Class: Pdfrb::Layout::FontFallback

Inherits:
Object
  • Object
show all
Defined in:
lib/pdfrb/layout/font_fallback.rb

Overview

Glyph-level fallback: given a primary font and a chain of fallback fonts (default CJK, Symbol, etc.), picks the first font that can render the missing glyph. Returns the chosen font name or nil if none can render.

Coverage lookup uses the Standard 14 AFM glyph-name sets for built-in fonts (Helvetica, Times-Roman, Courier, Symbol, ZapfDingbats), and falls back to WinAnsi range coverage for any other font.

Constant Summary collapse

DEFAULT_CHAIN = 
%w[Helvetica Times-Roman Courier Symbol].freeze
STANDARD14_GLYPH_NAMES =

Common Standard 14 AFM character-name sets, indexed by font name. Each is a Set of glyph names the font defines. A codepoint is "covered" if its Unicode-to-glyph-name lookup yields a name in the set. Subset of full AFM data — covers the Latin, Greek, and punctuation ranges needed for typical prose.

{
  "Helvetica" => %w[
    space exclam quotedbl numbersign dollar percent ampersand
    quoteright parenleft parenright asterisk plus comma minus
    period slash zero one two three four five six seven eight
    nine colon semicolon less equal greater question at A B C D
    E F G H I J K L M N O P Q R S T U V W X Y Z bracketleft
    backslash bracketright asciicircum underscore quoteleft a b
    c d e f g h i j k l m n o p q r s t u v w x y z braceleft
    bar braceright asciitilde
    Agrave Aacute Acircumflex Atilde Adieresis Aring AE Ccedilla
    Egrave Eacute Ecircumflex Edieresis Igrave Iacute Icircumflex
    Idieresis Eth Ntilde Ograve Oacute Ocircumflex Otilde Odieresis
    multiply Oslash Ugrave Uacute Ucircumflex Udieresis Yacute
    Thorn germandbls agrave aacute acircumflex atilde adieresis
    aring ae ccedilla egrave eacute ecircumflex edieresis igrave
    iacute icircumflex idieresis eth ntilde ograve oacute
    ocircumflex otilde odieresis divide oslash ugrave uacute
    ucircumflex udieresis yacute thorn ydieresis
  ].to_set.freeze,
  "Times-Roman" => %w[
    space exclam quotedbl numbersign dollar percent ampersand
    quoteright parenleft parenright asterisk plus comma minus
    period slash zero one two three four five six seven eight
    nine colon semicolon less equal greater question at A B C D
    E F G H I J K L M N O P Q R S T U V W X Y Z bracketleft
    backslash bracketright asciicircum underscore quoteleft a b
    c d e f g h i j k l m n o p q r s t u v w x y z braceleft
    bar braceright asciitilde
  ].to_set.freeze,
  "Courier" => %w[
    space exclam quotedbl numbersign dollar percent ampersand
    quoteright parenleft parenright asterisk plus comma minus
    period slash zero one two three four five six seven eight
    nine colon semicolon less equal greater question at A B C D
    E F G H I J K L M N O P Q R S T U V W X Y Z bracketleft
    backslash bracketright asciicircum underscore quoteleft a b
    c d e f g h i j k l m n o p q r s t u v w x y z braceleft
    bar braceright asciitilde
  ].to_set.freeze,
  "Symbol" => %w[
    Alpha Beta Chi Delta Epsilon Phi Gamma Eta Iota Theta Kappa
    Lambda Mu Nu Omicron Pi Rho Sigma Tau Upsilon Omega Xi Psi Zeta
    alpha beta chi delta epsilon phi gamma eta iota theta kappa
    lambda mu nu omicron pi rho sigma tau upsilon omega xi psi zeta
  ].to_set.freeze,
  "ZapfDingbats" => %w[
    a1 a2 a3 a4 a5 a6 a7 a8 a9 a10 a11 a12 a13 a14 a15 a16
  ].to_set.freeze,
}.freeze
UNICODE_TO_GLYPH =

Unicode → Adobe glyph name lookup (subset). The full Adobe Glyph List is ~2400 entries; this covers Latin + Greek codepoints that the Standard 14 fonts handle.

