Hint Conversion

Fontisan provides bidirectional hint conversion between TrueType and PostScript formats.

Overview

Direction	From	To	Process
TTF → OTF	TrueType instructions	PostScript hints	Analyze bytecode
OTF → TTF	PostScript hints	TrueType instructions	Generate bytecode

TTF → OTF Conversion

Process

TrueType Font → Instruction Analysis → PostScript Parameters → CFF Table
   (Input)       (prep/fpgm/cvt)      (Hint Dict)        (Output)

Analysis Steps

1. Parse prep (Control Value Program) bytecode
2. Analyze fpgm (Font Program) for complexity indicators
3. Extract blue zones from CVT (Control Value Table) values
4. Extract stem widths and alignment zones

Generated Parameters

Parameter	Source
blue_scale	CVT analysis
std_hw	CVT[0] (standard horizontal stem)
std_vw	CVT[1] (standard vertical stem)
stem_snap_h	CVT analysis
stem_snap_v	CVT analysis
blue_values	CVT analysis (baseline, cap)
other_blues	CVT analysis (descender)

CLI

fontisan convert font.ttf --to otf --hinting-mode preserve

API

converter = Fontisan::Converters::HintConverter.new

# Convert TrueType hints to PostScript
ps_hints = converter.truetype_to_postscript(font)

# Result contains:
# {
#   blue_scale: 0.039625,
#   std_hw: 80,
#   std_vw: 100,
#   blue_values: [-20, 0, 700, 720],
#   stem_snap_h: [80, 90],
#   stem_snap_v: [100, 110]
# }

OTF → TTF Conversion

Process

OTF Font → Parameter Extraction → Instruction Generation → TrueType Tables
 (Input)    (CFF Private Dict)    (prep/fpgm/cvt)        (Output)

Generation Steps

1. Generate prep program with CVT cut-in, single width settings
2. Build CVT table with stem widths and blue zone values
3. Create fpgm program (typically empty for converted fonts)
4. Encode instructions using optimal PUSH opcodes

Generated Tables

Table	Content
prep	Control Value Program with hint setup
cvt	Control Value Table with stems and blue zones
fpgm	Font Program (empty for converted fonts)

CLI

fontisan convert font.otf --to ttf --hinting-mode preserve

API

converter = Fontisan::Converters::HintConverter.new

# Convert PostScript hints to TrueType
ttf_tables = converter.postscript_to_truetype(font)

# Result contains:
# {
#   prep: "\xB0\x11\x1D",  # Bytecode
#   cvt: [80, 100, -20, 0, 700, 720],
#   fpgm: ""
# }

Round-Trip Conversion

Fontisan ensures hint integrity through round-trip conversion:

Original PS Hints → TrueType → PostScript → Validation
     (Input)         (Convert)   (Convert)    (Verify)

Round-Trip Guarantees

• CVT values preserved (sorted and deduplicated)
• Standard widths (std_hw, std_vw) maintained
• Blue zone values retained
• <10% loss tolerance for approximations

Known Limitations

• CVT sorting may change positions (optimization trade-off)
• blue_scale not perfectly round-trippable (conversion approximation)
• Standard widths extracted from CVT[0] and CVT[1] positions

Example

# Round-trip test
font = Fontisan::FontLoader.load('font.otf')
original_hints = font.tables['CFF'].top_dicts.first[:private]

# Convert to TrueType
ttf_converter = Fontisan::Converters::HintConverter.new
ttf_tables = ttf_converter.postscript_to_truetype(font)

# Convert back to PostScript
ps_hints = ttf_converter.truetype_to_postscript(ttf_font)

# Compare
puts "Original std_hw: #{original_hints[:std_hw]}"
puts "Round-trip std_hw: #{ps_hints[:std_hw]}"

Best Practices

1. Test round-trip — Verify hint preservation
2. Use autohint for cross-format — Consider --hinting-mode auto
3. Validate after conversion — Check hint validity
4. Test rendering — Verify visual quality at small sizes