Crate egui

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egui: an easy-to-use GUI in pure Rust!

Try the live web demo: https://www.egui.rs/#demo. Read more about egui at https://github.com/emilk/egui.

egui is in heavy development, with each new version having breaking changes. You need to have rust 1.76.0 or later to use egui.

To quickly get started with egui, you can take a look at eframe_template which uses eframe.

To create a GUI using egui you first need a Context (by convention referred to by ctx). Then you add a Window or a SidePanel to get a Ui, which is what you’ll be using to add all the buttons and labels that you need.

§Feature flags

§Using egui

To see what is possible to build with egui you can check out the online demo at https://www.egui.rs/#demo.

If you like the “learning by doing” approach, clone https://github.com/emilk/eframe_template and get started using egui right away.

§A simple example

Here is a simple counter that can be incremented and decremented using two buttons:

fn ui_counter(ui: &mut egui::Ui, counter: &mut i32) {
    // Put the buttons and label on the same row:
    ui.horizontal(|ui| {
        if ui.button("−").clicked() {
            *counter -= 1;
        }
        ui.label(counter.to_string());
        if ui.button("+").clicked() {
            *counter += 1;
        }
    });
}

In some GUI frameworks this would require defining multiple types and functions with callbacks or message handlers, but thanks to egui being immediate mode everything is one self-contained function!

§Getting a Ui

Use one of SidePanel, TopBottomPanel, CentralPanel, Window or Area to get access to an Ui where you can put widgets. For example:

egui::CentralPanel::default().show(&ctx, |ui| {
    ui.add(egui::Label::new("Hello World!"));
    ui.label("A shorter and more convenient way to add a label.");
    if ui.button("Click me").clicked() {
        // take some action here
    }
});

§Quick start

ui.label("This is a label");
ui.hyperlink("https://github.com/emilk/egui");
ui.text_edit_singleline(&mut my_string);
if ui.button("Click me").clicked() { }
ui.add(egui::Slider::new(&mut my_f32, 0.0..=100.0));
ui.add(egui::DragValue::new(&mut my_f32));

ui.checkbox(&mut my_boolean, "Checkbox");

#[derive(PartialEq)]
enum Enum { First, Second, Third }
ui.horizontal(|ui| {
    ui.radio_value(&mut my_enum, Enum::First, "First");
    ui.radio_value(&mut my_enum, Enum::Second, "Second");
    ui.radio_value(&mut my_enum, Enum::Third, "Third");
});

ui.separator();

ui.image((my_image, egui::Vec2::new(640.0, 480.0)));

ui.collapsing("Click to see what is hidden!", |ui| {
    ui.label("Not much, as it turns out");
});

§Viewports

Some egui backends support multiple viewports, which is what egui calls the native OS windows it resides in. See crate::viewport for more information.

§Coordinate system

The left-top corner of the screen is (0.0, 0.0), with X increasing to the right and Y increasing downwards.

egui uses logical points as its coordinate system. Those related to physical pixels by the pixels_per_point scale factor. For example, a high-dpi screen can have pixels_per_point = 2.0, meaning there are two physical screen pixels for each logical point.

Angles are in radians, and are measured clockwise from the X-axis, which has angle=0.

§Integrating with egui

Most likely you are using an existing egui backend/integration such as eframe, bevy_egui, or egui-miniquad, but if you want to integrate egui into a new game engine or graphics backend, this is the section for you.

You need to collect RawInput and handle FullOutput. The basic structure is this:

let mut ctx = egui::Context::default();

// Game loop:
loop {
    let raw_input: egui::RawInput = gather_input();

    let full_output = ctx.run(raw_input, |ctx| {
        egui::CentralPanel::default().show(&ctx, |ui| {
            ui.label("Hello world!");
            if ui.button("Click me").clicked() {
                // take some action here
            }
        });
    });
    handle_platform_output(full_output.platform_output);
    let clipped_primitives = ctx.tessellate(full_output.shapes, full_output.pixels_per_point);
    paint(full_output.textures_delta, clipped_primitives);
}

For a reference OpenGL renderer, see the egui_glow painter.

§Debugging your renderer

§Things look jagged
  • Turn off backface culling.
§My text is blurry
  • Make sure you set the proper pixels_per_point in the input to egui.
  • Make sure the texture sampler is not off by half a pixel. Try nearest-neighbor sampler to check.
§My windows are too transparent or too dark
  • egui uses premultiplied alpha, so make sure your blending function is (ONE, ONE_MINUS_SRC_ALPHA).
  • Make sure your texture sampler is clamped (GL_CLAMP_TO_EDGE).
  • egui prefers linear color spaces for all blending so:
    • Use an sRGBA-aware texture if available (e.g. GL_SRGB8_ALPHA8).
      • Otherwise: remember to decode gamma in the fragment shader.
    • Decode the gamma of the incoming vertex colors in your vertex shader.
    • Turn on sRGBA/linear framebuffer if available (GL_FRAMEBUFFER_SRGB).
      • Otherwise: gamma-encode the colors before you write them again.

