Linebender in March 2025

Daniel McNab, Bruce Mitchener, Raph Levien, April 4, 2025

Linebender is an informal open-source organization working on various projects to advance the state of the art in GUI for the Rust programming language.

Masonry

Masonry is the widget system developed by Linebender. It provides a non-opinionated retained widget tree, designed as a base layer for high-level GUI frameworks.

We have made a major change to the internal design of Masonry, in xilem#910 and xilem#914. This change splits Masonry into two packages, masonry_core and masonry. The Masonry crate provides a runner for Masonry core using Winit, and should be used for an integrated app development experience. Masonry Core contains the implementation of Masonry's widgets and passes, and is designed to be be embedded into existing apps (currently only those which use wgpu). This should be transparent to existing Masonry users.

We hoped to release a new alpha of Xilem and Masonry to crates.io in March. However, this was put on hold whilst the Properties experiment we mentioned last month continues:

A button with a red border, a red font, and placeholder text.

An example of a styled button.

We have also made significant progress outside of these highlighted areas:

A screenshot of a desktop window, showing a list of numbers around 999900. Every number divisible by 3 has been replaced with the word 'Fizz', 'Buzz' for 5, and 'FizzBuzz' for numbers divisible by both. The 'FizzBuzz' which would be item 999900 is twice as large as the other items.

xilem#882 added a virtual scrolling widget. This is the 'Infinite FizzBuzz' example, which also shows how the virtual scrolling can support items with arbitrary heights.

Xilem

Xilem is our flagship GUI project, inspired by SwiftUI, which uses Masonry for its widgets. It lets you build user interfaces declaratively by composing lightweight views together, and will diff them to provide minimal updates to a retained layer.

Vello

Vello is our GPU vector renderer. It can draw large 2D scenes with high performance, using GPU compute shaders for most of the work.

We have also renamed the #gpu channel on Zulip to #vello, which better reflects the purpose of the channel. Progress on the sparse strips renderer has also been continuing at pace:

A browser window showing a partial map of the center of Paris, France.

Paris-30k rendered using Vello's new hybrid renderer in Firefox on Linux, a browser which does not yet support WebGPU.

The current state of the sparse strip renderers is that they can reliably render scenes where all elements are a solid colour. Laurenz Stampfl started their master's project working on the CPU-only renderer full-time at the end of March, working towards this roadmap and on a write-up. The planned ordering of next steps is laid out in our loose roadmap. You can also follow the progress in #vello > Vello Hybrid and other threads in #vello. Our weekly renderer office hours specifically aimed at furthering this collaboration are public - for more information about how to join these, follow #office hours. Work on the renderers is continuing at pace, most recently in these open PRs:

Parley

Parley is a text layout library. It handles text layout, mostly at the level of line breaking and resolving glyph positions. The biggest news in March was that an egui community member has opened a PR to replace egui's bespoke text handling with Parley, with buy-in from egui's maintainer. The author of that change, valadaptive, has been doing fantastic work improving Parley, and has authored many of these changes.

A screenshot of an egui widget, titled 'Complex text 🔡'. It also contains emoji support, arabic text (which is partially selected across the left-to-right to right-to-left boundary, giving a logically contiguous but visually split selection), and a properly typeset fraction 37 over 54. The final line has the the text 'plus variable fonts', where the font weight is increasing smoothly from left to right.

egui#5784 significantly improves egui's text rendering by using Parley.

Kurbo

Kurbo provides data structures and algorithms for curves and vector paths.

One of the core features in kurbo is expanding a stroked outline to a filled path, which is based on computing accurate approximations to offset curves, a difficult problem in computational geometry. As tracked in kurbo#317, our existing solution, based on quartic solving, produced good results but was slow. We've been doing research into much faster techniques based on least-squares error minimization and accelerated Newton techniques, and kurbo#427 is the first step in landing that work. Over the coming months, we plan to refine robustness and performance of the stroke expansion algorithm, and will also explore generalizing it to variable width strokes, an often-requested feature.

Color

Color provides functionality for representing, converting, parsing, serializing, and manipulating colors in a variety of color spaces. It closely follows the CSS Color Module Level 4 draft spec.

Kompari

Kompari is a tool for visual inspection of snapshot tests.

Research and Future Directions

Linebender has an origin story in being a very research oriented group, looking to break new ground. While we are focused on shipping code today, we still have an eye on the future and how to be prepared for the new opportunities and technologies that are coming.

Matt Campbell has started work on a new platform integration for Rust code in Android, called Android View. This is designed to work around several issues from the activities available when using Winit, which use either NativeActivity or GameActivity. Both of these do not integrate well with text input, and struggle to interoperate inside existing apps. This work is in an early stage, and is taking inspiration from several existing solutions to these problems, with a special focus on getting accessibility integration correct.

We have also started work on a Styled Text library, which is designed to allow sharing rich text functionality between Rust libraries. This is in the very early stages - if you're working on a project which can use this, then input on your requirements will be welcome.

RustWeek

Many members of the Linebender community will be attending the RustWeek 2025 conference, in Utrecht in May. At the time of writing, there are still tickets available. Raph Levien will be giving a talk, titled Faster, easier 2D vector rendering. This will be covering a lot of the sparse strips work discussed in the Vello section.

Matt Campbell will give a talk, titled AccessKit: reusable UI accessibility.

Get Involved

We welcome collaboration on any of our crates. This can include improving the documentation, implementing new features, improving our test coverage, or using them within your own code.

We host an hour long office hours meeting each week where we discuss what's going on in our projects. See #office hours in Zulip for details. We've also started a separate office hours time dedicated to the renderer collaboration, details also available at that link.