Expand description

Tessellation of 2D fill and stroke operations.

This crate is reexported in lyon.


The most interesting types and traits of this crate are:

  • FillTessellator - Tessellator for complex path fill operations.
  • StrokeTessellator - Tessellator for complex path stroke operations.
  • GeometryBuilder - (See the documentation of the geometry_builder module) which the above two are built on. This trait provides an interface for types that help with building and assembling the vertices and triangles that form the tessellation, usually in the form of arbitrary vertex and index buffers.

The tessellation pipeline

FillTessellator GeometryBuilder output VertexConstructor Iterator<PathEvent> builder.add_vertex(FillVertex) -> VertexId;builder.add_triangle(VertexId, VertexId, VertexId); FillVertex -> CustomVertex MoveTo(Point)LineTo(Point)Close

The figure above shows a simplified summary of each step of the fill tessellation pipeline.

The input: iterators

The path tessellators are not tied to a particular data structure. Instead they consume iterators of flattened path events. A Path struct in the crate lyon_path is provided for convenience (but is optional).

The output: geometry builders

The tessellators are parametrized over a type implementing the GeometryBuilder trait. This trait provides some simple methods to add vertices and triangles, without enforcing any particular representation for the resulting geometry. This is important because each application will usually want to work with its own vertex type tailored a certain rendering model.

Applications can implement the GeometryBuilder<Point> trait in order to generate vertex buffers and index buffers with custom vertex types.

The structs VertexBuffers and geometry_builder::BuffersBuilder are provided for convenience. VertexBuffers<T> is contains a Vec<T> for the vertices and a Vec<u16> for the indices.

BuffersBuilder is generic over a VertexConstructor<InputVertex, OutputVertex> trait which creates the application’s output vertices from the tessellator input vertices (either FillVertex or StrokeVertex).

Rendering the tessellated geometry

The tessellators produce geometry in the form of vertex and index buffers which are expected to be rendered using the equivalent of OpenGL’s glDrawElements with mode GL_TRIANGLES available under various names in the different graphics APIs. There is an example showing how it can be done with wgpu.

Flattening and tolerance

Most tessellators in this crate currently operate on flattened paths (paths or shapes represented by sequences of line segments). when paths contain bézier curves or arcs, the latter need to be approximated with sequences of line segments. This approximation depends on a tolerance parameter which represents the maximum distance between a curve and its flattened approximation.

More explanation about flattening and tolerance in the lyon_geom crate.



  • pub use lyon_path as path;
  • pub use crate::path::geom;


  • Tools to help with generating vertex and index buffers.
  • f32 version of the lyon_geom types used everywhere. Most other lyon crates reexport them.


  • A temporary view on a VertexBuffers object which facilitate the population of vertex and index data.
  • A queue of sorted events for the fill tessellator’s sweep-line algorithm.
  • Parameters for the fill tessellator.
  • A Context object that can tessellate fill operations for complex paths.
  • Extra vertex information from the FillTessellator, accessible when building vertices.
  • A builder object that tessellates a stroked path via the PathBuilder interface.
  • Parameters for the tessellator.
  • A Context object that can tessellate stroke operations for complex paths.
  • Extra vertex information from the StrokeTessellator accessible when building vertices.
  • Structure that holds the vertex and index data.
  • A virtual vertex offset in a geometry.
  • An iterator over the sources of a given vertex.


  • The fill rule defines how to determine what is inside and what is outside of the shape.
  • An error that can happen while generating geometry.
  • Describes an unexpected error happening during tessellation.
  • Line cap as defined by the SVG specification.
  • Line join as defined by the SVG specification.
  • Vertical or Horizontal.
  • The positive or negative side of a vector or segment.
  • The fill tessellator’s error enumeration.
  • Where a vertex produced by a tessellator comes from in the original path.


Type Aliases