Add SPIFF correction

[putnokiabel]

Closes #695. Closes #413.

Feature

Based on https://www.nature.com/articles/s41598-017-14166-6?WT.feed_name=subjects_optical-manipulation-and-tweezers#Sec9 .
Use SPIFF to remove pixel-locking bias and improve sub-pixel accuracy of located features.
In the test cases shown (see test_spiff.py), you can see an order of magnitude better accuracy when the SPIFF correction is applied.

Sample results

Verify by adding the following line in test_spiff.py and running the tests:

2D
error before: 0.1300
error after SPIFF: 0.01998

3D
error before SPIFF: 0.2486
error after SPIFF: 0.0227

Note on documentation and further integration

I did not invest time into updating the documentation and further integrating it into the basic features yet, as I wanted to see what the maintainers think of the feature first.

Since it seems like SPIFF improves accuracy pretty much universally, I'd suggest the following (but feel free to ignore or suggest something else):

• Include an apply_spiff argument in tp.locate() and tp.batch() (could be enabled or disabled by default) and have those functions apply the SPIFF correction so the user doesn't have to know or think about it necessarily.
• Add documentation on using the new apply_spiff argument as well as the underlying apply_spiff_correction function.
• Update documentation (and walkthrough example) on subpixel bias to mention that using SPIFF is preferred to increasing the location radius (see the paper mentioned in the apply_spiff_correction method for more details on this).

[putnokiabel]

@nkeim
Hi, I'm new here!
Is there a process for contributing to trackpy (other than just submitting a PR and waiting)?

[nkeim]

Hi! Thanks for this PR!! I’ve seen SPIFF before and I’m glad it could be part of trackpy. The more “active” maintainers are also faculty, so our attention to trackpy (for non-urgent matters) is somewhat seasonal. I can’t give you a definite timeline, but obviously our activity tends to pick up in late Spring/early summer. We’ll do our best to take a look before then! Nathan On Feb 3, 2026, at 2:17 PM, Abel Putnoki ***@***.***> wrote:  [https://avatars.githubusercontent.com/u/29568469?s=20&v=4]putnokiabel left a comment (soft-matter/trackpy#799)<#799 (comment)> @nkeim<https://github.com/nkeim> Hi, I'm new here! Is there a process for contributing to trackpy (other than just submitting a PR and waiting)? — Reply to this email directly, view it on GitHub<#799 (comment)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/AAWY23ISYEZVWVCFIDEAVKL4KDX4HAVCNFSM6AAAAACS6XKRE6VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTQNBTGE3TKOBZGE>. You are receiving this because you were mentioned.Message ID: ***@***.***>