Comments about adding tablet support to Wayland

[Lyude]

On Wed, 2014-06-25 at 11:06 +0400, Dmitry Kazakov wrote:
> Hi, all!
> 
> 
> I am a developer from Krita painting application team. We recently did
> quite much work on incorporating better tablet support in Krita. I
> have several comments about your proposal of the tablet protocol
> (sorry for nor replying directly, since I wasn't subscribed to the
> list before).

Benjamin commented on this as I was writing this e-mail, and I figured I
should too: yes, it's awesome to see developers commenting on the
protocol. A lot of the quirks around this protocol are going to be
difficult to see without the help of people who have programmed on the
client side of things as opposed to the compositor side of things. So
yes, your input is very much appreciated and I thank you for it!

> 
> 1) Axes. There should also be an axis for rotation of the stylus
> (Artpen) and Tangential Pressure (for the wheel of the Airbrush).
> 
> 2) There is also Artpen type of stylus. In Qt it is called "Rotational
> Stylus".

Don't worry, we haven't forgotten about these. These will eventually be
added into the protocol. The reason why they're not in this draft is
because I'm doing this as my Google Summer of Code project, and as such
I'm on a deadline and I have to focus on just getting the basics done
first before I can focus on all of the other features.

> 
> 3) Fingers. There is a complication in XInput2 right now, since touch
> enabled Wacom devices have a special Finger XInput2 device, which
> provides both interfaces: tablet and touch and therefore generates
> both types of events. Right now Qt5 still cannot handle it properly,
> but the work is in progress. From Krita point of view, the main
> usecase for us is to distinguish whether the user paints with a finger
> of with the stylus. Because most of the users prefer to disable
> painting with fingers and use it for gestures/UI only (yes, palm
> detection works with non-100% probability).

Right now libinput handles the finger device as another touchpad, since
that's usually what it is. Your use-case sounds perfectly valid though,
but IMO a better approach would be to add something to the protocol for
touchpads on wayland so that it can be known that they belong to a
tablet and provide any other sorts of data you might be need, so
programs like yours can treat them differently.

> 
> 4) Button Press/Release events should come in both cases: when the
> user clicks on the stylus' buttons and when the stylus touches the
> surface of the tablet.

I'm not entirely sure that's a good idea. If I'm reading this right, you
mean that additional button presses should be sent when the tool touches
the surface of the tablet. This could be done, but this would result in
us either having to:
- Send release events for all the currently held down buttons when the
stylus makes contact with the surface of the tablet, then immediately
send press events for all of the buttons or
- Just send redundant press events when the tool makes contact with the
surface

The issue with this is that it's very misleading to a wayland client as
to when the buttons are actually being pressed or not, and it doesn't
provide much in the way of extra useful information for the majority of
use cases.

> 
> 6) It might be a good idea to define the physical properties of the
> axes. E.g. for tilt, rotation and tangential pressure. Afair, Wacom
> driver for XInput returns some not-very-obvious values right now. One
> would need to experiment to know what these numbers mean.

We would all love for this to be the case I promise you, but
unfortunately it's not that simple for all of the axes. The distance
axis reports a seemingly meaningless value that can't be converted to
millimeters very easily. That being said, I have come up with a few ways
that we could actually convert it to millimeters, but this will have to
wait until I've fulfilled the goals for my Google Summer of Code Project
(unless anyone else wants to implement this in the mean time in libinput
of course).

I'll write up the method I've come up with for converting this wild
value to actual millimeters at some point when I get the chance.

As for the tilt axes of the tools, this is something that could be
represented in a more meaningful value. We could normalize it to a
number between 0 and 180°, so you can get the actual tilt in degrees as
opposed to what we currently have. This is something we could pull off
rather easily, I'll make sure to discuss this with my mentor tomorrow.

As for tangential pressure, this is a term I've never actually heard of
before. I don't know what the values from the tablet are supposed to
correspond to in regards to pressure, so I've added two of our friends
at Wacom to the CC list help us out (I hope you two don't mind!) on
this.
(Jason and Ping, if you guys aren't on the list already, the original
protocol this e-mail is discussing can be found here: 
http://lists.freedesktop.org/archives/wayland-devel/2014-June/015583.html
)

In regards to rotation and any other axes, I haven't had any contact
with these yet. So I can't really say much on them.

Cheers,
	Lyude
> 
> Thanks!
> 
> 
> -- 
> Dmitry Kazakov
> _______________________________________________
> wayland-devel mailing list
> wayland-devel at lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/wayland-devel

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 836 bytes
Desc: This is a digitally signed message part
URL: <http://lists.freedesktop.org/archives/wayland-devel/attachments/20140625/0df8c80e/attachment.sig>