Class: TTFunk::TTFEncoder

Inherits:
Object
  • Object
show all
Defined in:
lib/ttfunk/ttf_encoder.rb

Overview

Encodes a TrueType font subset to its binary representation.

Direct Known Subclasses

OTFEncoder

Constant Summary collapse

OPTIMAL_TABLE_ORDER =

Optimal table order according to TrueType specification.

[
  'head', 'hhea', 'maxp', 'OS/2', 'hmtx', 'LTSH', 'VDMX',
  'hdmx', 'cmap', 'fpgm', 'prep', 'cvt ', 'loca', 'glyf',
  'kern', 'name', 'post', 'gasp', 'PCLT',
].freeze

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(original, subset, options = {}) ⇒ TTFEncoder

Returns a new instance of TTFEncoder.

Parameters:

Options Hash (options):

  • :kerning (Boolean) —

    whether to encode Kerning (kern) table.

Source Code
lib/ttfunk/ttf_encoder.rb, line 30
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def initialize(original, subset, options = {})

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  @original = original

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  @subset = subset

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  @options = options

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end

Instance Attribute Details

#options ⇒ Hash (readonly)

Encoding options.

Returns:

  • (Hash)
Source Code
lib/ttfunk/ttf_encoder.rb, line 23
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def options

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  @options

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end

#original ⇒ TTFunk::File (readonly)

Original font.

Returns:

Source Code
lib/ttfunk/ttf_encoder.rb, line 15
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def original

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  @original

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end

#subset ⇒ TTFunk::Subset (readonly)

Subset to encode.

Returns:

Source Code
lib/ttfunk/ttf_encoder.rb, line 19
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def subset

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  @subset

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end

Instance Method Details

#encode ⇒ String

Encode the font subset.

Returns:

  • (String)
Source Code
lib/ttfunk/ttf_encoder.rb, line 39
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def encode

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  # https://www.microsoft.com/typography/otspec/otff.htm#offsetTable

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  search_range = (2**Math.log2(tables.length).floor) * 16

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  entry_selector = Integer(Math.log2(2**Math.log2(tables.length).floor))

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  range_shift = (tables.length * 16) - search_range

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  range_shift = 0 if range_shift.negative?

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  newfont = EncodedString.new

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  newfont << [

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    original.directory.scaler_type,

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    tables.length,

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    search_range,

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    entry_selector,

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    range_shift,

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  ].pack('Nn*')

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  # Tables are supposed to be listed in ascending order whereas there is a

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  # known optimal order for table data.

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  tables.keys.sort.each do |tag|

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    newfont << [tag, checksum(tables[tag])].pack('A4N')

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    newfont << Placeholder.new(tag, length: 4)

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    newfont << [tables[tag].length].pack('N')

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  end

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  optimal_table_order.each do |optimal_tag|

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    next unless tables.include?(optimal_tag)

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    newfont.resolve_placeholder(optimal_tag, [newfont.length].pack('N'))

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    newfont << tables[optimal_tag]

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    newfont.align!(4)

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  end

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  sum = checksum(newfont)

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  adjustment = 0xB1B0AFBA - sum

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  newfont.resolve_placeholder(:checksum, [adjustment].pack('N'))

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  newfont.string

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end