You don't need to worry about the patent anymore!
See why in the patent section below.
This is a toy/experimental project showing off how to get started rendering glyphs with Eric Lengyel's Slug algorithm on both CPUs and GPUs. This old project was a quick prototype to evaluate the rendering algorithm. I've made it public in the hope it could serve as a useful learning resource.
I recommend reading at least the slides linked in the references section before any of the code.
- Windows x64
- Visual C++ 2013 or later for compiling
- OpenGL 3.2 for the hardware renderer
| Sub-project | Purpose |
|---|---|
| Font generator | Reads a .ttf TrueType font file and outputs a .sluggish file |
| Software renderer | Reads a .sluggish file and outputs a .tga image per specified code point |
| Hardware renderer | Reads a .sluggish file and renders up to 6 specified glyphs using OpenGL |
| Feature | Support |
|---|---|
| Curves texture (FP32) | YES |
| Bands texture (U16) | YES |
| Cutting glyphs into bands (performance) | YES |
| Sorting curves (performance) | YES |
| High-quality implementation of anything | NO |
| High performance | NO |
| 16-bit floating point encoding | NO |
| Data de-duplication and compression | NO |
| Text layouting | NO |
| Colored shapes | NO |
| Adaptive super-sampling | NO |
| Gamma correction | NO |
| Bounding polygons | NO |
The goal was to keep things pretty simple while still not having awful performance. For instance, cutting the glyphs into bands has been implemented because it's simple and improves performance massively.
By default, the debugger's target executable isn't in the output directory but a directory path specified by the SLUGGISH_APP_DIR environment variable.
The .exe and .pdb files are copied over to that directory after every build.
To make this process easy, you can launch Visual Studio through a simple batch script:
:: This file would sit next to the Sluggish directory
cd Sluggish\makefiles\vs2013
set SLUGGISH_APP_DIR=C:\important_stuff\sluggish
Sluggish.sln
To learn more about how the Slug algorithm works, refer to:
GPU-Centered Font Rendering Directly from Glyph Outlines by Eric Lengyel
Journal of Computer Graphics Techniques (JCGT), vol. 6, no. 2, 31-47, 2017
Paper: http://jcgt.org/published/0006/02/02
Slides: http://terathon.com/i3d2018_lengyel.pdf
Eric Lengyel's 2019 article about dynamic glyph dilation:
https://terathon.com/blog/glyph-dilation.html
Eric Lengyel's 2026 open sourced implementations of the vertex and pixel shaders:
https://github.com/EricLengyel/Slug
Eric Lengyel's 2026 article about rendering improvements:
https://terathon.com/blog/decade-slug.html
The changes are:
- added dynamic glyph dilation
- removed adaptive supersampling
- removed band split optimization
| Project/File | License(s) | License File(s) |
|---|---|---|
| This project | Unlicense (public domain) | UNLICENSE.MD |
| SDL 2 | zlib | libs/SDL2/COPYING.txt |
| GLEW | Modified BSD License Mesa 3-D License (MIT) Khronos License (MIT) |
libs/GLEW/LICENSE.txt |
| premake 5 | premake 5 | makefiles/PREMAKE_LICENSE.txt |
| stb_truetype.h | Pick-a-license: Unlicense (public domain) MIT |
code/generator/stb_truetype.h |
| stb_image_write.h | Pick-a-license: Unlicense (public domain) MIT |
code/renderer_sw/stb_image_write.h |
As of March 17 2026, Eric Lengyel announced that he is "permanently and irrevocably dedicating the Slug patent to the public domain". He also wrote that "anybody can freely implement the Slug algorithm from this day forward without a license for whatever purpose they want, and they don’t need to worry about infringing upon any intellectual property rights." For more details on the legalese of that change, refer to:
https://terathon.com/blog/decade-slug.html
Patent: https://patents.google.com/patent/US10373352B1
History:
- 2018-02-01: application filed
- 2019-08-06: application granted
- 2026-03-17: form SB/43 filed