Class: Fontisan::Hints::TrueTypeInstructionAnalyzer

Inherits:
Object
  • Object
show all
Defined in:
lib/fontisan/hints/truetype_instruction_analyzer.rb

Overview

Analyzes TrueType bytecode instructions to extract hint parameters

This analyzer parses fpgm (Font Program) and prep (Control Value Program) bytecode to extract semantic hint information that can be converted to PostScript Private dict parameters.

**Key Extracted Parameters:**

  • Blue zones (alignment zones for baseline, x-height, cap-height)

  • Stem widths (from CVT setup in prep)

  • Delta base and shift values

  • Twilight zone setup

Examples:

Analyze prep program

analyzer = TrueTypeInstructionAnalyzer.new
params = analyzer.analyze_prep(prep_bytecode, cvt_values)

Constant Summary collapse

NPUSHB =

TrueType instruction opcodes relevant for hint extraction

0x40
NPUSHW =

Push N bytes

0x41
PUSHB =

Push N words

(0xB0..0xB7).to_a
PUSHW =

Push 1-8 words

(0xB8..0xBF).to_a
SVTCA_Y =

Set freedom and projection vectors to Y-axis

0x00
SVTCA_X =

Set freedom and projection vectors to X-axis

0x01
RCVT =

Read CVT

0x45
WCVTP =

Write CVT (in Pixels)

0x44
WCVTF =

Write CVT (in FUnits)

0x70
MDAP =

Move Direct Absolute Point

[0x2E, 0x2F].freeze
SCVTCI =

Set Control Value Table Cut In

0x1D
SSWCI =

Set Single Width Cut In

0x1E
SSW =

Set Single Width

0x1F

Instance Method Summary collapse

Instance Method Details

#analyze_fpgm(fpgm) ⇒ Hash

Analyze Font Program (fpgm) for complexity indicators

The fpgm contains font-level function definitions. While we don’t fully decompile it, we can extract useful metadata about hint complexity.

Parameters:

  • fpgm (String) —

    Binary fpgm data

Returns:

  • (Hash) —

    Analysis results with complexity indicators



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# File 'lib/fontisan/hints/truetype_instruction_analyzer.rb', line 155

def analyze_fpgm(fpgm)
  return {} if fpgm.nil? || fpgm.empty?

  size = fpgm.bytesize

  # Estimate complexity based on size
  complexity = if size < 100
                 :simple
               elsif size < 200
                 :moderate
               else
                 :complex
               end

  {
    fpgm_size: size,
    has_functions: size > 0,
    complexity: complexity,
  }
rescue StandardError
  # Return empty hash on any error
  {}
end

#analyze_prep(prep, cvt = []) ⇒ Hash

Analyze prep program to extract hint parameters

Parameters:

  • prep (String) —

    Control value program bytecode

  • cvt (Array<Integer>) (defaults to: []) —

    Control values

Returns:

  • (Hash) —

    Extracted hint parameters



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# File 'lib/fontisan/hints/truetype_instruction_analyzer.rb', line 42

def analyze_prep(prep, cvt = [])
  return {} if prep.nil? && (cvt.nil? || cvt.empty?)

  params = {}

  # Parse prep bytecode if present
  if prep && !prep.empty?
    bytes = prep.bytes
    stack = []
    i = 0

    while i < bytes.length
      opcode = bytes[i]

      case opcode
      when NPUSHB
        # Push N bytes
        i += 1
        count = bytes[i]
        i += 1
        count.times do
          stack.push(bytes[i])
          i += 1
        end
        next

      when NPUSHW
        # Push N words (16-bit values)
        i += 1
        count = bytes[i]
        i += 1
        count.times do
          value = (bytes[i] << 8) | bytes[i + 1]
          # Convert to signed
          value = value - 65536 if value > 32767
          stack.push(value)
          i += 2
        end
        next

      when *PUSHB
        # Push 1-8 bytes
        count = opcode - 0xB0 + 1
        i += 1
        count.times do
          stack.push(bytes[i])
          i += 1
        end
        next

