Crate lyon [] [src]

2d rendering on the GPU in rust.


crate ci


  • crate doc - lyon_tessellation - Path tessellation routines.
  • crate doc - lyon_path_builder - Tools to facilitate building paths.
  • crate doc - lyon_path_iterator - Tools to facilitate iteratring over paths.
  • crate doc - lyon_path - A simple optional path data structure, provided for convenience.
  • crate doc - lyon_bezier - Cubic and quadratic 2d bézier math.
  • crate doc - lyon_svg - Create paths using SVG's path syntax.
  • crate doc - lyon_extra - Additional testing and debugging tools.
  • crate doc - lyon_core - Common types to most lyon crates (mostly for internal use, reexported by the other crates).

This crate is a meta-crate, reexporting the crates listed above.

Additional documentation and links


Tessellating a rounded rectangle

The lyon_tessellation crate provides a collection of tessellation routines for common shapes such as rectangles and circles. Let's have a look at how to obtain the fill tessellation a rectangle with rounded corners:

extern crate lyon;
use lyon::math::rect;
use lyon::tessellation::VertexBuffers;
use lyon::tessellation::basic_shapes::*;
use lyon::tessellation::geometry_builder::simple_builder;

fn main() {
    let mut geometry = VertexBuffers::new();

    let tolerance = 0.1;

        &rect(0.0, 0.0, 100.0, 50.0),
        &BorderRadii {
            top_left: 10.0,
            top_right: 5.0,
            bottom_left: 20.0,
            bottom_right: 25.0,
        &mut simple_builder(&mut geometry),

    // The tessellated geometry is ready to be uploaded to the GPU.
    println!(" -- {} vertices {} indices",

Building and tessellating an arbitrary path

extern crate lyon;
use lyon::math::point;
use lyon::path::Path;
use lyon::path_builder::*;
use lyon::path_iterator::PathIterator;
use lyon::tessellation::{FillTessellator, FillOptions, VertexBuffers};
use lyon::tessellation::geometry_builder::simple_builder;

fn main() {
    // Build a Path.
    let mut builder = Path::builder();
    builder.move_to(point(0.0, 0.0));
    builder.line_to(point(1.0, 0.0));
    builder.quadratic_bezier_to(point(2.0, 0.0), point(2.0, 1.0));
    builder.cubic_bezier_to(point(1.0, 1.0), point(0.0, 1.0), point(0.0, 0.0));
    let path =;

    // Will contain the result of the tessellation.
    let mut geometry = VertexBuffers::new();

    let mut tessellator = FillTessellator::new();

        let mut geom_builder = simple_builder(&mut geometry);

        let tolerance = 0.1;

        // Compute the tessellation.
            &mut geom_builder

    // The tessellated geometry is ready to be uploaded to the GPU.
    println!(" -- {} vertices {} indices",

What is the tolerance variable in these examples?

The tessellator operates on flattened paths (that only contains line segments) so we have to approximate the curves segments with sequences of line segments. To do so we pick a tolerance threshold which is the maximum distance allowed between the curve and its approximation. The documentation of the lyon_bezier crate provides more detailed explanations about this tolerance parameter.

Rendering the tessellated geometry

Lyon does not provide with any GPU abstraction or rendering backend (for now). It is up to the user of this crate to decide whether to use OpenGL, vulkan, gfx-rs, glium, or any low level graphics API and how to render it. The basic and advanced gfx-rs examples can be used to get an idea of how to render the geometry (in this case using gfx-rs).

Which crates do I need?

The meta-crate (lyon) mostly reexports the other lyon crates for convenience.

Be careful when using this code, it's not being tested!
extern crate lyon;
use lyon::tessellation::FillTessellator;

Is equivalent to:

Be careful when using this code, it's not being tested!
extern crate lyon_tessellation;
use lyon_tessellation::FillTessellator;
  • The lyon_tessellation crate is the most interesting crate so is what most people using lyon are interested in. The tessellation algorithms don't depend on a specific data structure. Instead they work on iterators of path. When using the lyon_tessellation crate you'll almost always want to use the lyon_path_iterator crate as well.
  • The lyon_path_iterator crate contains a colletion of tools to chain iterators of path events. These adapters are very useful to convert an iterator of SVG events (which contains various types of curves in relative and absolute coordinates) into iterator of simpler path events (every thing in absolute coordinates) all the way to flattened events (only line segments in absolute corrdinates).
  • The lyon_path crate is completely optional. It contains a path data structure which work with the lyon_path_iterator (and thus works with lyon_tessellation) and lyon_path_builder crates. Various examples use it but anyone can implement a custom path data structure that works with the tessellators as long as it provides an iterator of path events.
  • The lyon_path_builder crate is also optional, but provide useful abstractions to build path objects from sequences of function calls like move_to, cubic_bezier_to, etc. Just like lyon_path_iterator this crate provides adapters between the different types of path events, making it easy to use the full set of SVG events to build a path object that does not actually support all of them by converting events to lower level primitives on the fly.
  • The lyon_bezier crate is really standalone as it does not depend on any other lyon_* crate. It implements useful quadratic and cubic bezier curve math, including the flattening algorithm that is used by lyon_path_iterator and lyon_path_builder.
  • The lyon_svg crate contains utilities to interface with SVG. At the moment it is mostly a collection of wrappers around the excellent svgparser crate.
  • The lyon_core crate contains internal details that are useful to all other lyon crates (except lyon_bezier). It is reexported by all crates and you should not have to interact directly with it.

These crates are not very big, it's usually fine for most use-case to simply import the lyon meta-crate, unless you are only interested in the bezier tools.


pub extern crate lyon_core;
pub extern crate lyon_path;
pub extern crate lyon_path_builder;
pub extern crate lyon_path_iterator;
pub extern crate lyon_tessellation;
pub extern crate lyon_bezier;
pub extern crate lyon_extra;
pub extern crate lyon_svg;
pub use lyon_core::*;