# harfbuzz_rs

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`harfbuzz_rs` is a high-level interface to HarfBuzz, exposing its most important
functionality in a safe manner using Rust.

# What is HarfBuzz?

HarfBuzz is a library for performing complex text layout. It does not perform
any drawing. This is quite a low-level operation. If you want to simply draw
some text on the screen you should maybe choose another more high-level library.
However if you want to build a library for drawing text on some canvas or need a
lot of control on advanced text layout then this is the right library to use.

# Getting Started

To shape a simple string of text you just create a `Font` from a font file, fill
a `Buffer` with some text and call the `shape` function.

```rust
use harfbuzz_rs::*;

let path = "path/to/some/font_file.otf";
let index = 0; //< face index in the font file
let face = Face::from_file(path, index)?;
let mut font = Font::new(face);

let buffer = UnicodeBuffer::new().add_str("Hello World!");
let output = shape(&font, buffer, &[]);

// The results of the shaping operation are stored in the `output` buffer.

let positions = output.get_glyph_positions();
let infos = output.get_glyph_infos();

// iterate over the shaped glyphs
for (position, info) in positions.iter().zip(infos) {
    let gid = info.codepoint;
    let cluster = info.cluster;
    let x_advance = position.x_advance;
    let x_offset = position.x_offset;
    let y_offset = position.y_offset;

    // Here you would usually draw the glyphs.
    println!("gid{:?}={:?}@{:?},{:?}+{:?}", gid, cluster, x_advance, x_offset, y_offset);
}
```

This should print out something similar to the following:

```text
gid41=0@741,0+0
gid70=1@421,0+0
gid77=2@258,0+0
gid77=3@253,0+0
gid80=4@510,0+0
gid1=5@227,0+0
gid56=6@874,0+0
gid80=7@498,0+0
gid83=8@367,0+0
gid77=9@253,0+0
gid69=10@528,0+0
gid2=11@276,0+0
```

The values of `x_advance`, `x_offset`, `y_advance` and `y_offset` are all given in so-called _font units_ by default.
By calling `face.upem()` you get the number of font units per [EM](<https://en.wikipedia.org/wiki/Em_(typography)>) for
a specific `face`. This `upem` value can be used to scale the advances and offsets to a given font-size.
For example, if you want to display a font at 16 point (pt) size, that means that _1 EM = 16 pt_.
In this example, to convert a value, say `x_advance`, from font-units to points, we compute `((x_advance * font_size) as f64) / (upem as f64)`, where `font_size = 16` is a variable specifying the font size in points.

Note that harfbuzz internally supports scaling fonts itself as well (using `font.set_scale(...)`, etc.) but in my opinion it is easier to scale the results oneself as described in the paragraph above.

# Supported HarfBuzz versions

This crate is tested to work with harfbuzz versions 2.0 and higher. Older versions may work as well. I recommend statically linking the harfbuzz library provided by the `harfbuzz-sys` crate which is always up-to-date.

# Optional Features

If you want to use rusttype as font functions enable the `rusttype` feature.