[Interest] Astropad and Tablet events

[Rutledge Shawn]

> On 22 Mar 2016, at 02:30, Alexei Gilchrist <te100 at runbox.com> wrote:
> 
> Hi all,
> 
> I'm trying to use Astropad (app provides OsX tablet interface with an iPad) with an app I’ve written and I'm getting odd results with tablet events in QT. First of all, I've been in contact with the devs and they seem to think it's a QT bug rather than an Astropad bug, and in any case Astropad works with other graphics programs that I've tried.

Why do they think it’s a Qt bug?  Maybe they should tell us what we are doing wrong if they know.

I guess there must be a Mac daemon which tries to emulate a Wacom by sending the same events?  It’s not only for Qt applications, right?  But Qt applications work with a real Wacom but not Astropad, so they must not be emulating the events exactly the same.

You can enable the logging category qt.qpa.input.tablet to see how the events come in (the code is in qnsview.mm).

> I'm coding in python using pyqt and have Python 3.5.0 and QT 5.5.0 installed.
> 
> At the level of QWidget you get an initial TabletPress event with pressure=0 followed immediately by a TabletRelease event with pressure>0 at which point the rest of the stroke is only reported as mouse events, with are low frequency and don’t contain pressure info.
> 
> Astropad:
> 
> TabletPress: pointer=1 pressure=0.0 tilt=(27,12)
> TabletRelease: pointer=1 pressure=0.13333334028720856 tilt=(27,12)
> MousePress
> MouseMove
> MouseMove
> ...[ many MouseMove ]...
> MouseRelease
> 
> Works as you'd expect with Wacom tablet, though accepting the tablet events does't stop the mouse events being fired (minor niggle):

That’s https://bugreports.qt.io/browse/QTBUG-47007  It’s probably always been that way, at least in Qt 5, because the platform plugins are doing this mouse emulation when necessary.  (In some cases the window system will send a synthesized mouse event itself, in which case we don’t need to.)  Plugins don’t pay attention to whether the tablet event was accepted or not, because sending events from the platform plugin into the application is asynchronous, going through a queue.  We assume the tablet event is rejected because that’s the default policy (outside of QtQuick where we assume an event is accepted), so the default is that the application does nothing, and then it gets a mouse event (so that you can use the stylus to interact with widgets etc.)  AFAIK there isn’t a nice mechanism to wait and see whether the app accepted the tablet event or not; we’d have to wait until the asynchronous delivery is done, somehow.

In older Qt versions, you needed the mouse event (which means you should reject the tablet event, according to the docs) in order to find out what buttons on the stylus were pressed.  But if you reject the tablet press event, you don’t get movement updates as tablet events, and the mouse events are compressed (as you noticed).  Now, QTabletEvent has buttons too, so you no longer need to do that, but there may still be some legacy applications that are already written to work that way.  So I’m not sure whether we should fix that now, or will it be seen as a behavior change which breaks applications?  I guess fixing it in 5.7 or later ought to be OK at least.  But it’s nontrivial to fix.

QTBUG-51617 is about labelling the mouse events as synthesized so that you can at least easily know which mouse events to reject.  It should be fixed now on X11 (for 5.6.1), we just need to make sure the other platforms work consistently.  So you can have a mouse handler which ignores such events.

> TabletPress: pointer=1 pressure=0.4313725531101227 tilt=(0,0)
> MousePress
> TabletMove: pointer=1 pressure=0.45098039507865906 tilt=(0,0)
> MouseMove
> TabletMove: pointer=1 pressure=0.45098039507865906 tilt=(0,0)
> TabletMove: pointer=1 pressure=0.4470588266849518 tilt=(0,0)
> ...[ many MouseMove and many more TableMove ] ...
> TabletRelease: pointer=1 pressure=0.0 tilt=(0,0)
> MouseMove
> MouseRelease
> 
> At the level of QApplication I'm seeing TabletEnterProximity and TabletLeaveProximity events as you'd expect.

Cool.  Are you using an iPad Pro?  Otherwise it can’t detect proximity, but maybe it synthesizes those events when you press?