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Re: wayland tablet support redux

Hey Peter :),

On Fri, Apr 1, 2016 at 6:00 AM, Peter Hutterer <peter.hutterer@xxxxxxxxx> wrote:
> On Wed, Mar 30, 2016 at 04:46:43PM +0200, Carlos Garnacho wrote:
>> Hey all,
>> Now that 3.22 is open, and that the v1 of the wayland tablet protocol
>> is formalized, I'll be attempting to merge wip/wayland-tablet again.
>> Some random concepts:
>> =What's new in the branch=
>> The main new API feature is GdkDeviceTool, it (almost, more on this
>> ahead) univocally represents physical tablet tool (usually a pen),
>> these structs contain a GdkDeviceToolType saying the kind of tool it
>> is (pen, eraser, airbrush,...) and their serial number if available.
>> The idea behind this struct is that interested apps can do per-tool
>> tracking/configuration/etc, using a same tool will always result in
>> the same GdkDeviceTool pointer reported in events, etc. And the serial
>> number can be used to compose GSettings paths and whatnot.
>> The branch also resuscitates proximity events, they are a thing again,
>> and I added some kind of support for these for X11/XI2.
>> There's though low-range tablets that don't report serials (and most
>> usually, you can't use further/different tools than the one coming
>> with the device), in wayland it's the compositor which takes care of
>> creating the separate tool interfaces for those, in x11 we create
>> extra GdkDeviceTools with serial=0, which are shared for all devices
>> for all devices with these characteristics.
>> =...but we had already an eraser device?=
>> Yes, and in the branch we still do, all events from the eraser tool
>> will be sent with a GDK_SOURCE_ERASER device, and all other tools will
>> be always sent with a GDK_SOURCE_PEN device. That's kind of
>> unfortunate and something that'd be great to switch on 4.0.
> can you explain what exactly you mean here? IMO the various device types are
> a high-level approximation at best anyway and while we have it in libinput
> and the protocol, I'm not sure things like PENCIL/etc. need to be exported.
> if you can't use something like libwacom, you're pretty much lost anyway for
> the device type (short of pen, mouse, eraser).

Hmm, so you're saying that wp_tablet_tool.type is not completely
reliable per-se? This is mostly a note that the GdkDevice hierarchy
doesn't change wrt what we did earlier/on X11, where there are already
"pen" and "eraser" devices. Basically I'd expect this assert to pass
in an event handler:

GdkDeviceTool *tool = gdk_event_get_device_tool (event);
GdkDevice *device = gdk_event_get_source_device (event);

if (tool)
  g_assert ((gdk_device_tool_get_tool_type (tool) == GDK_DEVICE_TOOL_ERASER &&
             gdk_device_get_source (device) == GDK_SOURCE_ERASER) ||
            (gdk_device_tool_get_tool_type (tool) != GDK_DEVICE_TOOL_ERASER &&
             gdk_device_get_source (device) == GDK_SOURCE_PEN));

The typical app now roughly does:

if (gdk_device_get_source (gdk_event_get_source_device (event)) ==
  color = bg_color;
  color = fg_color;


I think a more future proof check would be looking up the tool, even
though that code will still work in the mean time.

>> Another option could have been going the other way around, and adding
>> extra GdkInputSources and devices. I don't think that option scales as
>> well.
>> =Relationship between tablets and tools=
>> The typical noncontroversial setups are those with one tablet+one
>> tool, or multiple tablets each with their own tool. Things complicate
>> soon though when you add multiple tools and multiple compatible
>> tablets, tools can then be used interchangeably across tablets.
>> This is one thing that IMO the wayland protocol got right, both
>> tablets and tools are announced by the seat, it is tools which do
>> "emit" the events, have axes, etc. And you can only relate a tool to a
>> tablet on proximity_in.
>> And this is something we kind of do too in gdk. In the end you
>> shouldn't expect that a given GdkDeviceTool will always be accompanied
>> with the same GdkDevice in events. The difference here is that in gdk
>> it is GdkDevices which have axes, "emit" events, etc. In order to keep
>> doing this, slave pen/eraser devices do change their axes in order to
>> resemble the tool in proximity_in, just like master devices currently
>> do in x11 on slave device switch. The end message is the same,
>> event->xxx.axes is only guaranteed to map correctly to the current
>> device axes at the time of dispatching the event.
>> This also affects the lifetime of objects, unplugging a tablet will
>> tear down the associated GdkDevices, but GdkDeviceTools may stay
>> indefinitely longer.
> I think it's also important to point out that in libinput (and thus by
> inflection the X11 driver and the compositor) create the tools on the first
> proximity in. This is a departure from the previous approach where the base
> set of tools, sufficient for 99% of the use-cases, were available as soon as
> the tablet was plugged in. This is not the case anymore, clients cannot
> rely on this anymore. I haven't looked yet how you solved this, but I
> suspect it's done in the straightforward manner (tool shows up, tool is no
> available in GTK :)

Right :). As a minor side note, this also means there's bumps ahead
with gnome-settings-daemon.

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