How to implement image diffing for visual regression testing

[rassie]

If I understand the documentation correctly, it's possible to implement a snapshot extension which would compare images visually (e.g. with pixelmatch) instead of byte-wise. Is it a matter of writing a custom SnapshotReporter and mix it into an extension or would I need to do more?

[noahnu]

Same as with the other discussion, sorry about the delayed response.

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The SnapshotReporter is only used for displaying diff results. You'd likely want to override it, specifically the diff_lines method to communicate the results from pixelmatch. The actual assertion unfortunately happens here and cannot be overridden at this time.

That being said, it'd be fairly trivial to update this logic to let a custom extension override the equality check. That'd also enable other sorts of "tolerance" assertions. If there is still interest in this, we can move this discussion to a feature request.