      when *PUSHW
        # Push 1-8 words
        count = opcode - 0xB8 + 1
        i += 1
        count.times do
          value = (bytes[i] << 8) | bytes[i + 1]
          value = value - 65536 if value > 32767
          stack.push(value)
          i += 2
        end
        next

      when WCVTP, WCVTF
        # Write to CVT - this shows which CVT indices are being set up
        # Pattern: value cvt_index WCVTP (stack top to bottom)
        if stack.length >= 2
          value = stack.pop
          cvt_index = stack.pop
          # Track CVT modifications (useful for understanding setup)
        end

      when SSW
        # Set Single Width - used for stem width control
        if stack.length >= 1
          width = stack.pop
          params[:single_width] = width unless params[:single_width]
        end

      when SSWCI
        # Set Single Width Cut In
        if stack.length >= 1
          params[:single_width_cut_in] = stack.pop
        end

      when SCVTCI
        # Set CVT Cut In
        if stack.length >= 1
          params[:cvt_cut_in] = stack.pop
        end
      end

      i += 1
    end
  end

  # Extract blue zones from CVT analysis (always do this if CVT is present)
  if cvt && !cvt.empty?
    params.merge!(extract_blue_zones_from_cvt(cvt))
  end

  params
rescue StandardError => e
  warn "Error analyzing prep program: #{e.message}"
  {}
end

#extract_blue_zones_from_cvt(cvt) ⇒ Hash

Extract blue zones from CVT values using heuristics

Blue zones in PostScript define alignment constraints for baseline, x-height, cap-height, ascender, and descender. TrueType doesn’t have explicit blue zones, but we can derive them from CVT values using common patterns.

Heuristics:

  • Negative values near -250 to -200: Descender zones

  • Values near 0: Baseline zones

  • Values near 500-550: X-height zones

  • Values near 700-750: Cap-height zones

  • For large UPM (>2000): Scale thresholds proportionally

Parameters:

  • cvt (Array<Integer>) —

    Control Value Table entries

Returns:

  • (Hash) —

    Extracted blue zone parameters



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# File 'lib/fontisan/hints/truetype_instruction_analyzer.rb', line 195

def extract_blue_zones_from_cvt(cvt)
  return {} if cvt.nil? || cvt.empty?

  zones = {}

  # Detect scale from maximum absolute value
  max_value = cvt.map(&:abs).max
  scale_factor = max_value > 1000 ? (max_value / 1000.0) : 1.0

  # Scaled thresholds
  descender_min = (-300 * scale_factor).to_i
  descender_max = (-150 * scale_factor).to_i
  baseline_range = (50 * scale_factor).to_i
  xheight_min = (450 * scale_factor).to_i
  xheight_max = (600 * scale_factor).to_i
  capheight_min = (650 * scale_factor).to_i
  capheight_max = (1500 * scale_factor).to_i  # Wider range for large UPM

  # Group CVT values by typical alignment zones
  descender_values = cvt.select { |v| v < descender_max && v > descender_min }
  baseline_values = cvt.select { |v| v >= -baseline_range && v <= baseline_range }
  xheight_values = cvt.select { |v| v >= xheight_min && v <= xheight_max }
  capheight_values = cvt.select { |v| v >= capheight_min && v <= capheight_max }

  # Build blue_values (baseline and top zones)
  blue_values = []

  # Add baseline zone if detected
  if baseline_values.any?
    min_baseline = baseline_values.min
    max_baseline = baseline_values.max
    blue_values << min_baseline << max_baseline
  end

  # Add cap-height zone if detected (or any top zone for large UPM)
  if capheight_values.any?
    min_cap = capheight_values.min
    max_cap = capheight_values.max
    blue_values << min_cap << max_cap
  end

  zones[:blue_values] = blue_values unless blue_values.empty?

  # Build other_blues (descender zones)
  if descender_values.any?
    min_desc = descender_values.min
    max_desc = descender_values.max
    zones[:other_blues] = [min_desc, max_desc]
  end

  zones
end