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#679 in Graphics APIs

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Tessellation of filled and stroked 2D paths.

crates.io documentation

lyon_tessellation can be used as a standalone crate or as part of lyon via the lyon::tessellation module.


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.



~35K SLoC