[PATCH libinput 0/24] Tablet support

[Chandler Paul]

On Tue, 2014-05-27 at 13:11 -0700, Jason Gerecke wrote:
> I've been away from my computer for most of the (long) weekend up
> here, so apologies for being a bit quiet :)

> There's a subtlety on the protocol side of things that can't be
> ignored. When normalizing data, you want to be careful to preserve
> information about the zero point. Without that, you can't meaningfully
> pass the data along. Lets imagine that we have some sensor that will
> report values between 10 and 100, with a resolution of 1 unit = 1
> elbow per square ounce. If we normalize that to the range [0,
> UINT32_MAX] we've lost information about where "zero" is. A normalized
> value of zero does not correspond to zero elbows per square ounce as
> you might expect, and the resolution info is insufficient to correct
> the offset.
> 
> Now, if we've done our jobs properly in libinput, that shouldn't be a
> problem. We would have normalized that sensor's values to [0.1, 1] and
> announced the axis to have a resolution of 1 unit = 100 elbows per
> square ounce. Because the zero point is offset like it originally was,
> it's preserved through the scaling done for the protocol and so the
> original 10-100 range can be recovered. The only amendment I'd make is
> to use a signed integer type rather than an unsigned one, since we may
> have negative normalized values that need to be sent through the
> protocol.
I just wrote code to normalize it to INT_MAX, but since everything's in
fixed point integers the actual values it's being scaled to are
0-8388607.99609375 when the fixed point axis value is converted back
into a double, which as I'm sure you probably realize is kind of a
strange value, and I'm starting to think something like 0.1-1.0 would be
a lot better, trying to normalize to INT_MAX results in something that
sounds really weird to work with.
Also, what exactly is a "zero-point" in this context?
> 
> >> Seems fine to me. As for normalizing values to units/mm or the like, is
> >> there any known conversion for the units the tablet returns for distance
> >> to metric?
> >
> > Benjamin answered that on IRC, but for the archives: the distance is in mm,
> > though in reality the data is inprecise.
> >
> > Cheers,
> >    Peter
> >
> 
> I would avoid attaching units or resolution to axes which do not
> already declare them. The distance values on our pens do roughly
> correspond to millimeters from the sensor (which itself is usually a
> few mm below the surface) but we should be reporting a non-zero
> resolution through evdev if the data were reasonably accurate :D
> 
> Also, libinput shouldn't generally be "normalizing values to units/mm
> or the like." Data should be normalized to some range within [-1, 1]
> so that the zero point is preserved. Resolution data should be
> provided through another means which relates normalized values to
> real-world units (and should probably be documented to be zero if the
> resolution is unknown). The only exception to this /might/ be
> something like tilt or rotation (though the more I think about it, the
> less I believe it to be worthy of exception given how apps actually
> use the data).
Just to get an idea, how many applications do you think would actually
need to get the resolution information for the tablet?

> 
> Jason
> ---
> Now instead of four in the eights place /
> you’ve got three, ‘Cause you added one  /
> (That is to say, eight) to the two,     /
> But you can’t take seven from three,    /
> So you look at the sixty-fours....

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