Class: Fontisan::Woff2::GlyfTransformer

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

Overview

Reconstructs glyf and loca tables from WOFF2 transformed format

WOFF2 glyf table transformation splits glyph data into separate streams for better compression. This transformer reconstructs the standard ‘glyf` and `loca` table formats from the transformed data.

Transformation format (Section 5 of WOFF2 spec):

  • Separate streams for nContour, nPoints, flags, x-coords, y-coords

  • Variable-length integer encoding (255UInt16)

  • Composite glyph components stored separately

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

Examples:

Reconstructing tables

result = GlyfTransformer.reconstruct(transformed_data, num_glyphs)
glyf_data = result[:glyf]
loca_data = result[:loca]

Constant Summary collapse

ON_CURVE_POINT =

Glyph flags

0x01
X_SHORT_VECTOR = 
0x02
Y_SHORT_VECTOR = 
0x04
REPEAT_FLAG = 
0x08
X_IS_SAME_OR_POSITIVE_X_SHORT_VECTOR = 
0x10
Y_IS_SAME_OR_POSITIVE_Y_SHORT_VECTOR = 
0x20
ARG_1_AND_2_ARE_WORDS =

Composite glyph flags

0x0001
ARGS_ARE_XY_VALUES = 
0x0002
ROUND_XY_TO_GRID = 
0x0004
WE_HAVE_A_SCALE = 
0x0008
MORE_COMPONENTS = 
0x0020
WE_HAVE_AN_X_AND_Y_SCALE = 
0x0040
WE_HAVE_A_TWO_BY_TWO = 
0x0080
WE_HAVE_INSTRUCTIONS = 
0x0100
USE_MY_METRICS = 
0x0200
OVERLAP_COMPOUND = 
0x0400
HAVE_VARIATIONS =

Variable font variation data follows

0x1000

Class Method Summary collapse

Class Method Details

.build_simple_glyph_data(num_contours, x_min, y_min, x_max, y_max, end_pts, instructions, flags, x_coords, y_coords) ⇒ String

Build simple glyph data in standard format

Returns:

  • (String) —

    Glyph data



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 610

def self.build_simple_glyph_data(num_contours, x_min, y_min, x_max, y_max,
                                 end_pts, instructions, flags, x_coords, y_coords)
  data = +""
  data << [num_contours].pack("n")
  data << [x_min, y_min, x_max, y_max].pack("n4")

  end_pts.each { |pt| data << [pt].pack("n") }

  data << [instructions.bytesize].pack("n")
  data << instructions

  flags.each { |flag| data << [flag].pack("C") }

  # Write x-coordinates
  prev_x = 0
  x_coords.each do |x|
    delta = x - prev_x
    prev_x = x

    data << if delta.abs <= 255
              [delta.abs].pack("C")
            else
              [delta].pack("n")
            end
  end

  # Write y-coordinates
  prev_y = 0
  y_coords.each do |y|
    delta = y - prev_y
    prev_y = y

    data << if delta.abs <= 255
              [delta.abs].pack("C")
            else
              [delta].pack("n")
            end
  end

  data
end

.build_tables(glyphs, index_format) ⇒ Hash

Build glyf and loca tables

Parameters:

  • glyphs (Array<String>) —

    Glyph data

  • index_format (Integer) —

    Loca format (0 = short, 1 = long)

Returns:

  • (Hash) —

    { glyf: String, loca: String }



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 657

def self.build_tables(glyphs, index_format)
  glyf_data = +""
  loca_offsets = [0]

  glyphs.each do |glyph|
    glyf_data << glyph

    # Add padding to 4-byte boundary
    padding = (4 - (glyph.bytesize % 4)) % 4
    glyf_data << ("\x00" * padding)

    loca_offsets << glyf_data.bytesize
  end

  # Build loca table
  loca_data = +""
  if index_format.zero?
    # Short format (divide offsets by 2)
    loca_offsets.each do |offset|
      loca_data << [offset / 2].pack("n")
    end
  else
    # Long format
    loca_offsets.each do |offset|
      loca_data << [offset].pack("N")
    end
  end