§Understanding immediate mode

egui is an immediate mode GUI library.

Immediate mode has its roots in gaming, where everything on the screen is painted at the display refresh rate, i.e. at 60+ frames per second. In immediate mode GUIs, the entire interface is laid out and painted at the same high rate. This makes immediate mode GUIs especially well suited for highly interactive applications.

It is useful to fully grok what “immediate mode” implies.

Here is an example to illustrate it:

if ui.button("click me").clicked() {
    take_action()
}

This code is being executed each frame at maybe 60 frames per second. Each frame egui does these things:

  • lays out the letters click me in order to figure out the size of the button
  • decides where on screen to place the button
  • check if the mouse is hovering or clicking that location
  • chose button colors based on if it is being hovered or clicked
  • add a Shape::Rect and Shape::Text to the list of shapes to be painted later this frame
  • return a Response with the clicked member so the user can check for interactions

There is no button being created and stored somewhere. The only output of this call is some colored shapes, and a Response.

Similarly, consider this code:

ui.add(egui::Slider::new(&mut value, 0.0..=100.0).text("My value"));

Here egui will read value (an f32) to display the slider, then look if the mouse is dragging the slider and if so change the value. Note that egui does not store the slider value for you - it only displays the current value, and changes it by how much the slider has been dragged in the previous few milliseconds. This means it is responsibility of the egui user to store the state (value) so that it persists between frames.

It can be useful to read the code for the toggle switch example widget to get a better understanding of how egui works: https://github.com/emilk/egui/blob/master/crates/egui_demo_lib/src/demo/toggle_switch.rs.

Read more about the pros and cons of immediate mode at https://github.com/emilk/egui#why-immediate-mode.

§Multi-pass immediate mode

By default, egui usually only does one pass for each rendered frame. However, egui supports multi-pass immediate mode. Another pass can be requested with Context::request_discard.

This is used by some widgets to cover up “first-frame jitters”. For instance, the Grid needs to know the width of all columns before it can properly place the widgets. But it cannot know the width of widgets to come. So it stores the max widths of previous frames and uses that. This means the first time a Grid is shown it will guess the widths of the columns, and will usually guess wrong. This means the contents of the grid will be wrong for one frame, before settling to the correct places. Therefore Grid calls Context::request_discard when it is first shown, so the wrong placement is never visible to the end user.

This is an example of a form of multi-pass immediate mode, where earlier passes are used for sizing, and later passes for layout.

See Context::request_discard and Options::max_passes for more.

§Misc

§How widgets works

if ui.button("click me").clicked() { take_action() }

is short for

let button = egui::Button::new("click me");
if ui.add(button).clicked() { take_action() }

which is short for

let button = egui::Button::new("click me");
let response = button.ui(ui);
if response.clicked() { take_action() }

Button uses the builder pattern to create the data required to show it. The Button is then discarded.

Button implements trait Widget, which looks like this:

pub trait Widget {
    /// Allocate space, interact, paint, and return a [`Response`].
    fn ui(self, ui: &mut Ui) -> Response;
}

§Widget interaction

Each widget has a Sense, which defines whether or not the widget is sensitive to clicking and/or drags.

For instance, a Button only has a Sense::click (by default). This means if you drag a button it will not respond with Response::dragged. Instead, the drag will continue through the button to the first widget behind it that is sensitive to dragging, which for instance could be a ScrollArea. This lets you scroll by dragging a scroll area (important on touch screens), just as long as you don’t drag on a widget that is sensitive to drags (e.g. a Slider).

When widgets overlap it is the last added one that is considered to be on top and which will get input priority.

The widget interaction logic is run at the start of each frame, based on the output from the previous frame. This means that when a new widget shows up you cannot click it in the same frame (i.e. in the same fraction of a second), but unless the user is spider-man, they wouldn’t be fast enough to do so anyways.

By running the interaction code early, egui can actually tell you if a widget is being interacted with before you add it, as long as you know its Id before-hand (e.g. using Ui::next_auto_id), by calling Context::read_response. This can be useful in some circumstances in order to style a widget, or to respond to interactions before adding the widget (perhaps on top of other widgets).

