Class: Fontisan::Variation::MetricsAdjuster

Inherits:
Object
  • Object
show all
Includes:
TableAccessor
Defined in:
lib/fontisan/variation/metrics_adjuster.rb

Overview

Applies metrics variation deltas to font metrics tables

This class handles applying variation deltas from HVAR, VVAR, and MVAR tables to the corresponding metrics tables (hmtx, vmtx, head, hhea, etc.).

Process:

  1. Parse ItemVariationStore from HVAR/VVAR/MVAR
  2. Calculate scalars for current coordinates using regions
  3. Apply deltas to base metrics
  4. Update metrics tables with adjusted values

Examples:

Applying HVAR deltas

adjuster = MetricsAdjuster.new(font, interpolator)
adjuster.apply_hvar_deltas({ "wght" => 700.0 })

Constant Summary collapse

METRIC_SOURCES =

Maps MVAR metric tags to [table_tag, method] for base value lookup.

{
  "hasc" => ["hhea", :ascender],
  "hdsc" => ["hhea", :descender],
  "hlgp" => ["hhea", :line_gap],
  "xhgt" => ["OS/2", :sx_height],
  "cpht" => ["OS/2", :s_cap_height],
}.freeze

Instance Attribute Summary collapse

Instance Method Summary collapse

Methods included from TableAccessor

#clear_variation_cache, #clear_variation_table, #has_variation_table?, #require_variation_table, #variation_table

Constructor Details

#initialize(font, interpolator) ⇒ MetricsAdjuster

Initialize metrics adjuster

Parameters:



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# File 'lib/fontisan/variation/metrics_adjuster.rb', line 32

def initialize(font, interpolator)
  @font = font
  @interpolator = interpolator
  @variation_tables = {}
end

Instance Attribute Details

#font ⇒ TrueTypeFont, OpenTypeFont (readonly)

Returns Font instance.

Returns:



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# File 'lib/fontisan/variation/metrics_adjuster.rb', line 23

def font
  @font
end

#interpolator ⇒ Interpolator (readonly)

Returns Coordinate interpolator.

Returns:



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# File 'lib/fontisan/variation/metrics_adjuster.rb', line 26

def interpolator
  @interpolator
end

Instance Method Details

#apply_hvar_deltas(coordinates) ⇒ Boolean

Apply HVAR deltas to horizontal metrics

Parameters:

  • coordinates (Hash<String, Float>) —

    Design space coordinates

Returns:

  • (Boolean) —

    True if deltas were applied



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# File 'lib/fontisan/variation/metrics_adjuster.rb', line 42

def apply_hvar_deltas(coordinates)
  return false unless has_variation_table?("HVAR")
  return false unless has_variation_table?("hmtx")

  hvar = variation_table("HVAR")
  return false unless hvar&.item_variation_store

  # Parse region list and calculate scalars
  regions = extract_regions_from_store(hvar.item_variation_store)
  return false if regions.empty?

  scalars = @interpolator.calculate_scalars(coordinates, regions)

  # Get hmtx table
  hmtx = variation_table("hmtx")
  return false unless hmtx&.parsed?

  # Get glyph count
  maxp = variation_table("maxp")
  glyph_count = maxp ? maxp.num_glyphs : 0
  return false if glyph_count.zero?

  # Apply deltas to each glyph
  adjusted_metrics = apply_horizontal_metrics_deltas(
    hvar, hmtx, glyph_count, scalars
  )

  # Rebuild hmtx table with adjusted metrics
  rebuild_hmtx_table(adjusted_metrics)

  true
end

#apply_mvar_deltas(coordinates) ⇒ Boolean

Apply MVAR deltas to font-wide metrics

Parameters:

  • coordinates (Hash<String, Float>) —

    Design space coordinates

Returns:

  • (Boolean) —

    True if deltas were applied



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# File 'lib/fontisan/variation/metrics_adjuster.rb', line 103

def apply_mvar_deltas(coordinates)
  return false unless has_variation_table?("MVAR")

  mvar = variation_table("MVAR")
  return false unless mvar&.item_variation_store

  # Parse region list and calculate scalars
  regions = extract_regions_from_store(mvar.item_variation_store)
  return false if regions.empty?