  { glyf: glyf_data, loca: loca_data }
end

.parse_n_contour_stream(io, num_glyphs) ⇒ Array<Integer>

Parse nContour stream

Parameters:

  • io (StringIO) —

    Input stream

  • num_glyphs (Integer) —

    Number of glyphs

Returns:

  • (Array<Integer>) —

    Number of contours per glyph (-1 for composite)



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 199

def self.parse_n_contour_stream(io, num_glyphs)
  n_contours = []
  num_glyphs.times do
    # For variable fonts, stream may be incomplete
    break if io.eof? || (io.size - io.pos) < 2

    value = read_int16(io)
    n_contours << value
  end

  # Pad with zeros if we have fewer contours than glyphs
  while n_contours.size < num_glyphs
    n_contours << 0
  end

  n_contours
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, or 0 if not enough data



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 159

def self.read_255_uint16(io)
  return 0 if io.eof? || (io.size - io.pos) < 1

  code_byte = io.read(1)
  return 0 unless code_byte && code_byte.bytesize == 1

  code = code_byte.unpack1("C")

  case code
  when 255
    return 0 if io.eof? || (io.size - io.pos) < 2

    value_bytes = io.read(2)
    return 0 unless value_bytes && value_bytes.bytesize == 2

    759 + value_bytes.unpack1("n")  # 253 * 3 + value
  when 254
    return 0 if io.eof? || (io.size - io.pos) < 2

    value_bytes = io.read(2)
    return 0 unless value_bytes && value_bytes.bytesize == 2

    506 + value_bytes.unpack1("n")  # 253 * 2 + value
  when 253
    return 0 if io.eof? || (io.size - io.pos) < 2

    value_bytes = io.read(2)
    return 0 unless value_bytes && value_bytes.bytesize == 2

    253 + value_bytes.unpack1("n")
  else
    code
  end
end

.read_coordinates(io, flags, short_flag, same_or_positive_flag) ⇒ Array<Integer>

Read coordinates

Parameters:

  • io (StringIO) —

    Glyph stream

  • flags (Array<Integer>) —

    Flag values

  • short_flag (Integer) —

    Flag bit for short vector

  • same_or_positive_flag (Integer) —

    Flag bit for same/positive

Returns:

  • (Array<Integer>) —

    Coordinate values



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 572

def self.read_coordinates(io, flags, short_flag, same_or_positive_flag)
  coords = []
  value = 0

  flags.each do |flag|
    # EOF protection
    if (flag & short_flag) != 0
      break if io.eof? || (io.size - io.pos) < 1

      # Short vector (one byte)
      delta = read_uint8(io)
      delta = -delta if (flag & same_or_positive_flag).zero?
    elsif (flag & same_or_positive_flag) != 0
      # Same as previous (delta = 0)
      delta = 0
    else
      break if io.eof? || (io.size - io.pos) < 2

      # Long vector (two bytes, signed)
      delta = read_int16(io)
    end

    value += delta
    coords << value
  end

  # Pad with last value if needed
  last_val = coords.last || 0
  while coords.size < flags.size
    coords << last_val
  end

  coords
end

.read_f2dot14(io) ⇒ Object 



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 711

def self.read_f2dot14(io)
  read_uint16(io)
end

.read_flags(io, count) ⇒ Array<Integer>

Read flags with repeat handling

Parameters:

  • io (StringIO) —

    Flag stream

  • count (Integer) —

    Number of flags to read

Returns:

  • (Array<Integer>) —

    Flag values



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 533

def self.read_flags(io, count)
  flags = []

  while flags.size < count
    # Safety check to prevent infinite loops with corrupted streams
    if flags.size > 200000
      # Stream appears corrupted, pad with zeros
      while flags.size < count
        flags << 0
      end
      break
    end

