NaN when `looking_at` is called with `Transform`'s origin as target

Bevy version

0.5 and 0.6

Operating system & version

Linux Mint 20.2

What you did

I modified the 3D Scene example so line 42 read as,
transform: Transform::from_xyz(0.0, 0.0, 0.0).looking_at(Vec3::ZERO, Vec3::Y),

As you can see, the origin of the transform is the same as where it's set to look.

What you expected to happen

Default to either current Quat or the unit matrix.

What actually happened

Printing the debug value of that particular transform produced,
Mut(Transform { translation: Vec3(NaN, NaN, NaN), rotation: Quat(NaN, NaN, NaN, NaN), scale: Vec3(1.0, 1.0, 1.0) })

and attempting to further modify it resulted in no change (as expected, given what's happening).

Additional information

[HackerFoo]

Why should this be defined?

Using the current or identity rotation is not correct if the up vectors don't match, such as:

Transform::identity().looking_at(Vec3::ZERO, Vec::Z)

You're right, there are an unlimited number of rotations that satisfy those constraints. Perhaps a bit of documentation for those who aren't aware of the effect and what the proper method for doing what they want is (they might just want a default Transform)?

[rparrett]

While we're here, is this also expected?

[examples/3d/lighting.rs:32] Transform::from_xyz(0.0, 5.5, 0.0).looking_at(Vec3::ZERO, Vec3::Y) = Transform {
    translation: Vec3(
        0.0,
        5.5,
        0.0,
    ),
    rotation: Quat(
        NaN,
        NaN,
        NaN,
        NaN,
    ),
    scale: Vec3(
        1.0,
        1.0,
        1.0,
    ),
}

edit: yes, of course.