Class: Fontisan::Woff2::HmtxTransformer

Inherits:
Object
  • Object
show all
Defined in:
lib/fontisan/woff2/hmtx_transformer.rb

Overview

Reconstructs hmtx table from WOFF2 transformed format

WOFF2 hmtx transformation optimizes horizontal metrics by:

  • Using variable-length encoding for advance widths

  • Optionally deriving LSB from glyf bounding boxes

  • Omitting redundant trailing advance widths

See: www.w3.org/TR/WOFF2/#hmtx_table_format

Examples:

Reconstructing hmtx table

hmtx_data = HmtxTransformer.reconstruct(
  transformed_data,
  num_glyphs,
  number_of_h_metrics
)

Constant Summary collapse

HMTX_FLAG_EXPLICIT_ADVANCE_WIDTHS =

Flags for hmtx transformation

0x01
HMTX_FLAG_EXPLICIT_LSB_VALUES = 
0x02
HMTX_FLAG_SYMMETRIC = 
0x04

Class Method Summary collapse

Class Method Details

.build_hmtx_table(advance_widths, lsbs, num_h_metrics, num_glyphs) ⇒ String

Build standard hmtx table format

Standard hmtx format:

  • longHorMetric (advanceWidth, lsb pairs)

  • int16[numGlyphs - numberOfHMetrics] (additional LSBs)

Parameters:

  • advance_widths (Array<Integer>) —

    Advance widths

  • lsbs (Array<Integer>) —

    Left side bearings

  • num_h_metrics (Integer) —

    Number of entries with full hMetrics

  • num_glyphs (Integer) —

    Total number of glyphs

Returns:

  • (String) —

    Standard hmtx table data



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# File 'lib/fontisan/woff2/hmtx_transformer.rb', line 126

def self.build_hmtx_table(advance_widths, lsbs, num_h_metrics, num_glyphs)
  data = +""

  # Write longHorMetric array (advanceWidth + lsb pairs)
  num_h_metrics.times do |i|
    advance_width = advance_widths[i] || advance_widths.last
    lsb = lsbs[i] || 0

    data << [advance_width].pack("n")  # uint16 advanceWidth
    data << [lsb].pack("n")            # int16 lsb
  end

  # Write remaining LSB values
  # These glyphs all share the last advance width from the array
  (num_h_metrics...num_glyphs).each do |i|
    lsb = lsbs[i] || 0
    data << [lsb].pack("n") # int16 lsb
  end

  data
end

.read_255_uint16(io) ⇒ Integer

Read variable-length 255UInt16 integer

Format from WOFF2 spec:

  • value < 253: one byte

  • value == 253: 253 + next uint16

  • value == 254: 253 * 2 + next uint16

  • value == 255: 253 * 3 + next uint16

Parameters:

  • io (StringIO) —

    Input stream

Returns:

  • (Integer) —

    Decoded value



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# File 'lib/fontisan/woff2/hmtx_transformer.rb', line 100

def self.read_255_uint16(io)
  code = read_uint8(io)

  case code
  when 255
    759 + read_uint16(io)  # 253 * 3 + value
  when 254
    506 + read_uint16(io)  # 253 * 2 + value
  when 253
    253 + read_uint16(io)
  else
    code
  end
end

.read_int16(io) ⇒ Object 



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# File 'lib/fontisan/woff2/hmtx_transformer.rb', line 158

def self.read_int16(io)
  value = read_uint16(io)
  value > 0x7FFF ? value - 0x10000 : value
end

.read_uint16(io) ⇒ Object 



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# File 'lib/fontisan/woff2/hmtx_transformer.rb', line 154

def self.read_uint16(io)
  io.read(2)&.unpack1("n") || raise(EOFError, "Unexpected end of stream")
end

.read_uint8(io) ⇒ Object

Helper methods for reading binary data



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# File 'lib/fontisan/woff2/hmtx_transformer.rb', line 150

def self.read_uint8(io)
  io.read(1)&.unpack1("C") || raise(EOFError, "Unexpected end of stream")
end

.reconstruct(transformed_data, num_glyphs, num_h_metrics, glyf_lsbs = nil) ⇒ String

Reconstruct hmtx table from transformed data

Parameters:

  • transformed_data (String) —

    The transformed hmtx table data

  • num_glyphs (Integer) —

    Number of glyphs

  • num_h_metrics (Integer) —

    From hhea.numberOfHMetrics

  • glyf_lsbs (Array<Integer>, nil) (defaults to: nil) —

    LSB values from glyf bboxes (optional)

Returns:

  • (String) —

    Standard hmtx table data

Raises:



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# File 'lib/fontisan/woff2/hmtx_transformer.rb', line 36

def self.reconstruct(transformed_data, num_glyphs, num_h_metrics, glyf_lsbs = nil)
  io = StringIO.new(transformed_data)

  # Read transformation flags
  flags = read_uint8(io)

  # Read advance widths
  advance_widths = []

  if (flags & HMTX_FLAG_EXPLICIT_ADVANCE_WIDTHS).zero?
    # Proportional encoding - read deltas
    # First advance width is explicit
    first_advance = read_255_uint16(io)
    advance_widths << first_advance

    # Remaining are deltas from previous
    (num_h_metrics - 1).times do
      delta = read_int16(io)
      advance_widths << (advance_widths.last + delta)
    end
  else
    # Explicit advance widths in transformed format
    num_h_metrics.times do
      advance_widths << read_255_uint16(io)
    end
  end

  # Read LSB values
  lsbs = []

  if (flags & HMTX_FLAG_EXPLICIT_LSB_VALUES) != 0
    # Explicit LSB values
    num_glyphs.times do
      lsbs << read_int16(io)
    end
  elsif glyf_lsbs
    # Use LSB values from glyf bounding boxes
    lsbs = glyf_lsbs
  else
    # Need to read LSB values for long metrics
    num_h_metrics.times do
      lsbs << read_int16(io)
    end

    # Remaining LSBs for glyphs that share the last advance width
    (num_glyphs - num_h_metrics).times do
      lsbs << read_int16(io)
    end
  end

  # Build standard hmtx table
  build_hmtx_table(advance_widths, lsbs, num_h_metrics, num_glyphs)
end