Slug Font Rendering Algorithm

Skill by ara.so — Daily 2026 Skills collection.

Slug is a reference implementation of the Slug font rendering algorithm — a GPU-accelerated technique for rendering vector fonts and glyphs at arbitrary scales with high quality anti-aliasing. It works by encoding glyph outlines as lists of quadratic Bézier curves and line segments, then resolving coverage directly in fragment shaders without pre-rasterized textures.

Paper: JCGT 2017 — Slug Algorithm
Blog (updates): A Decade of Slug
License: MIT — Patent dedicated to public domain. Credit required if distributed.

What Slug Does

• Renders TrueType/OpenType glyphs entirely on the GPU
• No texture atlases or pre-rasterization needed
• Scales to any resolution without quality loss
• Anti-aliased coverage computed per-fragment using Bézier math
• Works with any rendering API that supports programmable shaders (D3D11/12, Vulkan, Metal via translation)

Repository Structure

Slug/
├── slug.hlsl          # Core fragment shader — coverage computation
├── band.hlsl          # Band-based optimization for glyph rendering
├── curve.hlsl         # Quadratic Bézier and line segment evaluation
├── README.md

Installation / Integration

Slug is a reference implementation — you integrate the HLSL shaders into your own rendering pipeline.

Step 1: Clone the Repository

git clone https://github.com/EricLengyel/Slug.git

Step 2: Include the Shaders

Copy the .hlsl files into your shader directory and include them in your pipeline:

#include "slug.hlsl"
#include "curve.hlsl"

Step 3: Prepare Glyph Data on the CPU

You must preprocess font outlines (TrueType/OTF) into Slug's curve buffer format:

• Decompose glyph contours into quadratic Bézier segments and line segments
• Upload curve data to a GPU buffer (structured buffer or texture buffer)
• Precompute per-glyph "band" metadata for the band optimization

Core Concepts

Glyph Coordinate System

• Glyph outlines live in font units (typically 0–2048 or 0–1000 per em)
• The fragment shader receives a position in glyph space via interpolated vertex attributes
• Coverage is computed by counting signed curve crossings in the Y direction (winding number)

Curve Data Format

Each curve entry in the GPU buffer stores:

// Line segment: p0, p1
// Quadratic Bézier: p0, p1 (control), p2

struct CurveRecord
{
    float2 p0;   // Start point
    float2 p1;   // Control point (or end point for lines)
    float2 p2;   // End point (unused for lines — flagged via type)
    // Type/flags encoded separately or in padding
};

Band Optimization

The glyph bounding box is divided into horizontal bands. Each band stores only the curves that intersect it, reducing per-fragment work from O(all curves) to O(local curves).

Key Shader Code & Patterns

Fragment Shader Entry Point (Conceptual Integration)

// Inputs from vertex shader
struct PS_Input
{
    float4 position  : SV_Position;
    float2 glyphCoord : TEXCOORD0;  // Position in glyph/font units
    // Band index or precomputed band data
    nointerpolation uint bandOffset : TEXCOORD1;
    nointerpolation uint curveCount : TEXCOORD2;
};

// Glyph curve data buffer
StructuredBuffer<float4> CurveBuffer : register(t0);

float4 PS_Slug(PS_Input input) : SV_Target
{
    float coverage = ComputeGlyphCoverage(
        input.glyphCoord,
        CurveBuffer,
        input.bandOffset,
        input.curveCount
    );

    // Premultiplied alpha output
    float4 color = float4(textColor.rgb * coverage, coverage);
    return color;
}

Quadratic Bézier Coverage Computation

The heart of the algorithm — computing signed coverage from a quadratic Bézier:

// Evaluate whether a quadratic bezier contributes to coverage at point p
// p0: start, p1: control, p2: end
// Returns signed coverage contribution
float QuadraticBezierCoverage(float2 p, float2 p0, float2 p1, float2 p2)
{
    // Transform to canonical space
    float2 a = p1 - p0;
    float2 b = p0 - 2.0 * p1 + p2;

    // Find t values where bezier Y == p.y
    float2 delta = p - p0;
    
    float A = b.y;
    float B = a.y;
    float C = p0.y - p.y;

    float coverage = 0.0;

    if (abs(A) > 1e-6)
    {
        float disc = B * B - A * C;
        if (disc >= 0.0)
        {
            float sqrtDisc = sqrt(disc);
            float t0 = (-B - sqrtDisc) / A;
            float t1 = (-B + sqrtDisc) / A;

            // For each valid t in [0,1], compute x and check winding
            if (t0 >= 0.0 && t0 <= 1.0)
            {
                float x = (A * t0 + 2.0 * B) * t0 + p0.x + delta.x;
                // ... accumulate signed coverage
            }
            if (t1 >= 0.0 && t1 <= 1.0)
            {
                float x = (A * t1 + 2.0 * B) * t1 + p0.x + delta.x;
                // ... accumulate signed coverage
            }
        }
    }
    else
    {
        // Degenerate to linear case
        float t = -C / (2.0 * B);
        if (t >= 0.0 && t <= 1.0)
        {
            float x = 2.0 * a.x * t + p0.x;
            // ... accumulate signed coverage
        }
    }

    return coverage;
}

Line Segment Coverage

// Signed coverage contribution of a line segment from p0 to p1
float LineCoverage(float2 p, float2 p0, float2 p1)
{
    // Check Y range
    float minY = min(p0.y, p1.y);
    float maxY = max(p0.y, p1.y);

    if (p.y < minY || p.y >= maxY)
        return 0.0;

    // Interpolate X at p.y
    float t = (p.y - p0.y) / (p1.y - p0.y);
    float x = lerp(p0.x, p1.x, t);

    // Winding: +1 if p is to the left (inside), -1 if right
    float dir = (p1.y > p0.y) ? 1.0 : -1.0;
    return (p.x <= x) ? dir : 0.0;
}

Anti-Aliasing with Partial Coverage

For smooth ed