TrueType Hinting

TrueType hinting uses bytecode instructions to control glyph rendering.

Storage

TrueType hints are stored in three tables:

Table	Purpose
prep	Control Value Program
fpgm	Font Program
cvt	Control Value Table

Instructions

Stack Operations

Opcode	Name	Purpose
0x40	NPUSHB	Push n bytes onto stack
0x41	NPUSHW	Push n words onto stack
0xB0-0xB7	PUSHB[0-7]	Push 1-8 bytes
0xB8-0xBF	PUSHW[0-7]	Push 1-8 words

Control Value Instructions

Opcode	Name	Purpose
0x1D	SCVTCI	Set CVT cut-in
0x1E	SSWCI	Set single width cut-in
0x1F	SSW	Set single width
0x44	WCVTP	Write CVT in pixels
0x70	WCVTF	Write CVT in FUnits

Reading Instructions

font = Fontisan::FontLoader.load('font.ttf')

# Get prep table
prep = font.tables['prep']
if prep
  puts "Prep program: #{prep.bytecode.length} bytes"

  # Parse instructions
  parser = Fontisan::Hints::InstructionParser.new
  instructions = parser.parse(prep.bytecode)

  instructions.each do |inst|
    puts "#{inst.opcode.to_s(16)}: #{inst.name}"
  end
end

# Get CVT table
cvt = font.tables['cvt']
if cvt
  puts "CVT entries: #{cvt.values.length}"
  cvt.values.each_with_index do |value, i|
    puts "  CVT[#{i}] = #{value}"
  end
end

Validation

validator = Fontisan::Hints::HintValidator.new

# Validate prep instructions
prep = font.tables['prep']
result = validator.validate_truetype_instructions(prep.bytecode)

if result[:valid]
  puts "Valid instructions"
else
  result[:errors].each do |error|
    puts "Error: #{error}"
  end
end

Validation Checks

• Bytecode Structure — Validates instruction opcodes and parameters
• Stack Operations — Ensures proper stack depth management
• Parameter Counts — Verifies correct number of operands
• Truncation Detection — Identifies incomplete instructions
• Stack Neutrality — Checks if sequences maintain stack balance

Instruction Encoding

The generator automatically selects efficient encoding:

Byte Values (0-255)

Value 17: PUSHB[0] 17          (2 bytes)
Values [10,20,30]: NPUSHB 3 10 20 30  (5 bytes)

Word Values (256-65535)

Value 300: PUSHW[0] 0x01 0x2C   (3 bytes, big-endian)
Values [300,400]: NPUSHW 2 0x01 0x2C 0x01 0x90  (7 bytes)

CVT Values

The Control Value Table stores frequently used values:

cvt = font.tables['cvt']

# Typical CVT layout
# CVT[0] - Standard vertical stem width
# CVT[1] - Standard horizontal stem width
# CVT[2-7] - Blue zone values
# CVT[8+] - Other common values

puts "Standard V stem: #{cvt.values[0]}"
puts "Standard H stem: #{cvt.values[1]}"

Best Practices

1. Use CVT for common values — Reduces bytecode size
2. Keep prep simple — Complex programs can cause compatibility issues
3. Test cross-platform — Hinting behavior varies by rasterizer
4. Validate before release — Ensure instructions are well-formed