts in a page's resources map.

parse_w_array(w_array)

Parse a CIDFont /W array into a %{cid => width} map.

Types

widths_fn()

@type widths_fn() :: (binary() -> [non_neg_integer()])

Functions

build_widths_fn(font_ref, doc)

@spec build_widths_fn(
  map() | {:ref, pos_integer(), non_neg_integer()},
  Pdf.Reader.Document.t()
) ::
  {:ok, widths_fn(), Pdf.Reader.Document.t()} | {:error, term()}

Build a widths closure for a font dict or indirect reference.

Returns {:ok, widths_fn, updated_doc}.

Spec: § 9.6.2.1 (simple fonts), § 9.7.4.3 (CIDFonts).

build_widths_for_resources(resources, doc)

@spec build_widths_for_resources(map(), Pdf.Reader.Document.t()) ::
  {:ok, %{required(binary()) => widths_fn()}, Pdf.Reader.Document.t()}
  | {:error, term()}

Build widths closures for all fonts in a page's resources map.

Mirrors Pdf.Reader.Font.build_decoders_for_resources/2. Returns {:ok, %{font_name => widths_fn}, updated_doc}.

When doc.recover_mode is true: on per-font widths failure, installs a zero-width fallback (all glyphs advance 0) and continues. The font_skipped event is already logged by build_decoders_for_resources; no duplicate event is emitted here.

When doc.recover_mode is false: halts on first font widths failure (unchanged strict behavior).

parse_w_array(w_array)

@spec parse_w_array([term()]) :: %{required(non_neg_integer()) => non_neg_integer()}

Parse a CIDFont /W array into a %{cid => width} map.

Supports:

Form A: c [w1 w2 …] — CID c, c+1, … each get successive widths
Form B: c1 c2 w — all CIDs from c1 to c2 inclusive get width w

Both forms may be interleaved in the same array. (§ 9.7.4.3)