Crate twenty_twenty

The twenty-twenty crate allows for visual regression testing of H.264 frames (enabled with the h264 feature) as well as images. It makes it easy to update the contents when they should be updated to match the new results.

Each function takes a minimum permissible similarity, which is the lowest possible “score” you are willing for the image comparison to return. If the resulting score is less than the minimum, the test will fail. The score must be a number between 0 and 1. If the images are the exact same, the score will be 1.

The underlying algorithm is SSIM, which is a perceptual metric that quantifies the image quality degradation that is caused by processing such as data compression or by losses in data transmission. More information can be found here.

To compare H.264 frames you will need ffmpeg installed on your system and the h264 feature enabled to use this crate, which relies on the Rust ffmpeg bindings to convert the H.264 frames to images.

Use it like this for an H.264 frame:

let actual = get_h264_frame();
twenty_twenty::assert_h264_frame("tests/initial-grid.png", &actual, 0.9);

Use it like this for an image:

let actual = get_image();
twenty_twenty::assert_image("tests/dog1.png", &actual, 0.9);

If the output doesn’t match, the program will panic! and emit the difference in the score.

To accept the changes from get_h264_frame() or get_image(), run with TWENTY_TWENTY=overwrite.

Usage in tests

1. Write a test, for example:

// tests/twenty_twenty.rs
#[test]
fn example_test() {
    let actual = get_image();
    twenty_twenty::assert_image("tests/dog1.png", &actual, 0.9);
}

2. Run the test and have it write actual to disk next to the file the test is in, in this case tests/dog1.png:

TWENTY_TWENTY=overwrite cargo test example_test

3. Review the output image and ensure it is a correct reference image.

4. Run cargo test. If the generated image changes and differs from the image written to disk, the test will fail.

Storing artifacts in CI

Use either TWENTY_TWENTY=store-artifact or TWENTY_TWENTY=store-artifact-on-mismatch to save artifacts to the artifacts/ directory. The latter can be used to only store failing tests for review and repair.

Functions

assert_image

Compare the contents of the file to the image provided.