    # EOF protection for variable fonts
    break if io.eof? || (io.size - io.pos) < 1

    flag = read_uint8(io)
    flags << flag

    if (flag & REPEAT_FLAG) != 0
      break if io.eof? || (io.size - io.pos) < 1

      repeat_count = read_uint8(io)
      # Safety check on repeat count
      repeat_count = [repeat_count, 100].min
      repeat_count.times { flags << flag }
    end
  end

  flags
end

.read_int16(io) ⇒ Object 



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 702

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

.read_int8(io) ⇒ Object 



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 694

def self.read_int8(io)
  io.read(1)&.unpack1("c") || raise(EOFError, "Unexpected end of stream")
end

.read_stream_safely(io, _stream_name, variable_font: false) ⇒ String

Safely read a stream with bounds checking

Parameters:

  • io (StringIO) —

    Input stream

  • stream_name (String) —

    Name of stream for error messages

  • variable_font (Boolean) (defaults to: false) —

    Whether this is a variable font (allows incomplete streams)

Returns:

  • (String) —

    Stream data (empty if not available)



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 124

def self.read_stream_safely(io, _stream_name, variable_font: false)
  remaining = io.size - io.pos
  if remaining < 4
    # Not enough data for stream size - return empty stream
    return ""
  end

  # Read stream size safely
  size_bytes = io.read(4)
  return "" unless size_bytes && size_bytes.bytesize == 4

  stream_size = size_bytes.unpack1("N")
  remaining = io.size - io.pos

  if remaining < stream_size
    # Stream size extends beyond available data
    # Read what we can
    io.read(remaining) || ""
    # For variable fonts, we may have incomplete streams - just return what we have

  else
    io.read(stream_size) || ""
  end
end

.read_uint16(io) ⇒ Object 



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 698

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

.read_uint32(io) ⇒ Object 



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 707

def self.read_uint32(io)
  io.read(4)&.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/glyf_transformer.rb', line 690

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

.reconstruct(transformed_data, num_glyphs, variable_font: false) ⇒ Hash

Reconstruct glyf and loca tables from transformed data

Parameters:

  • transformed_data (String) —

    The transformed glyf table data

  • num_glyphs (Integer) —

    Number of glyphs from maxp table

  • variable_font (Boolean) (defaults to: false) —

    Whether this is a variable font with variation data

Returns:

  • (Hash) —

    { glyf: String, loca: String }

Raises:



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 53

def self.reconstruct(transformed_data, num_glyphs, variable_font: false)
  io = StringIO.new(transformed_data)

  # Check minimum size for header
  if io.size < 8
    raise InvalidFontError,
          "Transformed glyf data too small: #{io.size} bytes"
  end

  # Read header
  read_uint32(io)
  num_glyphs_in_data = read_uint16(io)
  index_format = read_uint16(io)

  if num_glyphs_in_data != num_glyphs
    raise InvalidFontError,
          "Glyph count mismatch: expected #{num_glyphs}, got #{num_glyphs_in_data}"
  end

  # Read nContour stream
  n_contour_data = read_stream_safely(io, "nContour",
                                      variable_font: variable_font)

  # Read nPoints stream
  n_points_data = read_stream_safely(io, "nPoints",
                                     variable_font: variable_font)

  # Read flag stream
  flag_data = read_stream_safely(io, "flag", variable_font: variable_font)

  # Read glyph stream (coordinates, instructions, composite data)
  glyph_data = read_stream_safely(io, "glyph",
                                  variable_font: variable_font)

  # Read composite stream
  composite_data = read_stream_safely(io, "composite",
                                      variable_font: variable_font)

  # Read bbox stream
  bbox_data = read_stream_safely(io, "bbox", variable_font: variable_font)

  # Read instruction stream
  instruction_data = read_stream_safely(io, "instruction",
                                        variable_font: variable_font)

  # Parse streams
  n_contours = parse_n_contour_stream(StringIO.new(n_contour_data),
                                      num_glyphs)

  # Reconstruct glyphs
  glyphs = reconstruct_glyphs(
    n_contours,
    StringIO.new(n_points_data),
    StringIO.new(flag_data),
    StringIO.new(glyph_data),
    StringIO.new(composite_data),
    StringIO.new(bbox_data),
    StringIO.new(instruction_data),
    variable_font: variable_font,
  )

