# [`sdf-winding`][]

[Glyph][] contour [winding number][] and [SDF][] OpenGL experiment.

This repository has a singular purpose: to roughly reproduce [Figure 7][] from
the [Apple TrueType Reference Manual - Chapter 2: Font Engine - Distinguishing
the inside from the outside of a glyph][TrueType]:

![Figure 7][]

I have not read a single word of that reference manual, proceed accordingly.

To build, run `make`. You need the following dependencies on your system:

-   OpenGL
-   [GLEW][]
-   [GLFW][]

If you're having trouble building or running this and just want to see the
result, some output is saved in [`windings.ppm`](windings.ppm) and
[`resolve.ppm`](resolve.ppm). If you want to compare them with [Figure 7][] you
can run `make FE6.gif` (sic) to download it to the repository root (with
`wget`).

The code prioritizes conciseness and takes a number of shortcuts:

-   We don't actually generate the SDFs and winding numbers from contours, we
    just hard code simple circle SDFs with given winding numbers.
-   We don't always take care to explicitly bind OpenGL state at the point of
    use if we happen to know that the currently bound thing is correct or
    harmless, which we often do for this simple program.
-   We abuse the blend mode to add winding numbers, to avoid juggling multiple
    textures. If you're using clever geometry tricks to generate the SDFs this
    might be a problem. You might be able to use some overdraw reducing
    technique, like a depth pre-pass, to mitigate that.

[`sdf-winding`]:  https://git.rcrnstn.net/rcrnstn/sdf-winding
[glyph]:          https://en.wikipedia.org/wiki/Glyph
[winding number]: https://en.wikipedia.org/wiki/Winding_number
[SDF]:            https://en.wikipedia.org/wiki/Signed_distance_function
[Figure 7]:       https://developer.apple.com/fonts/TrueType-Reference-Manual/RM02/FE6.gif
[TrueType]:       https://developer.apple.com/fonts/TrueType-Reference-Manual/RM02/Chap2.html#distinguishing
[GLEW]:           https://glew.sourceforge.net
[GLFW]:           https://www.glfw.org

## License

Licensed under the [ISC License][] unless otherwise noted, see the
[`LICENSE`][] file.

[ISC License]: https://choosealicense.com/licenses/isc
[`LICENSE`]: LICENSE