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Painting primitives at non-integer pixel coordinates produces blurry output. Pixel snapping converts layout coordinates into integer device-pixel coordinates so painted edges land exactly on physical pixel boundaries. Non-integer coordinates can arise for several reasons, including: - flex distribution, percentages, centering, and text measurement can produce fractional element sizes and positions; - at fractional scale factors (for example 125% or 150%), integer logical-pixel values can map to non-integer device-pixel values. We pixel-snap by rounding in device-pixel space, after multiplying by `scale_factor`, so that snapping targets physical pixels. Bounds are divided by `scale_factor` before being returned to GPUI. Midpoints are rounded toward zero. This is a stylistic choice: a 1-logical-pixel line at 150% scale should render as 1 dp rather than 2 dp. Pixel snapping is done in two phases: 1. Pre-layout metric snapping. Before Taffy computes layout, all authored absolute lengths are rounded in `to_taffy`. This includes borders, padding, gaps, and explicit sizes. Custom-measured leaf nodes have their measured sizes rounded up to integer device-pixel lengths. 2. Post-layout edge snapping. After Taffy resolves the tree, layout relationships such as flex shares, grid tracks, percentages, and centering can produce new fractional edge positions. Boxes now have edges in absolute coordinates, and snapping must decide where those edges land on the device-pixel grid. Ideally, post-layout snapping would satisfy: - Edge closure. Two raw layout edges at the same absolute position should snap to the same pixel column. - Translation stability. A component's internal geometry should not change when it moves to a new absolute position. These goals are in tension because rounding is not associative. The simple local schemes make different tradeoffs: - Absolute edge rounding gives each window coordinate one answer, so coincident edges always close globally. But a span's snapped length is `round(far) - round(near)`, which may change by 1 dp as its absolute origin moves. - Parent-relative edge rounding rounds each child inside its parent's coordinate space. This guarantees translation stability, but a shared edge reached through different parents can accumulate different rounding, causing non-closure between cousins. - Length rounding rounds each width, height, and thickness independently and then places boxes from those rounded lengths. Sizes stay stable under translation, but neighboring boxes derive their shared boundary from different sources, so closure is not guaranteed. We apply absolute edge rounding for each element's outer box in post-layout rounding to preserve closure. Border and padding widths are not touched by post-layout rounding; they keep their pre-layout rounded value so that they remain stable under translation. This gives both closure and translation stability in the case that all local metrics are integer device-pixel lengths. Pre-layout rounding covers that in most cases. The exception is metrics resolved by layout relationships, such as percentages. Outer box edges will still close globally, and painted border widths are still snapped independently, but the raw content-box origin can carry a 1 dp residual into descendants. --- Fixes https://github.com/zed-industries/zed/issues/46360 Fixes https://github.com/zed-industries/zed/issues/44528 Fixes https://github.com/zed-industries/zed/issues/40282 Fixes https://github.com/zed-industries/zed/issues/42257 --- Release Notes: - Fixed potentially blurry appearance of UI elements when using fractional display scaling. |
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Welcome to GPUI!
GPUI is a hybrid immediate and retained mode, GPU accelerated, UI framework for Rust, designed to support a wide variety of applications.
Getting Started
GPUI is still in active development as we work on the Zed code editor, and is still pre-1.0. There will often be breaking changes between versions. You'll also need to use the latest version of stable Rust and be on macOS or Linux. Add the following to your Cargo.toml:
gpui = { version = "*" }
Everything in GPUI starts with an Application. You can create one with Application::new(), and kick off your application by passing a callback to Application::run(). Inside this callback, you can create a new window with App::open_window(), and register your first root view. See gpui.rs for a complete example.
Dependencies
GPUI has various system dependencies that it needs in order to work.
macOS
On macOS, GPUI uses Metal for rendering. In order to use Metal, you need to do the following:
- Install Xcode from the macOS App Store, or from the Apple Developer website. Note this requires a developer account.
Ensure you launch Xcode after installing, and install the macOS components, which is the default option.
-
Install Xcode command line tools
xcode-select --install -
Ensure that the Xcode command line tools are using your newly installed copy of Xcode:
sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer
The Big Picture
GPUI offers three different registers depending on your needs:
-
State management and communication with
Entity's. Whenever you need to store application state that communicates between different parts of your application, you'll want to use GPUI's entities. Entities are owned by GPUI and are only accessible through an owned smart pointer similar to anRc. See theapp::contextmodule for more information. -
High level, declarative UI with views. All UI in GPUI starts with a view. A view is simply an
Entitythat can be rendered, by implementing theRendertrait. At the start of each frame, GPUI will call this render method on the root view of a given window. Views build a tree ofelements, lay them out and style them with a tailwind-style API, and then give them to GPUI to turn into pixels. See thedivelement for an all purpose swiss-army knife of rendering. -
Low level, imperative UI with Elements. Elements are the building blocks of UI in GPUI, and they provide a nice wrapper around an imperative API that provides as much flexibility and control as you need. Elements have total control over how they and their child elements are rendered and can be used for making efficient views into large lists, implement custom layouting for a code editor, and anything else you can think of. See the
elementmodule for more information.
Each of these registers has one or more corresponding contexts that can be accessed from all GPUI services. This context is your main interface to GPUI, and is used extensively throughout the framework.
Other Resources
In addition to the systems above, GPUI provides a range of smaller services that are useful for building complex applications:
-
Actions are user-defined structs that are used for converting keystrokes into logical operations in your UI. Use this for implementing keyboard shortcuts, such as cmd-q. See the
actionmodule for more information. -
Platform services, such as
quit the apporopen a URLare available as methods on theapp::App. -
An async executor that is integrated with the platform's event loop. See the
executormodule for more information., -
The
[gpui::test]macro provides a convenient way to write tests for your GPUI applications. Tests also have their own kind of context, aTestAppContextwhich provides ways of simulating common platform input. Seeapp::test_contextandtestmodules for more details.
Currently, the best way to learn about these APIs is to read the Zed source code or drop a question in the Zed Discord. We're working on improving the documentation, creating more examples, and will be publishing more guides to GPUI on our blog.