Add linear interpolation (LERP) for quats

[Kamilczak020]

Introducing Quaternion LERP as there is SLERP already. No need for NLERP since its just LERP result normalized. Haven't done e2helper desc yet.

[TomyLobo]

what is LERP needed for?

[Divran]

If I understand it correctly, slerp speeds up, reaches a max speed, and then slows down again. Lerp, on the other hand, rotates/moves stuff at a constant speed. Could be wrong tho

[TomyLobo]

slerp is spherical linear interpolation. it's linear on a sphere.
In other words, if anything, it's the other way round.

But I was asking @Kamilczak020 :)

[Divran]

but I've used slerp in gmod many times and it does speed up, and then slow down, if provided with a constant multiplier. You have to use an equation to change the multiplier a lot to get it to be the same speed throughout the whole rotation. So therefore I just assumed that lerp must be constant speed, otherwise it wouldn't make much sense to have both

[thegrb93]

From wikipedia

When Slerp is applied to unit quaternions, the quaternion path maps to a path through 3D rotations in a standard way. The effect is a rotation with uniform angular velocity around a fixed rotation axis

[Divran]

In that case, slerp is implemented incorrectly.

[TomyLobo]

then it should be fixed.

...so the question still stands: what is LERP used for?

[AbigailBuccaneer]

http://number-none.com/product/Understanding%20Slerp,%20Then%20Not%20Using%20It/ This is a good article that gives more info about slerp and similar. (Formatting's a bit broken though.)

[TomyLobo]

that article lists slerp, nlerp and log-quaternion lerp.
I'm missing the logs in this log-quaternion lerp

nlerp is noted mostly for its performance where constant velocity is not important.
log-quaternion lerp seems to be the interpolation of choice if you need a commutative constant-velocity interpolation

It seems commutative here means that you can stack the NLERPs and LQLERPs (but not SLERPs) in whatever order you want

[TomyLobo]

In conclusion, let's have slerp, nlerp and lqlerp functions and let's a have a table on the wiki comparing the three.

Having lerp might cause people to use that and wonder why those stupid useless quaternions are broken and useless

[oldmud0]

Merge?

[AbigailBuccaneer]

This needs an E2Helper description, and I won't be home tonight. Happy for this to be merged if somebody can immediately follow it up with adding the description.

[AbigailBuccaneer]

(If anybody's curious about the implementation, I was looking up a reference implementation and ended up here, where the code seems to have been translated from.)

[oldmud0]

I should read up on quats one day.