Class: Fontisan::Variable::TableUpdater

Inherits:
Object
  • Object
show all
Defined in:
lib/fontisan/variable/table_updater.rb

Overview

Updates font tables with applied variation deltas

This class is responsible for taking original table data and applying calculated deltas to create updated tables for static font instances. It handles:

  • Updating hmtx with varied advance widths and sidebearings
  • Updating hhea with varied ascent/descent/line gap
  • Updating OS/2 with varied metrics
  • Updating head table's modified timestamp

Each update method takes the original table and delta values, then reconstructs the table binary with updated values.

Examples:

Update hmtx table

updater = TableUpdater.new
new_hmtx = updater.update_hmtx(
  original_hmtx_data,
  varied_metrics,
  num_h_metrics,
  num_glyphs
)

Instance Method Summary collapse

Instance Method Details

#apply_updates(original_data, updates) ⇒ String

Build updated table with varied values

This is a generic helper for building updated table binaries

Parameters:

  • original_data (String) —

    Original table binary

  • updates (Hash) —

    Hash of offset => new_value pairs

Returns:

  • (String) —

    Updated table binary



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# File 'lib/fontisan/variable/table_updater.rb', line 191

def apply_updates(original_data, updates)
  data = original_data.dup

  updates.each do |offset, value|
    # Handle different value types
    packed_value = case value
                   when Integer
                     if value >= -32768 && value <= 32767
                       # int16
                       unsigned = value.negative? ? value + 0x10000 : value
                       [unsigned].pack("n")
                     else
                       # int32
                       [value].pack("N")
                     end
                   when String
                     value
                   else
                     value.to_s
                   end

    data[offset, packed_value.bytesize] = packed_value
  end

  data
end

#update_head_modified(original_data, timestamp = Time.now) ⇒ String

Update head table's modified timestamp

Parameters:

  • original_data (String) —

    Original head table binary

  • timestamp (Time) (defaults to: Time.now) —

    New modification time

Returns:

  • (String) —

    Updated head table binary



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# File 'lib/fontisan/variable/table_updater.rb', line 162

def update_head_modified(original_data, timestamp = Time.now)
  io = StringIO.new(original_data)
  io.set_encoding(Encoding::BINARY)

  # Read up to modified timestamp
  header = io.read(28) # version through created timestamp
  _old_modified = io.read(8) # Skip old modified timestamp
  rest = io.read # Remaining data

  # Convert Time to LONGDATETIME (seconds since 1904-01-01)
  # Difference between 1904 and 1970 (Unix epoch) is 2082844800 seconds
  longdatetime = timestamp.to_i + 2_082_844_800

  # Build updated head binary
  output = String.new(encoding: Encoding::BINARY)
  output << header
  output << [longdatetime].pack("q>") # 64-bit big-endian signed integer
  output << rest

  output
end

#update_hhea(original_data, varied_metrics) ⇒ String

Update hhea table with varied metrics

Parameters:

  • original_data (String) —

    Original hhea table binary

  • varied_metrics (Hash) —

    Varied font metrics { ascent: 2048, descent: -512, line_gap: 0 }

Returns:

  • (String) —

    Updated hhea table binary



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# File 'lib/fontisan/variable/table_updater.rb', line 99

def update_hhea(original_data, varied_metrics)
  io = StringIO.new(original_data)
  io.set_encoding(Encoding::BINARY)

  # Read all fields
  version = io.read(4)
  ascent = io.read(2)&.unpack1("n") || 0
  ascent = ascent >= 0x8000 ? ascent - 0x10000 : ascent
  descent = io.read(2)&.unpack1("n") || 0
  descent = descent >= 0x8000 ? descent - 0x10000 : descent
  line_gap = io.read(2)&.unpack1("n") || 0
  line_gap = line_gap >= 0x8000 ? line_gap - 0x10000 : line_gap

  # Read remaining fields
  rest = io.read

  # Apply varied metrics
  ascent = varied_metrics[:ascent] if varied_metrics[:ascent]
  descent = varied_metrics[:descent] if varied_metrics[:descent]
  line_gap = varied_metrics[:line_gap] if varied_metrics[:line_gap]

