[PATCH libinput 0/24] Tablet support

[Chandler Paul]

On Wed, 2014-05-28 at 12:48 +1000, Peter Hutterer wrote:
> On Tue, May 27, 2014 at 03:40:07PM -0700, Jason Gerecke wrote:
> > On Tue, May 27, 2014 at 3:20 PM, Peter Hutterer
> > <peter.hutterer at who-t.net> wrote:
> > > On Tue, May 27, 2014 at 04:32:14PM -0400, Chandler Paul wrote:
> > >> 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.
> > >
> > > we need a LI_FIXED_MAX then. Normalising to 0-1 in a 24.8 fixed point only
> > > leaves us with 256 value per axis.
> > >
> > Yeah, we don't want to pass a value like that through the fixed type.
> > It either needs to be re-scaled to use the full range (be that
> > [INT_MIN, INT_MAX] or [FIXED_MIN, FIXED_MAX]) or sent with a type that
> > won't loose quite as many bits :D
> 
> didn't think of it until after I sent the previous email:
> 
> there's a side-effect that we need to be aware of: if we scale an axis to
> LI_FIXED_MAX, we're effectively guaranteed to get 32-bit integer overflows
> on operations with that value. So we're effectively forcing the caller to
> work with int64_t to be on the safe side.
> 
> which isn't the worst of all things: on fixed 24.8 with current devices
> overflows aren't _that_ hard to trigger so we pretty much get to pick
> whether they happen sometimes for some devices or all the time, effectively
> forcing all callers to handle this correctly from day 1.
I've already implemented scaling to LI_FIXED_MAX in my tree, but I agree
this is something we should probably try to avoid. My thought here is
that we could scale it to a smaller value that's extremely unlikely to
ever overflow. My idea is to scale it to a 16.8 bit value. I don't think
it's likely that we'll ever be encountering anything larger then a 16
bit value with a wacom tablet. This does kind of seem like a bit of a
hack though.
> 
> > >> Also, what exactly is a "zero-point" in this context?
> > >
> > > whatever the neutral state of an axis is. e.g. tilt goes in both
> > > directions so the effective range is -value ... 0 ... +value.
> > >
> > Or as I like to think about it, the point where an axis would report a
> > value of "0". Most axes will report that value, and you'll just want
> > to be sure that an input of 0 turns into an output of 0. There's a
> > corner case where 0 could be outside the range of the axis, but if you
> > treat it as though it could you'll end up mapping the range
> > appropriately (e.g. treating an axis that only reports [10, 100] as
> > though it really reported [0, 100] will result in proper
> > normalization).
> 
> new question: what about a device that has an zero state that's not halfway
> between min/max? currently it'd advertise a range of [-N, M]. normalising
> those means we lose that information.
I don't think there's any devices like that right now, but if there are
it might be better just to pass the value to the client as-is without
normalizing it like we do with the pressure axis.
>  
> Cheers,
>    Peter

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