Re: deviceorientation calibration

Steve Block wrote:
> Hi Rich,
>
>> The starting question here is: what is the correct, normative device
>> orientation calibration to use in the three reference planes provided below?
> When you say 'calibration', I believe you're referring to the
> definition of the body frame xyz and the angles alpha, beta and gamma,
> not the calibration event in the spec. Is that right?

Yes. This is not related to the calibration event included in the spec 
but is instead for ensuring the readings produced by different user 
agents on the same or different devices return the same values when 
oriented in the same directional vector.

>
>> IIUC the specification does not yet provide enough information to answer
>> this question
> I think it does. The spec defines alpha, beta and gama in terms of the
> transformation from a coordinate frame fixed on the Earth to a
> coordinate frame fixed in the device. If you see any specific problems
> with the definition, please let me know.

I think its unclear in particular for beta and gamma values that to not 
tend to have a continuous gradation in 360 degree space.

>
>> [A1]
>> http://people.opera.com/richt/release/tests/orientation/calibration/alpha_A1.jpg
>> (pointing due north)
> You need to be more precise than 'pointing'. Which vector in the body
> frame is pointing north?

Based on the provided photo the device is in a [0, 0, 0] position based 
on the xyz system included in the spec. The fact that both z (beta) and 
x (gamma) values are zero is inferred from the fact that the device is 
resting on a (presumably level) table top. In reality the only important 
component is that y (alpha) is oriented towards the north (as determined 
by an independent compass).

>
>> To start, we must rotate the device 90 degrees clockwise as it sits on a table.
> Why is this?

It's a simple rotation so that, as we rotate the device towards us, we 
are able to observe changes to the gamma reading (rather than a beta 
reading if the device was not rotated 90 degrees as per the beta 
reference plane described).

>
> If you'd like to add specific examples to the non-normative section of
> the spec, I'm happy to help with that.

Each user agent has implemented this differently which hints that we 
could provide more information in this regard. I still think we should 
go further to include radar charts describing the rotation in each of 
three reference planes for alpha, beta and gamma.

- Rich

Received on Monday, 25 June 2012 12:56:11 UTC