Use Lanczos interpolation for affine warp

[Messier-16]

The Bicubic interpolation algorithm is not great for astronomy image subtraction; Lanczos is better. However, scikit does not support this, so I used the OpenCV implementation.

In any case, there should also be an option to pass in an argument to change the interpolation method (even to nearest neighbors instead of bicubic).

[martinberoiz]

Hello,

Thanks for your PR I am currently out of town for a conference. I will look into it next week.

In the meantime, it seems that your PR introduces a new dependency (OpenCV). While I'm all for adding interpolation options (see #58) I'm very reluctant to introduce new dependencies to the package. Maybe I can consider OpenCV as an optional dependency.

[Messier-16]

Got it -- let me know what you think once you get to it! I'm not sure that Lanczos is flux-preserving either.

[martinberoiz]

Hi, I checked up on scikit-image to see if they had a Lanczos option and they don't. But in the warp method, the inverse_map argument accepts a function that does the mapping. This gives enough flexibility to inject there an exact or Lanczos interp. It has the caveat that it will be slower than providing a matrix but I think it's worth a try.

There are several projects on GitHub with implementations of Lanczos interp.

Do you have any reference where I can read more on why bi-cubic is not good for astronomy image subtraction? or why Lanczos is better?

[Messier-16]

https://pixinsight.com/doc/tools/StarAlignment/StarAlignment.html

This tool might have some insight for that, as they use it. The reason I believe it is because I personally did some empirical testing on my own data and the alignment was better.