[zamalek]

One option would be to have an input replay alongside captured outputs (audio visual), at some fixed framerate. Capturing intermediates (scene graph etc.) would probably also be valuable, as that could help nail down why something is failing.

Or you could do it [as I recall the project being called] the scientist way. You still have the old code, so you could replay inputs against each and compare. Probably more realistic because uncompressed video would be a ridiculously huge dataset. This would be more resilient in the face of testing hardware and driver drift.

Historically game engines are the worst offenders when it comes to unit testing. I'm not sure if that's still the case - but that's why I erred on the side of integration tests.

[hdjrudni]

Box3D just showcased some stuff including deterministic replays. If you wanted to port that, you could probably import the replay and make sure it plays back the same way in your new language. I think it captures the inputs and forces applied, not the pixels.

I suppose rendering is a component of a game engine too though, not just physics. I don't know how to do that reliably. Even if you captured pixels, it'd be annoying. If you've ever tried doing screenshot based diffing on web you will know that slight changes in aliasing in Chrome bugger everything up. Things that should be equivalent randomly aren't but not in a way that any human would care.