On 22 Mar 2016, at 20:18, Rutledge Shawn wrote:
On 22 Mar 2016, at 02:30, Alexei Gilchrist <te...@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.
It is running a daemon that handles communication with the corresponding
app on an iPad. One reason to believe it’s a QT bug is that Astropad
works fine with other applications such as Gimp, SketchbookEpress,
InKist, Photoshop, all respond to pressure variations so multiple
toolkits are able to interpret the synthesised events correctly.
You can enable the logging category qt.qpa.input.tablet to see how the
events come in (the code is in qnsview.mm).
Cool. Didn’t know that. I Need to figure out how to display the
logging events in pyqt as setting
export QT_LOGGING_RULES='qt.qpa.input=true'
or editing ~/.config/QtProject/qtlogging.ini is not reporting to the
console, maybe the python logging module which I’m using is taking
over stderr. Will play around and see if I can get some feedback.
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.
This is not much of a problem as a TabletPress is delivered before a
MousePress so it’s easy to toggle a pen_down variable and ignore mouse
events.
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.
Will take a look at those, it would be a more robust solution in case
the ordering of events should change.
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?
Yes, using an iPad pro. Astropad mirrors the mac screen to the iPad pro
and captures pencil and finger events synthesising corresponding events
on the mac. Effectively creating a Cintiq like tablet. I’m guessing
they add the proximity events for compatibility with apps that are coded
for a wacom tablet and looking for the hover (iPad pro does’t detect
pencil proximity).
When I first contacted the Astropad devs you could trick their code into
sending some sequence that QT transmitted with TabletMove events for one
stroke after doing a stroke on a wacom tablet. I pointed out the problem
and sent then a test app and got back:
I wanted to let you know that we found a workaround for this. It does
appear to be a bug in QT, but we figured out what event from Astropad
was confusing it. By removing this event (which doesn’t appear to be
necessary) QT once again works with Astropad.
Since then, they’ve updated their code and no TabletMove events are
generated at all. Not sure why they concluded that they’d created a
workaround, but they’re no longer responding to me pestering them :P
Alexei
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