[encoding] Bump buffer estimation for segments

The segments (and segment counts) buffers contain intermediate data
structures that associate lines (post-flatten, post-binning) with tiles.
Their overall count depends on the number tile crossings for each line.

These counts are now estimated as follows:

1. Explict lines with known endpoints are estimated precisely by
   guessing the worst case rasterization of the hypothenuse (i.e. the
   sum of the tiling estimate for the x and y coordinates.

2. Explicit lines with unknown endpoints (e.g. the caps on a stroke)
   are estimated based on the length of the segment.

3. Flattened curves are estimated by tiling a line whose length is
   derived based on a conservative estimate of the curve's arc length.
   The segment count is forced to be at least as large as the Wang's
   estimate used for the LineSoup count.

This implementation currently has two major shortcomings:

I. Clipping is not taken into account, so the count is overstimated when
   layers are present or when geometry lies outside the viewport. There
   are ways to address this but they will require some changes:

   a) Most of the count accummulation happens at encode time but the
      bounds of the viewport are only known at render time. This could
      be changed so that most of the accummulation happens at render
      time and additional data (like the bounding box of each shape) get
      tracked during encoding.

   b) It might make sense to track the clip stack to test each shape's
      bounding box against the clip geometry while resolving the counts
      and use that as input to a heuristic. It is also possible to
      discard shapes that lie completely outside the bounds of the clip
      geometry. All of this would require additional tracking that
      impacts CPU time and memory usage.

II. A rotation that is present in the transform has an impact on tile
    crossings. We can precisely account for this for explicit lines with
    known endpoints but we have to use a heuristic for curves. The
    estimator doesn't track detailed shape data, so a heuristic must be
    used when appending a scene fragment. We currently inflate all
    segment counts as if they have a 90 degree rotation whenever a
    transform should apply.

Overall the segment count tends to be overestimated 3x-10x. There is one
known failure mode when the count is underestimated with _very_ small
scale factors ("conflation artifacts" test scene).
1 file changed
tree: 6131e59aec326b9dc07aaf284e0eca57364f75b2
  1. .cargo/
  2. .github/
  3. .vscode/
  4. crates/
  5. doc/
  6. examples/
  7. integrations/
  8. shader/
  9. src/
  10. .clippy.toml
  11. .gitignore
  14. Cargo.toml
  17. README.md
  18. rustfmt.toml


An experimental GPU compute-centric 2D renderer

Linebender Zulip dependency status MIT/Apache 2.0 wgpu version

Vello is an experimental 2D graphics rendering engine written in Rust, with a focus on GPU compute. It can draw large 2D scenes with interactive or near-interactive performance, using wgpu for GPU access.

Quickstart to run an example program:

cargo run -p with_winit


It is used as the rendering backend for Xilem, a Rust GUI toolkit.

[!WARNING] Vello can currently be considered in an alpha state. In particular, we're still working on the following:


Vello is meant to fill the same place in the graphics stack as other vector graphics renderers like Skia, Cairo, and its predecessor project Piet. On a basic level, that means it provides tools to render shapes, images, gradients, text, etc, using a PostScript-inspired API, the same that powers SVG files and the browser <canvas> element.

Vello's selling point is that it gets better performance than other renderers by better leveraging the GPU. In traditional PostScript-style renderers, some steps of the render process like sorting and clipping either need to be handled in the CPU or done through the use of intermediary textures. Vello avoids this by using prefix-sum algorithms to parallelize work that usually needs to happen in sequence, so that work can be offloaded to the GPU with minimal use of temporary buffers.

This means that Vello needs a GPU with support for compute shaders to run.

Getting started

Vello is meant to be integrated deep in UI render stacks. While drawing in a Vello scene is easy, actually rendering that scene to a surface requires setting up a wgpu context, which is a non-trivial task.

To use Vello as the renderer for your PDF reader / GUI toolkit / etc, your code will have to look roughly like this:

// Initialize wgpu and get handles
let (width, height) = ...;
let device: wgpu::Device = ...;
let queue: wgpu::Queue = ...;
let surface: wgpu::Surface<'_> = ...;
let texture_format: wgpu::TextureFormat = ...;
let mut renderer = Renderer::new(
   RendererOptions {
      surface_format: Some(texture_format),
      use_cpu: false,
      antialiasing_support: vello::AaSupport::all(),
      num_init_threads: NonZeroUsize::new(1),
).expect("Failed to create renderer");

// Create scene and draw stuff in it
let mut scene = vello::Scene::new();
   vello::Color::rgb8(242, 140, 168),
   &vello::Circle::new((420.0, 200.0), 120.0),

// Draw more stuff

// Render to your window/buffer/etc.
let surface_texture = surface.get_current_texture()
   .expect("failed to get surface texture");
      &vello::RenderParams {
         base_color: Color::BLACK, // Background color
         antialiasing_method: AaConfig::Msaa16,
   .expect("Failed to render to surface");

See the examples/ folder to see how that code integrates with frameworks like winit and bevy.