  # Build glyf and loca tables
  build_tables(glyphs, index_format)
end

.reconstruct_composite_glyph(composite_io, bbox_io, instruction_io, variable_font: false) ⇒ String

Reconstruct a composite glyph

Parameters:

  • composite_io (StringIO) —

    Composite stream

  • bbox_io (StringIO) —

    Bounding box stream

  • instruction_io (StringIO) —

    Instruction stream

  • variable_font (Boolean) (defaults to: false) —

    Whether this is a variable font

Returns:

  • (String) —

    Glyph data in standard format



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 358

def self.reconstruct_composite_glyph(composite_io, bbox_io,
instruction_io, variable_font: false)
  # Track available bytes to prevent EOF errors
  composite_size = composite_io.size - composite_io.pos

  # Validate minimum size (at least flags + glyph_index + args)
  return "" if composite_size < 8

  # Read bounding box safely
  bbox_remaining = bbox_io.size - bbox_io.pos
  if bbox_remaining < 8
    # Not enough data for bounding box, return empty glyph
    return ""
  end

  bbox_bytes = bbox_io.read(8)
  unless bbox_bytes && bbox_bytes.bytesize == 8
    return ""
  end

  x_min, y_min, x_max, y_max = bbox_bytes.unpack("n4")
  # Convert to signed
  x_min = x_min > 0x7FFF ? x_min - 0x10000 : x_min
  y_min = y_min > 0x7FFF ? y_min - 0x10000 : y_min
  x_max = x_max > 0x7FFF ? x_max - 0x10000 : x_max
  y_max = y_max > 0x7FFF ? y_max - 0x10000 : y_max

  # Read composite data
  composite_data = +""
  has_instructions = false
  has_variations = false

  loop do
    # Check if we have enough bytes for flags and glyph_index
    remaining = composite_io.size - composite_io.pos
    break if composite_io.eof? || remaining < 4

    # Read flags and glyph_index safely
    component_header = composite_io.read(4)
    break unless component_header && component_header.bytesize == 4

    flags, glyph_index = component_header.unpack("n2")

    # Write flags and index
    composite_data << [flags].pack("n")
    composite_data << [glyph_index].pack("n")

    # Read arguments (depend on flags)
    remaining = composite_io.size - composite_io.pos
    if (flags & ARG_1_AND_2_ARE_WORDS).zero?
      break if composite_io.eof? || remaining < 2

      arg_bytes = composite_io.read(2)
      break unless arg_bytes && arg_bytes.bytesize == 2

      arg1, arg2 = arg_bytes.unpack("c2")
      composite_data << [arg1, arg2].pack("c2")
    else
      break if composite_io.eof? || remaining < 4

      arg_bytes = composite_io.read(4)
      break unless arg_bytes && arg_bytes.bytesize == 4

      arg1, arg2 = arg_bytes.unpack("n2")
      # Convert to signed
      arg1 = arg1 > 0x7FFF ? arg1 - 0x10000 : arg1
      arg2 = arg2 > 0x7FFF ? arg2 - 0x10000 : arg2
      composite_data << [arg1, arg2].pack("n2")
    end

    # Read transformation matrix (depends on flags) with bounds checking
    if (flags & WE_HAVE_A_SCALE) != 0
      remaining = composite_io.size - composite_io.pos
      break if composite_io.eof? || remaining < 2

      scale_bytes = composite_io.read(2)
      break unless scale_bytes && scale_bytes.bytesize == 2

      scale = scale_bytes.unpack1("n")
      composite_data << [scale].pack("n")
    elsif (flags & WE_HAVE_AN_X_AND_Y_SCALE) != 0
      remaining = composite_io.size - composite_io.pos
      break if composite_io.eof? || remaining < 4

      scale_bytes = composite_io.read(4)
      break unless scale_bytes && scale_bytes.bytesize == 4

      x_scale, y_scale = scale_bytes.unpack("n2")
      composite_data << [x_scale, y_scale].pack("n2")
    elsif (flags & WE_HAVE_A_TWO_BY_TWO) != 0
      remaining = composite_io.size - composite_io.pos
      break if composite_io.eof? || remaining < 8

      matrix_bytes = composite_io.read(8)
      break unless matrix_bytes && matrix_bytes.bytesize == 8

      x_scale, scale01, scale10, y_scale = matrix_bytes.unpack("n4")
      composite_data << [x_scale, scale01, scale10, y_scale].pack("n4")
    end