§Auto-sizing panels and windows

In egui, all panels and windows auto-shrink to fit the content. If the window or panel is also resizable, this can lead to a weird behavior where you can drag the edge of the panel/window to make it larger, and when you release the panel/window shrinks again. This is an artifact of immediate mode, and here are some alternatives on how to avoid it:

  1. Turn off resizing with Window::resizable, SidePanel::resizable, TopBottomPanel::resizable.
  2. Wrap your panel contents in a ScrollArea, or use Window::vscroll and Window::hscroll.
  3. Use a justified layout:
ui.with_layout(egui::Layout::top_down_justified(egui::Align::Center), |ui| {
    ui.button("I am becoming wider as needed");
});
  1. Fill in extra space with emptiness:
ui.allocate_space(ui.available_size()); // put this LAST in your panel/window code

§Sizes

You can control the size of widgets using Ui::add_sized.

ui.add_sized([40.0, 20.0], egui::DragValue::new(&mut my_value));

§Code snippets

// Miscellaneous tips and tricks

ui.horizontal_wrapped(|ui| {
    ui.spacing_mut().item_spacing.x = 0.0; // remove spacing between widgets
    // `radio_value` also works for enums, integers, and more.
    ui.radio_value(&mut some_bool, false, "Off");
    ui.radio_value(&mut some_bool, true, "On");
});

ui.group(|ui| {
    ui.label("Within a frame");
    ui.set_min_height(200.0);
});

// A `scope` creates a temporary [`Ui`] in which you can change settings:
ui.scope(|ui| {
    ui.visuals_mut().override_text_color = Some(egui::Color32::RED);
    ui.style_mut().override_text_style = Some(egui::TextStyle::Monospace);
    ui.style_mut().wrap_mode = Some(TextWrapMode::Truncate);

    ui.label("This text will be red, monospace, and won't wrap to a new line");
}); // the temporary settings are reverted here

§Installing additional fonts

The default egui fonts only support latin and cryllic characters, and some emojis. To use egui with e.g. asian characters you need to install your own font (.ttf or .otf) using Context::set_fonts.

Re-exports§

Modules§

  • containers
    Containers are pieces of the UI which wraps other pieces of UI. Examples: Window, ScrollArea, Resize, SidePanel, etc.
  • debug_text
    This is an example of how to create a plugin for egui.
  • gui_zoom
    Helpers for zooming the whole GUI of an app (changing Context::pixels_per_point.
  • introspection
    Showing UI:s for egui/epaint types.
  • layers
    Handles paint layers, i.e. how things are sometimes painted behind or in front of other things.
  • load
    Image loading
  • menu
    Menu bar functionality (very basic so far).
  • mutex
    Helper module that adds extra checks when the deadlock_detection feature is turned on.
  • os
  • output
    All the data egui returns to the backend at the end of each frame.
  • special_emojis
    The default egui fonts supports around 1216 emojis in total. Here are some of the most useful: ∞⊗⎗⎘⎙⏏⏴⏵⏶⏷ ⏩⏪⏭⏮⏸⏹⏺■▶📾🔀🔁🔃 ☀☁★☆☐☑☜☝☞☟⛃⛶✔ ↺↻⟲⟳⬅➡⬆⬇⬈⬉⬊⬋⬌⬍⮨⮩⮪⮫ ♡ 📅📆 📈📉📊 📋📌📎📤📥🔆 🔈🔉🔊🔍🔎🔗🔘 🕓🖧🖩🖮🖱🖴🖵🖼🗀🗁🗋🗐🗑🗙🚫❓
  • style
    egui theme (spacing, colors, etc).
  • text
  • text_selection
    Helpers regarding text selection for labels and text edit.
  • util
    Miscellaneous tools used by the rest of egui.
  • viewport
    egui supports multiple viewports, corresponding to multiple native windows.
  • widget_text
  • widgets
    Widgets are pieces of GUI such as Label, Button, Slider etc.

Macros§

Structs§

Enums§

Constants§

Traits§

  • NumExt
    Extends f32, Vec2 etc with at_least and at_most as aliases for max and min.

Functions§

  • __run_test_ctx
    For use in tests; especially doctests.
  • __run_test_ui
    For use in tests; especially doctests.
  • lerp
    Linear interpolation.
  • pos2
    pos2(x, y) == Pos2::new(x, y)
  • remap
    Linearly remap a value from one range to another, so that when x == from.start() returns to.start() and when x == from.end() returns to.end().
  • remap_clamp
    Like remap, but also clamps the value so that the returned value is always in the to range.
  • vec2
    vec2(x, y) == Vec2::new(x, y)
  • warn_if_debug_build
    Helper function that adds a label when compiling with debug assertions enabled.

Type Aliases§

  • IdMap
    IdMap<V> is a HashMap<Id, V> optimized by knowing that Id has good entropy, and doesn’t need more hashing.