Slug Font Rendering Algorithm Skill by [ara.so](https://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](https://jcgt.org/published/0006/02/02/) Blog (updates): [A Decade of Slug](https://terathon.com/blog/decade slug.html) 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 Installation / Integration Slug is a reference implementation — you integrate the HLSL shaders into your own rendering pipeline. Step 1: Clone the Repository Step 2: Include the Shaders Copy the .hlsl files into your shader directory and include them in your pipeline: 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: 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) Quadratic Bézier Coverage Computation The heart of the algorithm — computing signed coverage from a quadratic Bézier: Line Segment Coverage Anti Aliasing with Partial Coverage For smooth edges, use the distance to the nearest curve for sub pixel anti aliasing: Vertex Shader Pattern CPU Side Data Preparation (Pseudocode) Render State Requirements Common Patterns Rendering a String Scaling Text Scaling is handled entirely on the CPU side by transforming the screen space quad. The glyph space coordinates stay constant — the fragment shader always works in font units. Troubleshooting Problem Cause Fix Glyph appears hollow/inverted Winding order reversed Check contour orientation; TrueType uses clockwise for outer contours Jagged edges Anti aliasing not applied Ensure distance to edge is computed and used in final coverage Performance poor Band optimization not active Verify per fragment curve count is small (< ~20); increase band count Cubic curves not rendering OTF cubic Béziers unsupported natively Split cubics into quadratic approximations on CPU Artifacts at glyph overlap Curves not clipped to band Clip curve Y range to band extents before upload Black box instead of glyph Blend state wrong Use premultiplied alpha blend (ONE, INV SRC ALPHA) Missing glyphs Band offset incorrect Validate bandOffset indexing aligns with buffer layout Credits & Attribution Per the license: if you distribute software using this code, you must give credit to Eric Lengyel and the Slug algorithm. Suggested attribution: Font rendering uses the Slug Algorithm by Eric Lengyel (https://jcgt.org/published/0006/02/02/) References [Slug Algorithm Paper (JCGT 2017)](https://jcgt.org/published/0006/02/02/) [A Decade of Slug — Blog Post](https://terathon.com/blog/decade slug.html) [GitHub Repository](https://github.com/EricLengyel/Slug)

slug-font-rendering