egui_kittest

Ui testing library for egui, based on kittest (an AccessKit based testing library).

Example usage

use egui::accesskit::Toggled;
use egui_kittest::{Harness, kittest::Queryable};

fn main() {
    let mut checked = false;
    let app = |ui: &mut egui::Ui| {
        ui.checkbox(&mut checked, "Check me!");
    };

    let mut harness = Harness::new_ui(app);

    let checkbox = harness.get_by_label("Check me!");
    assert_eq!(checkbox.toggled(), Some(Toggled::False));
    checkbox.click();

    harness.run();

    let checkbox = harness.get_by_label("Check me!");
    assert_eq!(checkbox.toggled(), Some(Toggled::True));

    // Shrink the window size to the smallest size possible
    harness.fit_contents();

    // You can even render the ui and do image snapshot tests
    #[cfg(all(feature = "wgpu", feature = "snapshot"))]
    harness.snapshot("readme_example");
}

Snapshot testing

There is a snapshot testing feature. To create snapshot tests, enable the snapshot and wgpu features. Once enabled, you can call Harness::snapshot to render the ui and save the image to the tests/snapshots directory.

To update the snapshots, run your tests with UPDATE_SNAPSHOTS=true, so e.g. UPDATE_SNAPSHOTS=true cargo test. Running with UPDATE_SNAPSHOTS=true will cause the tests to succeed. This is so that you can set UPDATE_SNAPSHOTS=true and update all tests, without cargo test failing on the first failing crate.

If you want to have multiple snapshots in the same test, it makes sense to collect the results in a Vec (look here for an example). This way they can all be updated at the same time.

You should add the following to your .gitignore:

**/tests/snapshots/**/*.diff.png
**/tests/snapshots/**/*.new.png

Guidelines for writing snapshot tests

• Whenever possible prefer regular Rust tests or insta snapshot tests over image comparison tests because…
  • …compared to regular Rust tests, they can be relatively slow to run
  • …they are brittle since unrelated side effects (like a change in color) can cause the test to fail
  • …images take up repo space
• images should…
  • …be checked in or otherwise be available (egui use git LFS files for this purpose)
  • …depict exactly what's tested and nothing else
  • …have a low resolution to avoid growth in repo size
  • …have a low comparison threshold to avoid the test passing despite unwanted differences (the default threshold should be fine for most usecases!)

What do do when CI / another computer produces a different image?

The default tolerance settings should be fine for almost all gui comparison tests. However, especially when you're using custom rendering, you may observe images difference with different setups leading to unexpected test failures.

First check whether the difference is due to a change in enabled rendering features, potentially due to difference in hardware (/software renderer) capabilitites. Generally you should carefully enforcing the same set of features for all test runs, but this may happen nonetheless.

Once you validated that the differences are miniscule and hard to avoid, you can try to carefully adjust the comparison tolerance setting (SnapshotOptions::threshold, TODO(#5683): as well as number of pixels allowed to differ) for the specific test.

⚠️ WARNING ⚠️ Picking too high tolerances may mean that you are missing actual test failures. It is recommended to manually verify that the tests still break under the right circumstances as expected after adjusting the tolerances.

In order to avoid image differences, it can be useful to form an understanding of how they occur in the first place.

Discrepancies can be caused by a variety of implementation details that depend on the concrete GPU, OS, rendering backend (Metal/Vulkan/DX12 etc.) or graphics driver (even between different versions of the same driver).

Common issues include:

• multi-sample anti-aliasing
  • sample placement and sample resolve steps are implementation defined
  • alpha-to-coverage algorithm/pattern can wary wildly between implementations
• texture filtering
  • different implementations may apply different optimizations even for simple linear texture filtering
• out of bounds texture access (via textureLoad)
  • implementations are free to return indeterminate values instead of clamping
• floating point evaluation, for details see WGSL spec § 15.7. Floating Point Evaluation. Notably:
  • rounding mode may be inconsistent
  • floating point math "optimizations" may occur
    • depending on output shading language, different arithmetic optimizations may be performed upon floating point operations even if they change the result
  • floating point denormal flush
    • even on modern implementations, denormal float values may be flushed to zero
  • NaN/Inf handling
    • whenever the result of a function should yield NaN/Inf, implementations may free to yield an indeterminate value instead
  • builtin-function function precision & error handling (trigonometric functions and others)
• partial derivatives (dpdx/dpdx)
  • implementations are free to use either dpdxFine or dpdxCoarse
• [...]

From this follow a few simple recommendations (these may or may not apply as they may impose unwanted restrictions on your rendering setup):

• avoid enabling mult-sample anti-aliasing whenever it's not explicitly tested or needed
• do not rely on NaN, Inf and denormal float values
• consider dedicated test paths for texture sampling
• prefer explicit partial derivative functions