Bitmap Color Fonts

Bitmap color fonts store pixel images at specific sizes.

Formats

Format	Tables	Platform
sbix	sbix	Apple
CBDT/CBLC	CBDT, CBLC	Google/Android

sbix (Apple)

sbix stores bitmap images at multiple resolutions.

Structure

sbix = font.tables['sbix']

# Strikes (image sets at different sizes)
sbix.strikes.each do |strike|
  puts "Strike at #{strike.ppem} ppem"

  strike.glyphs.each do |glyph_id, image|
    puts "  Glyph #{glyph_id}: #{image.format}"
  end
end

Image Formats

• png — PNG image
• jpg — JPEG image
• tiff — TIFF image
• dupe — Duplicate of another glyph

Accessing Images

sbix = font.tables['sbix']
strike = sbix.strikes.first

glyph_id = 42
image = strike.image_for(glyph_id)

puts "Format: #{image.format}"
puts "Data size: #{image.data.length} bytes"

CBDT/CBLC (Android)

CBDT/CBLC stores bitmap images for Android.

Structure

cbdt = font.tables['CBDT']
cblc = font.tables['CBLC']

# CBLC has location data
cblc.strikes.each do |strike|
  puts "Strike at #{strike.ppem}x#{strike.ppem}"

  strike.glyph_ranges.each do |range|
    puts "  Glyphs #{range.start}..#{range.end}"
  end
end

# CBDT has actual bitmap data

Bitmap Formats

• Small metrics — For small glyphs
• Big metrics — For larger glyphs
• Bit aligned — 1, 2, 4, or 8 bits per pixel

Accessing Bitmaps

cbdt = font.tables['CBDT']
cblc = font.tables['CBLC']

glyph_id = 42
strike_index = 0

# Get bitmap location
location = cblc.location(strike_index, glyph_id)

if location
  # Get bitmap data
  bitmap = cbdt.bitmap(location)
  puts "Size: #{bitmap.width}x#{bitmap.height}"
  puts "Data: #{bitmap.data.length} bytes"
end

Working with Bitmaps

List Available Sizes

sbix = font.tables['sbix']

if sbix
  puts "Available sizes:"
  sbix.strikes.each do |strike|
    puts "  #{strike.ppem} ppem"
  end
end

Export Bitmaps

sbix = font.tables['sbix']
strike = sbix.strikes.first

sbix.glyphs.each do |glyph_id, image|
  if image.format == 'png '
    File.write("glyph-#{glyph_id}.png", image.data)
  end
end

Conversion

Preserve Bitmaps

# Bitmaps preserved during same-format conversion
fontisan convert bitmap-font.ttf --to ttf --output bitmap-font.ttf

Remove Bitmaps

# Create outline-only version
fontisan convert bitmap-font.ttf --to ttf --no-bitmaps --output outline-only.ttf

Subsetting color-emoji fonts

The web subsetting profile retains CBDT and CBLC tables and rewrites them to keep only the bitmaps for retained glyphs. Use it when subsetting a color-emoji source for browser delivery:

# Keep only 😀 + 😁 bitmaps; output WOFF2 for web
fontisan subset NotoColorEmoji.ttf \
  --chars "😀😁" \
  --profile web \
  --output-format woff2 \
  --output emoji-subset.woff2

Other profiles (pdf, minimal, full) drop CBDT/CBLC — pick them when the subset doesn't need color-emoji rendering (e.g. PDF embedding where the subsetter is just selecting glyphs for outlines).

See the subset CLI docs and docs/STITCHER_GUIDE.adoc (CBDT/CBLC passthrough section) for the full story, including the GID-stability caveat when CBDT and outline donors cover overlapping codepoint ranges in stitch mode.

Comparison

Feature	sbix	CBDT/CBLC
Platform	Apple	Android
Image formats	PNG, JPEG, TIFF	Raw bitmap
Scalability	No	No
File size	Larger	Smaller
Quality	Photo-realistic	Limited

Limitations

Bitmap Scaling

Bitmap fonts don't scale well:

• Each strike is designed for a specific ppem
• Scaling up causes pixelation
• Scaling down causes aliasing

Variable Fonts

Bitmap fonts don't work with variation:

• No interpolation possible
• Must provide strikes for each instance

Best Practices

1. Provide multiple strikes — Cover common sizes
2. Use PNG for sbix — Best quality/size tradeoff
3. Test on target platforms — Rendering varies
4. Consider outline fallback — For unavailable sizes