beamterm-atlas

A font atlas generator for WebGL terminal renderers, optimized for GPU texture memory and rendering efficiency.

Overview

beamterm-atlas generates tightly-packed 2D texture array atlases from TTF/OTF font files, producing a binary format optimized for GPU upload. The system supports multiple font styles, full Unicode including emoji, and automatic grapheme clustering.

Architecture

The crate consists of:

• Font rasterization engine using cosmic-text for high-quality text rendering
• 2D texture array packer organizing glyphs into 32×1 grids per texture layer
• Binary serializer with zlib compression for efficient storage
• Atlas verification tool for debugging and visualization

Glyph ID Assignment System

ID Structure

The system uses a 16-bit glyph ID that encodes both the base character and its style variations:

The atlas only encodes glyphs with the first 13 bits. Bits 13 and 14 are applied at runtime for text decoration effects, while bit 15 is reserved for future extensions.

Font Style Encoding

Each base glyph automatically generates four style variants by combining the bold and italic flags:

This encoding allows the shader to compute texture coordinates directly from the glyph ID without lookup tables.

Character Category Assignment

The generator assigns IDs based on three character categories:

1. ASCII Characters (0x00-0x7F)

• Direct mapping: character code = base glyph ID
• Guarantees fast lookup for common characters
• Occupies first 8 texture layers (128 chars ÷ 16 per layer)

2. Unicode Characters

• Fill unused slots in the 0x00-0x1FF range
• Sequential assignment starting from first available ID
• Constrained to 512 glyphs (0x000-0x1FF)

3. Emoji Characters

• Start at ID 0x800 (bit 11 set)
• Sequential assignment: 0x800, 0x801, 0x802...
• No style variants (emoji are always rendered as-is)
• Can extend beyond the 512 base glyph limit

Texture Layer Calculation

With the ID assignment scheme:

• Regular glyphs with styles: IDs 0x0000-0x07FF (first 128 layers)
• Emoji glyphs: IDs 0x0800+ (layers 128+)

For a typical atlas with ~500 base glyphs + 100 emoji:

• Base glyphs × 4 styles = 2000 IDs → 125 layers
• Emoji = 100 IDs → 7 additional layers
• Total = 132 layers

2D Texture Array Organization

Layer Layout

Each texture layer contains a 32×1 grid of glyphs:

Position in layer = ID & 0x1F (modulo 32)
Grid X = Position (0-31)
Grid Y = 0 (always single row)
Layer = ID ÷ 32

Memory Layout

The 2D texture array uses RGBA format with dimensions:

• Width: cell_width × 32
• Height: cell_height × 1
• Layers: max_glyph_id ÷ 32

The RGBA format is required for emoji support - while monochrome glyphs could use a single channel, emoji glyphs need full color information.

This layout ensures:

• Efficient GPU memory alignment
• Cache-friendly access pattern (sequential glyphs in same row)
• Simple coordinate calculation using bit operations

Rasterization Process

Cell Dimension Calculation

The system determines cell size by measuring the full block character █ to ensure all glyphs fit within the cell boundaries. Additional padding of 1px on all sides prevents texture bleeding.

Font Style Handling

Each glyph is rendered four times, one for each of the styles (normal, bold, italic, bold+italic).

Emoji Special Handling

Emoji glyphs require special processing:

1. Rendered at 2× size for measurement
2. Scaled down to fit within cell boundaries
3. Centered within the cell
4. Color information preserved in texture

The presence of emoji is the primary reason the atlas uses RGBA format instead of a single-channel texture. While monochrome glyphs only need an alpha channel, emoji require full color information to render correctly.

Binary Atlas Format

File Structure

The atlas uses a versioned binary format with header validation:

Header (5 bytes)
├─ Magic: [0xBA, 0xB1, 0xF0, 0xA7]
└─ Version: 0x01

Metadata Section
├─ Font name (u8 length + UTF-8 string)
├─ Font size (f32)
├─ Texture width (i32)
├─ Texture height (i32)
├─ Texture layers (i32)
├─ Cell width (i32)
├─ Cell height (i32)
├─ Underline position (f32)
├─ Underline thickness (f32)
├─ Strikethrough position (f32)
├─ Strikethrough thickness (f32)
└─ Glyph count (u16)

Glyph Definitions
└─ Per glyph:
   ├─ ID (u16 - includes style bits)
   ├─ Style (u8) - ordinal: 0=Normal, 1=Bold, 2=Italic, 3=BoldItalic
   ├─ Is emoji (u8) - 0=false, 1=true
   ├─ Pixel X (i32)
   ├─ Pixel Y (i32)
   └─ Symbol (u8 length + UTF-8 string)

Compressed Texture Data
├─ Data length (u32)
└─ zlib-compressed RGBA data

Serialization Properties

• Endianness: Little-endian for cross-platform compatibility
• Compression: zlib level 9 (typically 75% size reduction)
• String encoding: Length-prefixed UTF-8 (u8 for strings, max 255 bytes)
• Texture data: Length-prefixed compressed data (u32 length)
• Alignment: Natural alignment without padding

Usage

Installation

cargo install beamterm-atlas

Command-Line Interface

beamterm-atlas [OPTIONS] <FONT>

Arguments

• <FONT> - Font selection by name (partial match) or 1-based index

Options

• -s, --font-size <SIZE> - Font size in points (default: 15.0)
• -l, --line-height <MULTIPLIER> - Line height multiplier (default: 1.0)
• -o, --output <PATH> - Output file path (default: "./bitmap_font.atlas")
• --underline-position <FRACTION> - Underline position from 0.0 (top) to 1.0 (bottom) of cell (default: 0.85)
• --underline-thickness <PERCENT> - Underline thickness as percentage of cell height (default: 5.0)
• --strikethrough-position <FRACTION> - Strikethrough position from 0.0 (top) to 1.0 (bottom) of cell (default: 0.5)
• --strikethrough-thickness <PERCENT> - Strikethrough thickness as percentage of cell height (default: 5.0)
• -L, --list-fonts - List available fonts and exit

Examples

List all available monospace fonts with complete style variants:

beamterm-atlas --list-fonts

Generate an atlas using JetBrains Mono at 16pt:

beamterm-atlas "JetBrains Mono" -s 16 -o jetbrains-16.atlas

Generate with custom text decoration settings:

beamterm-atlas "Fira Code" \
  --underline-position 0.9 \
  --underline-thickness 7.5 \
  --strikethrough-position 0.45

Select font by index (useful for scripting):

# First, list fonts to see indices
beamterm-atlas -L

# Then select by number
beamterm-atlas 5 -s 14

Character Set

The tool generates an atlas from a predefined character set including:

• Full ASCII and Latin-1 supplement
• Box drawing characters
• Mathematical symbols
• Arrows and geometric shapes
• Braille patterns
• Extensive emoji set

Verification

The verify-atlas binary visualizes the texture layout, showing:

• Layer organization
• Character placement
• Grid boundaries
• Glyph distribution

verify-atlas

Font Requirements

The generator requires monospace fonts with all four style variants:

• Regular
• Bold
• Italic
• Bold+Italic

Fonts missing any variant will not appear in the font list. The system automatically discovers all installed system fonts that meet these requirements.