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

[David Lechner <dlechner@baylibre.com>]

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_quatern
>>> 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?