Diffing.image(precision:) should be scale aware

[moritzsternemann]

While working with snapshot-tests on a large scale, we resorted to lowering the snapshot resolution in some cases to improve performance. This was achieved by reducing the displayScale of the trait collection passed to the Snapshotting.image() method.

While the results are as expected, when the snapshot-test fails, the attached diff is incorrect.

Example

Notice the plus icon in the corner, which has the correct size while the i icon is scaled up.

Solution

I noticed that the image diffing strategy uses the UIScreen.main.scale constant to load the reference snapshot from disk (Sources/SnapshotTesting/Snapshotting/UIImage.swift#L16).
This causes the reference image being a different size the new snapshot when creating the diff, which results in a pretty much useless XCTestAttachment.

I suggest to resolve this by passing the displayScale from the trait collection through to the image diffing strategy. I prepared the necessary changes and would like to open a PR as soon as possible if nobody finds anything wrong with my approach.

[stephencelis]

This sounds good to us! Thanks for exploring the solution.

[stephencelis]

@moritzsternemann This also appears to be a dupe of #243, right? I'm going to close this, but feel free to reopen if you think the issues are distinct.

[moritzsternemann]

@stephencelis yes, I totally missed that!

Though my solution won't allow snapshot tests to be run on different simulators than when the references were created because of the font rendering difference @kirillsh described in #243 (comment).