Re: How to indicate hover touch when exact distance unknown?

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

Hi Benjamin,

On Tue, Sep 23, 2014 at 10:59:16AM -0400, Benjamin Tissoires wrote:
> Hi Andrew,
> 
> On Sep 23 2014 or thereabouts, Andrew de los Reyes wrote:
> > Hi folks,
> > 
> > More and more we're seeing touch input devices that support detection of
> > fingers that are above the surface and not touching. We were thinking of
> > writing a kernel patch for one such device. Normally we would indicate
> > distance with ABS_MT_DISTANCE, but we ran into the problem that sometimes
> > the hardware will not be able to report a distance (or will stop reporting
> > a distance when it's too great a number), and we had the (bad) idea to
> > simply report a value or 1 or 255 or something like that when distance is
> > unknown.
> 
> FWIW, hid-multitouch already support those devices.
> ABS_MT_DISTANCE is set with a min/max of 0/1 when we detect win8
> certified panels with hovering capability. By default the spec does not
> provide the distance IIRC, and you only have one byte: InRange.

Hmm, I missed that and this is unfortunate. The ABS capabilities
advertised by the devices should match their real capabilities. If
device can't properly report distance it should not be using
ABS_MT_DISTANCE/ABS_DISTANCE...

> 
> I think Xorg can deal with that (the touch emulation discards the
> ABS_MT_DISTANCE, but the xorg-evdev driver should be smart enough to not
> take BTN_TOUCH into account).
> 
> > 
> > Dmitry warned that a similar thing happened with PRESSURE a while ago, and
> > it was a mess as different drivers did different things.
> 
> As I said, IIRC, xorg-evdev is fine with that. For generic desktop, we
> have to make sure libinput is aware, and then you only have to handle
> your own xorg chromebook driver. This is not something which scares me
> that much, especially given that this is what hid-multitouch reports
> since the v3.9 kernel.
> libinput is based on a per slot device information, so it is really easy
> to implement if it is not already in place. xorg-Synaptics and xorg-Wacom
> should not be ported to wayland from what I understood, so the mess with
> several implementations can be solved easily.
> 
> > 
> > We are now wondering if we should come up with a standard way to indicate
> > hover with or without distance (or with distance being optional). Dmitry
> > had the idea for a new tool type, HOVER; FINGER would be for touches that
> > are actually touching.
> 
> The tool is still the finger.

Do we reliably know that it is a finger and not a pen or eraser or some
other tool? If we do then obviously introducing new tool will not fly.

> So I am not very happy with having a new
> tool. In the end, it may also disturb older clients which will not know
> what to do with HOVER.
> And the ABS_MT_DISTANCE approach used to be fully retro-compatible
> (assuming that the hovering distance is small enough for the user not to
> detect it).

Except that if we start getting devices that can actually tell the
distance we'd need special casing 0/1 handling, no?

Thanks.

-- 
Dmitry