Re: [PATCH v7 5/6] iio: ABI: Add partial quaternion modifier

[Andy Shevchenko]

On Thu, Mar 05, 2026 at 08:40:26AM -0600, David Lechner wrote:
> On 3/5/26 2:50 AM, Francesco Lavra wrote:
> > On Wed, 2026-03-04 at 16:42 -0600, David Lechner wrote:
> >> On 3/4/26 8:21 AM, Francesco Lavra wrote:
> >>> On Wed, 2026-03-04 at 13:51 +0200, Andy Shevchenko wrote:
> >>>> On Wed, Mar 04, 2026 at 09:07:05AM +0100, Francesco Lavra wrote:

...

> >>>>> +What:          /sys/bus/iio/devices/iio:deviceX/in_rot_partial_qua
> >>>>> tern
> >>>>> ion_raw
> >>>>
> >>>>> +               Raw value of {x, y, z} components of the quaternion
> >>>>> vector. These
> >>>>> +               components represent the axis about which a
> >>>>> rotation
> >>>>> occurs, and are
> >>>>> +               subject to the following costraints:
> >>>>> +               - the quaternion vector is normalized, i.e. x^2 +
> >>>>> y^2 +
> >>>>> z^2 + w^2 = 1
> >>>>> +               - the rotation angle is within the [-180, 180]
> >>>>> range,
> >>>>> i.e. the w
> >>>>> +                 component (which represents the amount of
> >>>>> rotation)
> >>>>> is non-negative
> >>>>> +               These constraints allow the w value to be
> >>>>> calculated
> >>>>> from the other
> >>>>> +               components: w = sqrt(1 - (x^2 + y^2 + z^2)).
> >>>>
> >>>> Just to double check if we really do not have a special mathematical
> >>>> term
> >>>> for
> >>>> that already. If we do, I prefer to have that over odd "partial
> >>>> quaternion".
> >>>
> >>> A quaternion is often represented as w + xi + yj + zk, i.e. it's
> >>> composed
> >>> of a real coefficient (w) and 3 imaginary coefficients (x, y, z). With
> >>> this
> >>> notation, the (x, y, z) components are referred to as the imaginary
> >>> part of
> >>> the quaternion.
> >>> Alternatively, a quaternion is represented as the combination of a
> >>> scalar
> >>> value w and a 3D vector value (x, y, z). With this notation, the (x, y,
> >>> z)
> >>> components are referred to as the vector part of the quaternion; this
> >>> can
> >>> be confusing, since the quaternion as a whole is often considered as a
> >>> 4D
> >>> vector.
> >>
> >> I'm surprised there isn't a common name for this. When I went looking,
> >> the
> >> thing that came up most often is that Doom 3 used this in their MD5 file
> >> format. So maybe IIO_MOD_DOOM3_QUATERNION? (joking)
> >>
> >> I do think we could come up with something better than "partial" though.
> >>
> >> My first thought is IIO_MOD_3D_QUATERNION since it is a 3-dimentional
> >> number,
> >> but I could see that being confusing to some since quaternions are
> >> generally
> >> used for rotations in 3-D space.
> >>
> >> Maybe something like IIO_MOD_3VALUE_QUATERNION or
> >> IIO_MOD_COMPRESSED_QUATERNION
> >> or just IIO_MOD_3QUATERNION?
> > 
> > Or, since this represents the axis of rotation, perhaps something like
> > IIO_MOD_QUATERNION_AXIS?
> 
> Sounds OK to me.

Agree, this is at least better than opaque "partial".

-- 
With Best Regards,
Andy Shevchenko