  # Build updated hhea binary
  output = String.new(encoding: Encoding::BINARY)
  output << version

  # Convert signed values to unsigned for packing
  ascent_unsigned = ascent.negative? ? ascent + 0x10000 : ascent
  descent_unsigned = descent.negative? ? descent + 0x10000 : descent
  line_gap_unsigned = line_gap.negative? ? line_gap + 0x10000 : line_gap

  output << [ascent_unsigned].pack("n")
  output << [descent_unsigned].pack("n")
  output << [line_gap_unsigned].pack("n")
  output << rest

  output
end

#update_hmtx(original_data, varied_metrics, num_h_metrics, num_glyphs) ⇒ String

Update hmtx table with varied metrics

Parameters:

  • original_data (String) —

    Original hmtx table binary

  • varied_metrics (Hash<Integer, Hash>) —

    Varied metrics by glyph ID { glyph_id => { advance_width: 500, lsb: 50 } }

  • num_h_metrics (Integer) —

    Number of hMetrics from hhea

  • num_glyphs (Integer) —

    Total glyphs from maxp

Returns:

  • (String) —

    Updated hmtx table binary



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# File 'lib/fontisan/variable/table_updater.rb', line 37

def update_hmtx(original_data, varied_metrics, num_h_metrics, num_glyphs)
  io = StringIO.new(original_data)
  io.set_encoding(Encoding::BINARY)

  # Parse original hMetrics
  h_metrics = []
  num_h_metrics.times do
    advance_width = io.read(2)&.unpack1("n") || 0
    lsb = io.read(2)&.unpack1("n") || 0
    lsb = lsb >= 0x8000 ? lsb - 0x10000 : lsb # Convert to signed
    h_metrics << { advance_width: advance_width, lsb: lsb }
  end

  # Parse additional LSBs
  lsb_count = num_glyphs - num_h_metrics
  left_side_bearings = []
  lsb_count.times do
    lsb = io.read(2)&.unpack1("n") || 0
    lsb = lsb >= 0x8000 ? lsb - 0x10000 : lsb
    left_side_bearings << lsb
  end

  # Apply varied metrics
  varied_metrics.each do |glyph_id, metrics|
    if glyph_id < num_h_metrics
      if metrics[:advance_width]
        h_metrics[glyph_id][:advance_width] =
          metrics[:advance_width]
      end
      h_metrics[glyph_id][:lsb] = metrics[:lsb] if metrics[:lsb]
    else
      lsb_index = glyph_id - num_h_metrics
      left_side_bearings[lsb_index] = metrics[:lsb] if metrics[:lsb]
    end
  end

  # Build updated hmtx binary
  output = String.new(encoding: Encoding::BINARY)

  # Write hMetrics
  h_metrics.each do |metric|
    output << [metric[:advance_width]].pack("n")
    # Convert signed LSB to unsigned for packing
    lsb_unsigned = metric[:lsb].negative? ? metric[:lsb] + 0x10000 : metric[:lsb]
    output << [lsb_unsigned].pack("n")
  end

  # Write additional LSBs
  left_side_bearings.each do |lsb|
    lsb_unsigned = lsb.negative? ? lsb + 0x10000 : lsb
    output << [lsb_unsigned].pack("n")
  end

  output
end

#update_os2(original_data, varied_metrics) ⇒ String

Update OS/2 table with varied metrics

Parameters:

  • original_data (String) —

    Original OS/2 table binary

  • varied_metrics (Hash) —

    Varied font metrics from MVAR

Returns:

  • (String) —

    Updated OS/2 table binary



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# File 'lib/fontisan/variable/table_updater.rb', line 142

def update_os2(original_data, varied_metrics)
  return original_data if varied_metrics.empty?

  io = StringIO.new(original_data)
  io.set_encoding(Encoding::BINARY)

  # Read version to determine table size
  io.read(2)&.unpack1("n") || 0
  io.rewind

  # For simplicity, return original if no specific OS/2 metrics to update
  # This would need to be expanded based on MVAR tags present
  original_data
end