Fix lerpVectors for when v1 === this

[sirxemic]

When calling v.lerpVectors(v1, v2, alpha) with v1 being equal to v, the function gives incorrect output. I don't think we should have to do a check like

if (v1 === v) v.lerp(v2, alpha)
else v.lerpVectors(v1, v2, alpha)

so I decided to fix the function.

[WestLangley]

How does the value of v, prior to calling the method, affect the result?

[sirxemic]

Not sure if I understand your question (or if you understand what I'm trying to point out).

But yeah, when v1 === v, then v.lerpVectors(v1, v2, alpha) basically returns 2 * (v2 - v1) * alpha, because the current implementation then basically boils down to this.subVectors( v2, this ).multiplyScalar( alpha ).add( this ), which hopefully you can see is clearly not correct.

[WestLangley]

So, instead of calling

v.lerp( v2, alpha );

you are calling:

v.lerpVectors( v, v2, alpha ); // v repeated

I see.

[sirxemic]

Well, I'm calling v.lerpVectors( v1, v2, alpha ). It just so happens that v and v1 are pointing to the same object, which is why I said I shouldn't have to be doing the following.

if (v1 === v) v.lerp(v2, alpha)
else v.lerpVectors(v1, v2, alpha)

[mrdoob]

Well, I'm calling v.lerpVectors( v1, v2, alpha ). It just so happens that v and v1 are pointing to the same object

Hmmm, okay...

[mrdoob]

Thanks!