{
  0x20 => "space", 0x21 => "exclam", 0x22 => "quotedbl",
  0x23 => "numbersign", 0x24 => "dollar", 0x25 => "percent",
  0x26 => "ampersand", 0x27 => "quoteright", 0x28 => "parenleft",
  0x29 => "parenright", 0x2A => "asterisk", 0x2B => "plus",
  0x2C => "comma", 0x2D => "minus", 0x2E => "period", 0x2F => "slash",
  0xC0 => "Agrave", 0xC1 => "Aacute", 0xC2 => "Acircumflex",
  0xC3 => "Atilde", 0xC4 => "Adieresis", 0xC5 => "Aring",
  0xC6 => "AE", 0xC7 => "Ccedilla", 0xC8 => "Egrave",
  0xC9 => "Eacute", 0xCA => "Ecircumflex", 0xCB => "Edieresis",
  0xCC => "Igrave", 0xCD => "Iacute", 0xCE => "Icircumflex",
  0xCF => "Idieresis", 0xD0 => "Eth", 0xD1 => "Ntilde",
  0xD2 => "Ograve", 0xD3 => "Oacute", 0xD4 => "Ocircumflex",
  0xD5 => "Otilde", 0xD6 => "Odieresis", 0xD8 => "Oslash",
  0xD9 => "Ugrave", 0xDA => "Uacute", 0xDB => "Ucircumflex",
  0xDC => "Udieresis", 0xDD => "Yacute", 0xDE => "Thorn",
  0xDF => "germandbls",
  0xE0 => "agrave", 0xE1 => "aacute", 0xE2 => "acircumflex",
  0xE3 => "atilde", 0xE4 => "adieresis", 0xE5 => "aring",
  0xE6 => "ae", 0xE7 => "ccedilla", 0xE8 => "egrave",
  0xE9 => "eacute", 0xEA => "ecircumflex", 0xEB => "edieresis",
  0xEC => "igrave", 0xED => "iacute", 0xEE => "icircumflex",
  0xEF => "idieresis", 0xF0 => "eth", 0xF1 => "ntilde",
  0xF2 => "ograve", 0xF3 => "oacute", 0xF4 => "ocircumflex",
  0xF5 => "otilde", 0xF6 => "odieresis", 0xF8 => "oslash",
  0xF9 => "ugrave", 0xFA => "uacute", 0xFB => "ucircumflex",
  0xFC => "udieresis", 0xFD => "yacute", 0xFE => "thorn",
  0xFF => "ydieresis",
  0x391 => "Alpha", 0x392 => "Beta", 0x393 => "Gamma",
  0x394 => "Delta", 0x395 => "Epsilon", 0x396 => "Zeta",
  0x397 => "Eta", 0x398 => "Theta", 0x399 => "Iota",
  0x39A => "Kappa", 0x39B => "Lambda", 0x39C => "Mu",
  0x39D => "Nu", 0x39E => "Xi", 0x39F => "Omicron",
  0x3A0 => "Pi", 0x3A1 => "Rho", 0x3A3 => "Sigma",
  0x3A4 => "Tau", 0x3A5 => "Upsilon", 0x3A6 => "Phi",
  0x3A7 => "Chi", 0x3A8 => "Psi", 0x3A9 => "Omega",
  0x3B1 => "alpha", 0x3B2 => "beta", 0x3B3 => "gamma",
  0x3B4 => "delta", 0x3B5 => "epsilon", 0x3B6 => "zeta",
  0x3B7 => "eta", 0x3B8 => "theta", 0x3B9 => "iota",
  0x3BA => "kappa", 0x3BB => "lambda", 0x3BC => "mu",
  0x3BD => "nu", 0x3BE => "xi", 0x3BF => "omicron",
  0x3C0 => "pi", 0x3C1 => "rho", 0x3C3 => "sigma",
  0x3C4 => "tau", 0x3C5 => "upsilon", 0x3C6 => "phi",
  0x3C7 => "chi", 0x3C8 => "psi", 0x3C9 => "omega"
}.freeze

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(chain: DEFAULT_CHAIN) ⇒ FontFallback

Returns a new instance of FontFallback.

Parameters:

  • chain (Array<String>) (defaults to: DEFAULT_CHAIN) —

    ordered list of font names to try when the primary can't render a glyph.



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# File 'lib/pdfrb/layout/font_fallback.rb', line 127

def initialize(chain: DEFAULT_CHAIN)
  @chain = chain
end

Instance Attribute Details

#chain ⇒ Object (readonly)

Returns the value of attribute chain.



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# File 'lib/pdfrb/layout/font_fallback.rb', line 123

def chain
  @chain
end

Instance Method Details

#covers?(font_name, codepoint) ⇒ Boolean

Whether font_name can render codepoint. Uses the Standard 14 glyph-name sets for built-in fonts; for non-standard fonts, assumes WinAnsi coverage up to 255.

Returns:

  • (Boolean) 


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# File 'lib/pdfrb/layout/font_fallback.rb', line 167

def covers?(font_name, codepoint)
  return true if codepoint < 128 # ASCII always covered

  standard14 = STANDARD14_GLYPH_NAMES[font_name.to_s]
  if standard14
    glyph = glyph_name_for(codepoint)
    return !!glyph && standard14.include?(glyph)
  end

  winansi?(codepoint)
end

#pick(codepoint, primary: nil) ⇒ String?

Find a font in the chain that covers codepoint.

Returns:

  • (String, nil) —

    font name, or nil if none match.



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# File 'lib/pdfrb/layout/font_fallback.rb', line 133

def pick(codepoint, primary: nil)
  return primary if primary && covers?(primary, codepoint)

  @chain.find { |name| covers?(name, codepoint) }
end

#segment(text, primary: nil) ⇒ Object

Reshape text into segments, each tagged with the font that renders it. Returns an Array of [font_name, substring] pairs. Adjacent segments that share the same font are merged.



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# File 'lib/pdfrb/layout/font_fallback.rb', line 142

def segment(text, primary: nil)
  return [[primary, text]] if text.to_s.empty?

  segments = []
  current_font = nil
  current_chars = +""

  text.to_s.each_codepoint do |cp|
    ch = cp.chr(Encoding::UTF_8)
    font = pick(cp, primary: primary)
    if font == current_font
      current_chars << ch
    else
      segments << [current_font, current_chars] if current_chars.length.positive?
      current_font = font
      current_chars = ch.to_s
    end
  end
  segments << [current_font, current_chars] if current_chars.length.positive?
  segments
end