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

[Francesco Lavra <flavra@baylibre.com>]

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:
> > > > This modifier applies to the IIO_ROT channel type, and indicates a
> > > > data
> > > > representation that specifies the {x, y, z} components of the
> > > > normalized
> > > > quaternion vector.
> > > 
> > > ...
> > > 
> > > > +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?