    # Check for variable font variation data
    # Only parse if this is a variable font and the flag is set
    if variable_font && (flags & HAVE_VARIATIONS) != 0
      has_variations = true
      # Read tuple variation count and data
      remaining = composite_io.size - composite_io.pos
      if !composite_io.eof? && remaining >= 2
        # Read tuple count safely
        tuple_bytes = composite_io.read(2)
        if tuple_bytes && tuple_bytes.bytesize == 2
          tuple_count = tuple_bytes.unpack1("n")
          composite_data << [tuple_count].pack("n")

          # Each tuple has variation data - read and preserve it
          tuple_count.times do
            remaining = composite_io.size - composite_io.pos
            break if composite_io.eof? || remaining < 4

            # Read variation data (2 int16 values per tuple)
            var_bytes = composite_io.read(4)
            break unless var_bytes && var_bytes.bytesize == 4

            var1, var2 = var_bytes.unpack("n2")
            # Convert to signed if needed
            var1 = var1 > 0x7FFF ? var1 - 0x10000 : var1
            var2 = var2 > 0x7FFF ? var2 - 0x10000 : var2
            composite_data << [var1, var2].pack("n2")
          end
        end
      end
    end

    has_instructions = (flags & WE_HAVE_INSTRUCTIONS) != 0

    break if (flags & MORE_COMPONENTS).zero?
  end

  # Add instructions if present
  instructions = +""
  if has_instructions
    # Read instruction length safely
    remaining = instruction_io.size - instruction_io.pos
    if !instruction_io.eof? && remaining >= 2
      length_bytes = instruction_io.read(2)
      if length_bytes && length_bytes.bytesize == 2
        instruction_length = length_bytes.unpack1("n")
        if instruction_length.positive?
          remaining = instruction_io.size - instruction_io.pos
          instructions = if remaining >= instruction_length
                           instruction_io.read(instruction_length) || ""
                         else
                           # Read what we can
                           instruction_io.read(remaining) || ""
                         end
        end
      end
    end
  end

  # Build composite glyph data
  data = +""
  data << [-1].pack("n") # numberOfContours = -1
  data << [x_min, y_min, x_max, y_max].pack("n4")
  data << composite_data
  data << [instructions.bytesize].pack("n") if has_instructions
  data << instructions if has_instructions

  data
end

.reconstruct_glyphs(n_contours, n_points_io, flag_io, glyph_io, composite_io, bbox_io, instruction_io, variable_font: false) ⇒ Array<String>

Reconstruct all glyphs

Parameters:

  • n_contours (Array<Integer>) —

    Contour counts

  • n_points_io (StringIO) —

    Points stream

  • flag_io (StringIO) —

    Flag stream

  • glyph_io (StringIO) —

    Glyph data stream

  • composite_io (StringIO) —

    Composite glyph stream

  • bbox_io (StringIO) —

    Bounding box stream

  • instruction_io (StringIO) —

    Instruction stream

  • variable_font (Boolean) (defaults to: false) —

    Whether this is a variable font

Returns:

  • (Array<String>) —

    Reconstructed glyph data



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 228

def self.reconstruct_glyphs(n_contours, n_points_io, flag_io, glyph_io,
                             composite_io, bbox_io, instruction_io, variable_font: false)
  glyphs = []

  n_contours.each do |num_contours|
    if num_contours.zero?
      # Empty glyph
      glyphs << ""
    elsif num_contours.positive?
      # Simple glyph
      glyphs << reconstruct_simple_glyph(
        num_contours, n_points_io, flag_io,
        glyph_io, bbox_io, instruction_io
      )
    elsif num_contours == -1
      # Composite glyph
      glyphs << reconstruct_composite_glyph(
        composite_io, bbox_io, instruction_io, variable_font: variable_font
      )
    else
      raise InvalidFontError, "Invalid nContours value: #{num_contours}"
    end
  end

  glyphs
end

.reconstruct_simple_glyph(num_contours, n_points_io, flag_io, glyph_io, bbox_io, instruction_io) ⇒ String