  scalars = @interpolator.calculate_scalars(coordinates, regions)

  # Apply deltas to each metric tag
  apply_font_wide_metrics_deltas(mvar, scalars)

  true
end

#apply_vvar_deltas(coordinates) ⇒ Boolean

Apply VVAR deltas to vertical metrics

Parameters:

  • coordinates (Hash<String, Float>) —

    Design space coordinates

Returns:

  • (Boolean) —

    True if deltas were applied



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# File 'lib/fontisan/variation/metrics_adjuster.rb', line 79

def apply_vvar_deltas(coordinates)
  return false unless has_variation_table?("VVAR")
  return false unless has_variation_table?("vmtx")

  vvar = variation_table("VVAR")
  return false unless vvar&.item_variation_store

  # Parse region list and calculate scalars
  regions = extract_regions_from_store(vvar.item_variation_store)
  return false if regions.empty?

  @interpolator.calculate_scalars(coordinates, regions)

  # Apply deltas to vmtx
  # Similar to HVAR but for vertical metrics
  # Placeholder for full implementation

  true
end

#build_hmtx_data(metrics) ⇒ String?

Build hmtx binary data from metrics

Parameters:

  • metrics (Array<Hash>) —

    Metrics to encode

Returns:

  • (String, nil) —

    Binary data



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# File 'lib/fontisan/variation/metrics_adjuster.rb', line 262

def build_hmtx_data(metrics)
  return nil if metrics.empty?

  # Find last unique advance width
  last_advance = metrics.last[:advance_width]
  number_of_h_metrics = metrics.length

  # Optimize: count from end while advance width is same
  (metrics.length - 1).downto(1) do |i|
    break if metrics[i][:advance_width] != last_advance

    number_of_h_metrics = i
  end

  # Build binary data
  data = String.new("", encoding: Encoding::BINARY)

  # Write hMetrics array
  number_of_h_metrics.times do |i|
    metric = metrics[i]
    data << [metric[:advance_width]].pack("n")  # uint16
    data << [metric[:lsb]].pack("n")            # int16 (as uint16, will be interpreted as signed)
  end

  # Write remaining LSBs
  (number_of_h_metrics...metrics.length).each do |i|
    data << [metrics[i][:lsb]].pack("n") # int16
  end

  # Update hhea's numberOfHMetrics
  update_hhea_number_of_h_metrics(number_of_h_metrics)

  data
end

#get_base_metric_value(tag) ⇒ Object 



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# File 'lib/fontisan/variation/metrics_adjuster.rb', line 227

def get_base_metric_value(tag)
  source = METRIC_SOURCES[tag]
  return nil unless source

  table_tag, method = source
  variation_table(table_tag)&.public_send(method)
end

#rebuild_hmtx_table(metrics) ⇒ Object

Rebuild hmtx table with adjusted metrics

Parameters:

  • metrics (Array<Hash>) —

    Adjusted metrics



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# File 'lib/fontisan/variation/metrics_adjuster.rb', line 250

def rebuild_hmtx_table(metrics)
  # Build new hmtx binary data
  data = build_hmtx_data(metrics)

  # Update font's table data
  @font.table_data["hmtx"] = data if data
end

#update_font_metric(tag, value) ⇒ Object

Update font metric in appropriate table

Parameters:

  • tag (String) —

    Metric tag

  • value (Integer) —

    New metric value



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# File 'lib/fontisan/variation/metrics_adjuster.rb', line 239

def update_font_metric(tag, value)
  # This is a placeholder - full implementation would:
  # 1. Modify the appropriate table's binary data
  # 2. Update the font's table data
  # For now, we just log the update
  # In production, this would rebuild the affected tables
end

#update_hhea_number_of_h_metrics(count) ⇒ Object

Update hhea table's numberOfHMetrics field

Parameters:

  • count (Integer) —

    New numberOfHMetrics value



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# File 'lib/fontisan/variation/metrics_adjuster.rb', line 300

def update_hhea_number_of_h_metrics(count)
  return unless has_variation_table?("hhea")

  hhea = variation_table("hhea")
  return unless hhea

  # Update the field if hhea supports it
  hhea.number_of_h_metrics = count
end