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

[Peter Hutterer <peter.hutterer@who-t.net>]

On Tue, Sep 23, 2014 at 09:28:11AM -0700, Dmitry Torokhov wrote:
> 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 the hid-mt behaviour makes sense. Without explicit resolution (and
no device sets that anyway, IIRC) any distance value > min tells us only that a
tool is within detectable range, but not yet touching. Anything between
min/max is only useful as a relative scale, but effectively that [min,max]
range could be a metre or a millimeter, we can't know. So a device with a
0/1 range simply has low granularity and is only able to detect whether
something is within range or touching the surface.

Cheers,
   Peter

> 
> > 
> > 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