Vmath.quat_from_to and nan

Questions
1

Short question: should the resulting quaternion between rotation of two vectors (0, 0, 1) to (0, 0, -1) being a undefined values (nan)? Or this is a bug.

    local v1 = vmath.vector3(0, 0, 1)
    local v2 = vmath.vector3(0, 0, -1)
    local rot = vmath.quat_from_to(v1, v2)
    print(rot) -- vmath.quat(-nan, -nan, -nan, 0)

https://defold.com/codepad/#?c=#cp_sprite&s1=GYVwdgxgLglg9mABDMMoAoDOBTANsASgCgBIXOCAQ10QDcBGRAXjoFtKoALAOlu2jgAnAMzoADABpEkxPWKIFiclRq0ATMzYcefASPFSZAWjlFFSitUSC4UTbXZduARxAcA+sBut3UOOgYpdXlFAAdBFAwbKGJsMAATIiJQSFgERGAUaiw8QiS4xOTwaHgkEFD4jmwc/Cl4mPyEpJSS9IR3VmxMTEoAc2qcWsRO7r7sdxh4qRGe/qkcBOxBYiIC5uK0pHaUUJAMQeApSlawCanEY82VtaLU0sR2wTw4SniavNWmoA===

2 Likes
2

The vectors are directly opposing, which isn’t allowed.

1 Like
3

It’s a nuisance, but it is in the documentation.

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4

Perhaps we should fix this and instead of nan return an identity quat? This is what Three.js does (https://github.com/mrdoob/three.js/blob/dev/src/math/Quaternion.js#L352). I’m not sure what other engines do?

1 Like
5

If it’s going to change, I would just return nil or false. It can’t possibly give a specific value because it doesn’t know what plane to rotate around, so you’ll always need to check for it. nil or false would at least be easier to handle than a quat with NaN values, whereas an identity quat would mislead you into thinking that no rotation was necessary. :scream:

You can check for nan by comparing it to itself. It will not be equal to itself.

if rot.x ~= rot.x then ... (is NaN)

4 Likes
6

yep, this comparing was in my code, but I don’t really like post-checking. Besides, sometimes the results were not nan but inf or -inf.

now using that code:

if vmath.dot(v1, v2) ~= -1 then
        local q = vmath.quat_from_to(v1, v2)
...
3 Likes