Thanks for working on this! According to ResampleScope, the 2013 variant seems incorrectly(?) implemented, see:
https://github.com/kleisauke/libvips-pr-4237#mks2013

Any thoughts on why? If not, I'd be glad to take a closer look.

Thank you @akimon658 for the PR. Do you think it's worth including both kernels, or would we be better to focus on the more recent 2021 "edition" only? (In addition, I note this kernel is rather useful for edge detection, so a future possible improvement there also.)

Do you think it's worth including both kernels, or would we be better to focus on the more recent 2021 "edition" only?

Currently, my team is using the 2013 version. I think adding the 2013 edition would be beneficial as it allows us to migrate to libvips without altering application's behavior.
Additionally, it would not significantly increase the maintenance burden on the libvips team.

Thanks for doing this work, @akimon658 !

Could you also:

• add it to the set of kernels in the test suite: https://github.com/libvips/libvips/blob/master/test/test-suite/test_resample.py#L77-L101
• add a line to ChangeLog.md, and credit yourself

In the ChangeLog file, which version should I put my change under?

This is an API addition, so it'll be in the next stable version (8.17). Thanks!

👍

Ah, sorry I missed this—my son Jack just flagged it to me. I would have been happy to provide advice, but it looks like you have it sorted.

Hopefully! Thanks for checking @j-p-c

Hopefully! Thanks for checking @j-p-c

For extreme clarity: I have not checked the correctness of your implementation; I assume that that has been covered off.