Class: Fontisan::Tables::Hhea

Inherits:
Binary::BaseRecord show all
Extended by:
Registered
Defined in:
lib/fontisan/tables/hhea.rb

Overview

BinData structure for the 'hhea' (Horizontal Header) table

The hhea table contains horizontal layout metrics for the entire font. It defines font-wide horizontal metrics such as ascent, descent, line gap, and the number of horizontal metrics in the hmtx table.

Reference: OpenType specification, hhea table https://docs.microsoft.com/en-us/typography/opentype/spec/hhea

Examples:

Reading an hhea table

data = File.binread("font.ttf", 36, hhea_offset)
hhea = Fontisan::Tables::Hhea.read(data)
puts hhea.ascent           # => 2048
puts hhea.descent          # => -512
puts hhea.version_number   # => 1.0

Constant Summary collapse

TABLE_SIZE =

Table size in bytes (fixed size)

36

Instance Attribute Summary

Attributes inherited from Binary::BaseRecord

#raw_data

Instance Method Summary collapse

Methods included from Registered

register_tag

Methods inherited from Binary::BaseRecord

read

Instance Method Details

#valid? ⇒ Boolean

Check if the table is valid

Returns:

  • (Boolean) —

    True if valid, false otherwise



88
89
90
91
92
93
94
95
96
97
98
99
 
# File 'lib/fontisan/tables/hhea.rb', line 88

def valid?
  # Version should be 1.0 (0x00010000)
  return false unless version_raw == 0x00010000

  # Metric data format must be 0
  return false unless metric_data_format.zero?

  # Number of metrics must be at least 1
  return false unless number_of_h_metrics >= 1

  true
end

#valid_advance_width_max? ⇒ Boolean

Validation helper: Check if advance width max is positive

Maximum advance width should be > 0

Returns:

  • (Boolean) —

    True if advance_width_max > 0



151
152
153
 
# File 'lib/fontisan/tables/hhea.rb', line 151

def valid_advance_width_max?
  advance_width_max&.positive?
end

#valid_ascent_descent? ⇒ Boolean

Validation helper: Check if ascent/descent values are reasonable

Ascent should be positive, descent should be negative

Returns:

  • (Boolean) —

    True if ascent/descent have correct signs



133
134
135
 
# File 'lib/fontisan/tables/hhea.rb', line 133

def valid_ascent_descent?
  ascent.positive? && descent.negative?
end

#valid_caret_slope? ⇒ Boolean

Validation helper: Check if caret slope is valid

For vertical text: rise=1, run=0 For horizontal italic: both should be non-zero

Returns:

  • (Boolean) —

    True if caret slope values are sensible



161
162
163
164
 
# File 'lib/fontisan/tables/hhea.rb', line 161

def valid_caret_slope?
  # At least one should be non-zero
  caret_slope_rise != 0 || caret_slope_run != 0
end

#valid_line_gap? ⇒ Boolean

Validation helper: Check if line gap is non-negative

Line gap should be >= 0

Returns:

  • (Boolean) —

    True if line_gap >= 0



142
143
144
 
# File 'lib/fontisan/tables/hhea.rb', line 142

def valid_line_gap?
  line_gap >= 0
end

#valid_metric_data_format? ⇒ Boolean

Validation helper: Check if metric data format is valid

Must be 0 for current format

Returns:

  • (Boolean) —

    True if format is 0



115
116
117
 
# File 'lib/fontisan/tables/hhea.rb', line 115

def valid_metric_data_format?
  metric_data_format.zero?
end

#valid_number_of_h_metrics? ⇒ Boolean

Validation helper: Check if number of h metrics is valid

Must be at least 1

Returns:

  • (Boolean) —

    True if number_of_h_metrics >= 1



124
125
126
 
# File 'lib/fontisan/tables/hhea.rb', line 124

def valid_number_of_h_metrics?
  number_of_h_metrics && number_of_h_metrics >= 1
end

#valid_version? ⇒ Boolean

Validation helper: Check if version is valid

OpenType spec requires version to be 1.0

Returns:

  • (Boolean) —

    True if version is 1.0



106
107
108
 
# File 'lib/fontisan/tables/hhea.rb', line 106

def valid_version?
  version_raw == 0x00010000
end

#valid_x_max_extent? ⇒ Boolean

Validation helper: Check if extent is reasonable

x_max_extent should be positive

Returns:

  • (Boolean) —

    True if x_max_extent > 0



171
172
173
 
# File 'lib/fontisan/tables/hhea.rb', line 171

def valid_x_max_extent?
  x_max_extent.positive?
end

#validate! ⇒ Object

Validate the table and raise error if invalid

Raises:



178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
 
# File 'lib/fontisan/tables/hhea.rb', line 178

def validate!
  unless version_raw == 0x00010000
    message = "Invalid hhea version: expected 0x00010000 (1.0), " \
              "got 0x#{version_raw.to_i.to_s(16).upcase}"
    raise Fontisan::CorruptedTableError, message
  end

  unless metric_data_format.zero?
    message = "Invalid metric data format: expected 0, " \
              "got #{metric_data_format.to_i}"
    raise Fontisan::CorruptedTableError, message
  end

  unless number_of_h_metrics >= 1
    message = "Invalid number of h metrics: must be >= 1, " \
              "got #{number_of_h_metrics.to_i}"
    raise Fontisan::CorruptedTableError, message
  end
end

#version ⇒ Float

Convert version from fixed-point to float

Returns:

  • (Float) —

    Version number (typically 1.0)



81
82
83
 
# File 'lib/fontisan/tables/hhea.rb', line 81

def version
  fixed_to_float(version_raw)
end