Easy interpolation across splines

Easy interpolation across splines

interpolation, spline

2018-08-05 19:00:00 UTC, by Dimitri Sabadie


This article introduces the splines crate.

Preliminary work

I’ve been using interpolation (linear, bilinear, trilinear, B-splines, etc.) for a while now because of my demoscene productions. My spectra crate features some interpolation primitives and I’ve been wanting to move them out of spectra for a while.

Basically, my need is simple:

That’s a lot of things. For what it’s worth, I’ll just use as reference a library I wrote years ago in Haskell, smoothie, doing exactly that.

However, because I didn’t want to bloat the ecosystem, I had a look around to see whether I could simply drop my code and add a simple dependency instead of extracting things from spectra and creating, again, new crates. Here are the reasons why.

bspline

This crate is about B-spline only and the interface is highly unsafe (panics if out of control points for instance). My current spectra code is already safer than that, so no reason to lose features here.

spline

This is… a… placeholder crate – i.e. it doesn’t have anything exposed. People do this to book a crate name for later. I highly dislike that and people doing this should really be warned not to do this, because they prevent others – who actually have some code to put there – from using the name. Plus, for this spline crate, I had already have a look years ago. In years, the so-called “author” hasn’t even uploaded anything and the name is just reserved… for nothing.

I highly suggest people from the Rust team to forbid people from doing this. This is just useless, childish and brings zero value to the community. Just remove that crate as no one has ever had the chance to ever depend on it.

trajectory

This is an interesting crate, but it’s way too much specialized to me. It could be very interesting to use it for camera and objects paths, but I also need splines for a lot of other situations, so this is too much restrictive (you can see functions like acceleration, position, velocity that works only for T: Float).

nbez

A – looks like – very comprehensive Bezier crate, but doesn’t provide anything for all the other interpolation I need.

Let’s build yet another crate!

Looks like the crate I’m looking for doesn’t exist yet – oh yes it does: it’s spectra!, but I want a crate that does interpolation only.

I could be using some of the crates listed above, add them as a bunch of dependencies to a glue crate and just expose the whole thing. However, I’m not sure they will compose quite correctly and I might just end up re-coding the whole thing.

So the crate I’ll be building will be about interpolation. You will find step (constant) interpolation, linear / bilinear / trilinear interpolation, cosine, cubice, Catmull-Rom and Bézier spline interpolations. The crate will be as abstract as possible to enable developers to plug in their own types. Because the spline name is already taken – duh! – I’ll use the splines name…

I’ll take my chance to try to take over spline first, though. I really don’t like that kind of things in an ecosystem, it’s just unaesthetic.

Note²: Ok, I tried, and seems like it’s more complicated than I thought. I’ll just go with the splines name then.

So this blogpost serves as an introduction to splines. I added a few unit tests to start off with and a very few examples in the documentation. Most of the interface is sufficient for now but more complex needs might show up. If you have any, please shoot an issue or even better, open a PR!

Keep the vibes and happy hacking!