We've observed 177 fps for the paris-30k test scene on an M1 Max, at a resolution of 1600 pixels square, which is excellent performance and represents something of a best case for the engine.

More formal benchmarks are on their way.



This repository also includes vello_svg, which supports converting a usvg Tree into a Vello scene.

This is currently incomplete; see its crate level documentation for more information.

This is used in the winit example for the SVG rendering.


A separate integration for playing Lottie animations is available through the velato crate.


Our examples are provided in separate packages in the examples folder. This allows them to have independent dependencies and faster builds. Examples must be selected using the --package (or -p) Cargo flag.


Our winit example (examples/with_winit) demonstrates rendering to a winit window. By default, this renders the GhostScript Tiger as well as all SVG files you add in the examples/assets/downloads/ directory using vello_svg. A custom list of SVG file paths (and directories to render all SVG files from) can be provided as arguments instead. It also includes a collection of test scenes showing the capabilities of vello, which can be shown with --test-scenes.

cargo run -p with_winit 

Some default test scenes can be downloaded from Wikimedia Commons using the download subcommand. This also supports downloading from user-provided URLS.

cargo run -p with_winit -- download


The Bevy example (examples/with_bevy) demonstrates using Vello within a Bevy application. This currently draws to a wgpu Texture using vello, then uses that texture as the faces of a cube.

cargo run -p with_bevy

There is also a separate community integration for rendering lottie and SVG files through bevy_vello.


We aim to target all environments which can support WebGPU with the default limits. We defer to wgpu for this support. Other platforms are more tricky, and may require special building/running procedures.


Because Vello relies heavily on compute shaders, we rely on the emerging WebGPU standard to run on the web. Until browser support becomes widespread, it will probably be necessary to use development browser versions (e.g. Chrome Canary) and explicitly enable WebGPU.

The following command builds and runs a web version of the winit demo. This uses cargo-run-wasm to build the example for web, and host a local server for it

# Make sure the Rust toolchain supports the wasm32 target
rustup target add wasm32-unknown-unknown

# The binary name must also be explicitly provided as it differs from the package name
cargo run_wasm -p with_winit --bin with_winit_bin

There is also a web demo available here on supporting web browsers.

[!WARNING] The web is not currently a primary target for Vello, and WebGPU implementations are incomplete, so you might run into issues running this example.


The with_winit example supports running on Android, using cargo apk.

cargo apk run -p with_winit

cargo apk doesn't support running in release mode without configuration. See their crates page docs (around package.metadata.android.signing.<profile>).

See also cargo-apk#16. To run in release mode, you must add the following to examples/with_winit/Cargo.toml (changing $HOME to your home directory):

path = "$HOME/.android/debug.keystore"
keystore_password = "android"

As cargo apk does not allow passing command line arguments or environment variables to the app when ran, these can be embedded into the program at compile time (currently for Android only) with_winit currently supports the environment variables:

  • VELLO_STATIC_LOG, which is equivalent to RUST_LOG
  • VELLO_STATIC_ARGS, which is equivalent to passing in command line arguments

For example (with unix shell environment variable syntax):

VELLO_STATIC_LOG="vello=trace" VELLO_STATIC_ARGS="--test-scenes" cargo apk run -p with_winit --lib


Discussion of Vello development happens in the Linebender Zulip, specifically the #gpu stream. All public content can be read without logging in.

Contributions are welcome by pull request. The Rust code of conduct applies.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be licensed as noted in the “License” section, without any additional terms or conditions.


Vello was previously known as piet-gpu. This prior incarnation used a custom cross-API hardware abstraction layer, called piet-gpu-hal, instead of wgpu.

An archive of this version can be found in the branches custom-hal-archive-with-shaders and custom-hal-archive. This succeeded the previous prototype, piet-metal, and included work adapted from piet-dx12.

The decision to lay down piet-gpu-hal in favor of WebGPU is discussed in detail in the blog post Requiem for piet-gpu-hal.

A vision document dated December 2020 explained the longer-term goals of the project, and how we might get there. Many of these items are out-of-date or completed, but it still may provide some useful background.

Related projects

Vello takes inspiration from many other rendering projects, including:


Licensed under either of

at your option.

In addition, all files in the shader and src/cpu_shader directories and subdirectories thereof are alternatively licensed under the Unlicense (shader/UNLICENSE or http://unlicense.org/). For clarity, these files are also licensed under either of the above licenses. The intent is for this research to be used in as broad a context as possible.

The files in subdirectories of the examples/assets directory are licensed solely under their respective licenses, available in the LICENSE file in their directories.