textshape

README

textshape

A pure Go implementation of text shaping, inspired by HarfBuzz.

Features

• OpenType Shaping: GSUB (substitution) and GPOS (positioning)
• Ligatures: Standard ligatures (fi, fl, ffi, ffl, etc.)
• Kerning: Pair adjustment positioning
• Mark positioning: Base-to-mark, mark-to-mark attachment
• Font Subsetting: Create minimal fonts for PDF embedding
• CFF Support: OpenType/CFF font subsetting with subroutine optimization
• HarfBuzz-compatible API: Similar concepts and data structures

Installation

go get github.com/boxesandglue/textshape

Quick Start

package main

import (
    "fmt"
    "os"

    "github.com/boxesandglue/textshape/ot"
)

func main() {
    // Load font
    data, _ := os.ReadFile("Roboto-Regular.ttf")
    font, _ := ot.ParseFont(data, 0)

    // Create shaper
    shaper, _ := ot.NewShaper(font)

    // Shape text
    buf := ot.NewBuffer()
    buf.AddString("office")
    buf.GuessSegmentProperties()
    shaper.Shape(buf, nil) // nil = default features

    // Read results
    for i, info := range buf.Info {
        fmt.Printf("glyph=%d advance=%d\n", info.GlyphID, buf.Pos[i].XAdvance)
    }
}

API Overview

Buffer

buf := ot.NewBuffer()
buf.AddString("Hello")           // Add text
buf.AddCodepoints([]Codepoint{}) // Or add codepoints directly
buf.GuessSegmentProperties()     // Auto-detect direction/script
buf.SetDirection(ot.DirectionRTL)
buf.Flags = ot.BufferFlagRemoveDefaultIgnorables

Shaper

shaper, err := ot.NewShaper(font)
shaper.Shape(buf, nil)           // Use default features
shaper.Shape(buf, features)      // Use specific features
shaper.ShapeString("text")       // Convenience method

Features

// Create features programmatically
features := []ot.Feature{
    ot.NewFeatureOn(ot.TagLiga),   // Enable ligatures
    ot.NewFeatureOff(ot.TagKern),  // Disable kerning
    ot.NewFeature(ot.TagAalt, 2),  // Alternate #2
}

// Or parse from string (HarfBuzz-compatible syntax)
f, ok := ot.FeatureFromString("kern")      // kern=1
f, ok := ot.FeatureFromString("-liga")     // liga=0
f, ok := ot.FeatureFromString("aalt=2")    // aalt=2
f, ok := ot.FeatureFromString("kern[3:5]") // kern for clusters 3-5

// Parse comma-separated list
features := ot.ParseFeatures("liga,kern,-smcp")

Convenience Function

// One-liner (caches shapers internally)
err := ot.Shape(font, buf, features)

Font Subsetting

import "github.com/boxesandglue/textshape/subset"

// Simple subsetting
result, err := subset.SubsetString(font, "Hello World")

// With options
input := subset.NewInput()
input.AddString("Hello World")
input.Flags = subset.FlagDropLayoutTables // For PDF embedding

plan, _ := subset.CreatePlan(font, input)
result, _ := plan.Execute()

Supported OpenType Features

GSUB (Glyph Substitution)

Type	Name	Status
1	Single Substitution	✓
2	Multiple Substitution	✓
3	Alternate Substitution	✓
4	Ligature Substitution	✓
5	Context Substitution	✓
6	Chaining Context	✓
7	Extension	✓
8	Reverse Chaining	✓

GPOS (Glyph Positioning)

Type	Name	Status
1	Single Adjustment	✓
2	Pair Adjustment (Kerning)	✓
3	Cursive Attachment	✓
4	Mark-to-Base	✓
5	Mark-to-Ligature	✓
6	Mark-to-Mark	✓
7	Context Positioning	✓
8	Chaining Context	✓
9	Extension	✓

Comparison with HarfBuzz

go-hb produces identical results to HarfBuzz for Latin scripts:

Text      HarfBuzz                    go-hb
-----     --------                    -----
Hello     [44 73 80 80 83]           [44 73 80 80 83]    ✓
office    [83 446 71 73]             [83 446 71 73]      ✓
fi        [444]                       [444]               ✓
AV        [37+1249 58+1303]          [37+1249 58+1303]   ✓

Limitations

• Complex scripts: Arabic, Indic, Thai shapers not yet implemented
• Variable fonts: Not yet supported
• Graphite: Not supported

For complex scripts, consider using the full HarfBuzz via cgo or the textlayout package.

License

MIT License - see LICENSE

Acknowledgments

• HarfBuzz - The original text shaping engine
• textlayout - Go port that inspired this implementation