PostScript Hinting

PostScript hints are declarative values stored in the CFF Private dictionary.

Hint Parameters

Blue Values

Alignment zones for vertical positioning:

Parameter	Description	Max Values
blue_values	Baseline and top zones	14 (7 pairs)
other_blues	Descender zones	10 (5 pairs)

# Example blue values
blue_values = [-20, 0, 700, 720]  # Baseline zone and cap height zone
other_blues = [-250, -230]        # Descender zone

Stem Widths

Standard stem dimensions:

Parameter	Description	Max Values
std_hw	Standard horizontal stem	1
std_vw	Standard vertical stem	1
stem_snap_h	Horizontal stem snap values	12
stem_snap_v	Vertical stem snap values	12

# Example stem values
std_hw = 80    # Standard horizontal stem width
std_vw = 100   # Standard vertical stem width
stem_snap_h = [80, 90, 100]  # Common H stem widths
stem_snap_v = [100, 110, 120] # Common V stem widths

Other Parameters

Parameter	Description	Default
blue_scale	Threshold for alignment zones	0.039625
blue_shift	Overshoot threshold	7
blue_fuzz	Blue zone expansion	1
force_bold	Force bold rendering	false
language_group	0=Latin, 1=CJK	0

Reading Hints

font = Fontisan::FontLoader.load('font.otf')
cff = font.tables['CFF']

# Get Private DICT
private = cff.top_dicts.first[:private]

# Read hint parameters
puts "Blue scale: #{private[:blue_scale]}"
puts "Std H width: #{private[:std_hw]}"
puts "Std V width: #{private[:std_vw]}"
puts "Blue values: #{private[:blue_values]}"
puts "Other blues: #{private[:other_blues]}"
puts "Stem snap H: #{private[:stem_snap_h]}"
puts "Stem snap V: #{private[:stem_snap_v]}"

Validation

validator = Fontisan::Hints::HintValidator.new

hints = {
  blue_scale: 0.039625,
  std_hw: 80,
  blue_values: [-20, 0, 700, 720]
}

result = validator.validate_postscript_hints(hints)

if result[:valid]
  puts "Valid PostScript hints"
else
  result[:errors].each { |err| puts "Error: #{err}" }
end

Validation Checks

• Value Ranges — Validates hint parameter bounds
• Pair Validation — Ensures blue zones are in pairs (even count)
• Array Limits — Enforces CFF specification limits
• Positive Values — Verifies stem widths are positive
• Language Group — Validates value is 0 or 1

Blue Zone Examples

Latin Font

blue_values = [
  -20, 0,      # Baseline zone (-20 to 0)
  470, 490,    # x-height zone (470 to 490)
  700, 720     # Cap height zone (700 to 720)
]

other_blues = [
  -250, -230   # Descender zone (-250 to -230)
]

CJK Font

blue_values = [
  -20, 0,      # Baseline
  880, 900     # Top zone
]

other_blues = []

language_group = 1  # CJK

Stem Snap Values

Stem snaps tell the rasterizer which widths are "standard":

# If stems in the font are primarily 80, 90, or 100 units
stem_snap_h = [80, 90, 100]

# The rasterizer will snap stems to these values
# when they're close enough

Best Practices

1. Keep blue zones in pairs — Each zone needs top and bottom
2. Use realistic values — Stems should match actual glyph stems
3. Don't over-specify — 4-5 blue zones are usually enough
4. Test at small sizes — Verify hints improve rendering
5. Consider font size — Adjust blue_scale for text vs display