Reconstruct a simple glyph

Parameters:

  • num_contours (Integer) —

    Number of contours

  • n_points_io (StringIO) —

    Points stream

  • flag_io (StringIO) —

    Flag stream

  • glyph_io (StringIO) —

    Glyph data stream

  • bbox_io (StringIO) —

    Bounding box stream

  • instruction_io (StringIO) —

    Instruction stream

Returns:

  • (String) —

    Glyph data in standard format



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# File 'lib/fontisan/woff2/glyf_transformer.rb', line 264

def self.reconstruct_simple_glyph(num_contours, n_points_io, flag_io,
                                  glyph_io, bbox_io, instruction_io)
  # Read end points of contours
  end_pts_of_contours = []
  max_points_per_glyph = 100000 # Sanity limit

  num_contours.times do
    if end_pts_of_contours.empty?
      val = read_255_uint16(n_points_io)
      end_pts_of_contours << val
    else
      delta = read_255_uint16(n_points_io)
      next_end_pt = end_pts_of_contours.last + delta + 1

      # Sanity check to prevent explosion from corrupted data
      if delta > 10000 || next_end_pt > max_points_per_glyph
        # Data appears corrupted, stop reading
        break
      end

      end_pts_of_contours << next_end_pt
    end
  end

  # Handle case where stream was corrupted
  if end_pts_of_contours.empty?
    # Return minimal empty glyph
    return ["\x00\x00"].pack("n") * 5 # Empty glyph: num_contours=0, bbox=0
  end

  total_points = [end_pts_of_contours.last + 1, max_points_per_glyph].min

  # Read flags
  flags = read_flags(flag_io, total_points)

  # Read coordinates
  x_coordinates = read_coordinates(glyph_io, flags, X_SHORT_VECTOR,
                                   X_IS_SAME_OR_POSITIVE_X_SHORT_VECTOR)
  y_coordinates = read_coordinates(glyph_io, flags, Y_SHORT_VECTOR,
                                   Y_IS_SAME_OR_POSITIVE_Y_SHORT_VECTOR)

  # Read bounding box safely
  bbox_remaining = bbox_io.size - bbox_io.pos
  if bbox_remaining < 8
    # Not enough data, use default bounding box
    x_min = y_min = x_max = y_max = 0
  else
    bbox_bytes = bbox_io.read(8)
    if bbox_bytes && bbox_bytes.bytesize == 8
      x_min, y_min, x_max, y_max = bbox_bytes.unpack("n4")
      # Convert to signed
      x_min = x_min > 0x7FFF ? x_min - 0x10000 : x_min
      y_min = y_min > 0x7FFF ? y_min - 0x10000 : y_min
      x_max = x_max > 0x7FFF ? x_max - 0x10000 : x_max
      y_max = y_max > 0x7FFF ? y_max - 0x10000 : y_max
    else
      x_min = y_min = x_max = y_max = 0
    end
  end

  # Read instructions safely
  instruction_length = 0
  instructions = ""

  inst_remaining = instruction_io.size - instruction_io.pos
  if inst_remaining >= 2
    inst_length_data = read_255_uint16(instruction_io)
    if inst_length_data
      instruction_length = inst_length_data
      if instruction_length.positive?
        inst_remaining = instruction_io.size - instruction_io.pos
        instructions = if inst_remaining >= instruction_length
                         instruction_io.read(instruction_length) || ""
                       else
                         # Read what we can
                         instruction_io.read(inst_remaining) || ""
                       end
      end
    end
  end

  # Build glyph data in standard format
  build_simple_glyph_data(num_contours, x_min, y_min, x_max, y_max,
                          end_pts_of_contours, instructions, flags,
                          x_coordinates, y_coordinates)
end