egui-screenshot-testing

Helper functions to test egui applications using screenshots and comparing them to a saved version. The idea is to take an application state and render it using the TestBackend. Then, you can compare a saved screenshot and compare that the generated visuals are the same.

use egui_screenshot_testing::TestBackend;

let mut backend = TestBackend::new("src/tests/expected", "src/tests/actual", |_ctx| {
    // You could do any initialization here.
});
backend.assert_screenshot_after_n_frames("test_case_a.png", (150, 100), 5, move |ctx| {
    // Add any egui elements
    egui::CentralPanel::default().show(ctx, |ui| {
        ui.heading("Hello World");
    });
});

The screenshots are compared to an image file that is stored in a given directory.

Cargo.toml
src/
  [...]
  tests/
    expected/
      test_case_a.png
      test_case_b.png
      [...]

On failure, the generated screenshot is written to a folder that contains all actual screenshots. You can compare the images by hand or with an image diff tool (eg. the ImageMagick compare tool) and decide whether you want to update the snapshot by copying the file to the expected folder. If you set the environment variable EGUI_SCREENSHOT_REPLACE, all expected files will be replaced with the actual ones without failing the tests. This is e.g. useful when creating an initial set of tests where no snapshot exists yet.

Also see the examples/ folder in the git repo for a usage example.

3rd party dependencies

This software depends on several 3rd party projects. In particular, it bundles the egui_skia crate from Lucas Merlin Meurer to render screenshots in headless environment in the src/egui_skia subfolder. It also incorporates the changes from Frans Skarman that update egui_skia to work with newer egui versions. The original license statement for egui_skia is given below.

MIT License

Copyright (c) 2022 Lucas Merlin Meurer

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.

Additional libraries are documented in the "third-party-licenses.html" file in this folder.