wl_tablet specification draft
Pekka Paalanen
ppaalanen at gmail.com
Wed Jun 25 23:48:37 PDT 2014
Hi,
it seems you forgot to reply-to-all, so I have re-added everyone and
not trimmed the quotation.
On Wed, 25 Jun 2014 18:37:08 -0400
Lyude <thatslyude at gmail.com> wrote:
> On Wed, 2014-06-25 at 15:19 +0300, Pekka Paalanen wrote:
> > On Tue, 24 Jun 2014 21:56:09 -0400
> > Chandler Paul <thatslyude at gmail.com> wrote:
> >
> > > Hello! As you all know I've been working on adding drawing tablet
> > > support to the Wayland protocol. Now that we've added support for
> > > tablets to libinput, the next step is writing the actual protocol that
> > > will be implemented by the compositor. Following this blurb is the
> > > current draft of the tablet protocol we have. Feel free to critique it.
> > >
> > > Cheers,
> > > Lyude
> > >
> > > ----------------------------------------------------------------------------
> > >
> > > wl_tablet specifications
> > >
> > > General notes:
> > > - Many of the axis values in this are normalized to either 0-65535 or
> > > (-65535)-65535. I would leave the axis values as-is since libinput reports
> > > them as doubles, but since we only have 8 bits of precision we'd lose way
> > > too many values. 65535 seemed like the best choice since it's the maximum
> > > length of a signed short, it's a whole number, it's way higher then the
> > > range of any of the axes (with the exception of X and Y, but these aren't
> > > normalized anyway) and we can do just about any basic arithmatic with it
> > > without having to worry about overflowing. plus, all we have to do is
> > > multiply the value by 65535 after we get it from libinput.
> > >
> > > Definitions:
> > > - WL_TABLET_AXIS_MAX = 65535
> > > - WL_TABLET_AXIS_MIN = (-65535)
> > >
> > > Enumerators:
> > > - wl_tablet_axis:
> > > - WL_TABLET_AXIS_X
> > > - WL_TABLET_AXIS_Y
> > > Represents the X and Y axes respectively. Only used in bitfields to
> > > indicate whether or not they've changed since the last event.
> > >
> > > - WL_TABLET_AXIS_DISTANCE
> > > Represents the distance axis on a tablet. Normalized from 0 to
> > > WL_TABLET_AXIS_MAX. For tablets that do not support reporting the
> > > distance, this will always be 0.
> > >
> > > - WL_TABLET_AXIS_PRESSURE
> > > Represents the pressure axis on a tablet. Normalized from 0 to
> > > WL_TABLET_AXIS_MAX. For tablets that do not support reporting the
> > > pressure, this will always be WL_TABLET_AXIS_MAX.
> > >
> > > - WL_TABLET_AXIS_TILT_VERTICAL
> > > - WL_TABLET_AXIS_TILT_HORIZONTAL
> > > Each represents the vertical and horizontal tilt axes respectfully.
> > > Normalized from WL_TABLET_AXIS_MIN to WL_TABLET_AXIS_MAX. For
> > > tablets that do not support this, this value will always be 0.
> > >
> > > - WL_TABLET_AXIS_CNT
> > > Represents the number of axes
> > > - wl_tablet_tool_type:
> > > - pen
> > > - eraser
> > > - brush
> > > - pencil
> > > - airbrush
> > > - finger
> > > - mouse
> > > - lens
> > > - wl_tablet_button_state
> > > - pressed
> > > - released
> > >
> > > Events:
> > > - proximity_in
> > > Sent when the tool comes into proximity above the client surface, either by
> > > the tool coming into proximity or a tool being in-proximity and moving to
> > > the client surface. If a tablet tool makes contact with the tablet at the
> > > same time that the tool comes into proximity, the proximity event comes
> > > first and the down event comes afterwards.
> > > Arguments:
> > > - Name: id
> > > Type: uint
> > > the id of the tablet sending this event.
> > >
> > > - Name: type
> > > Type: uint
> > > The type of tool that came into proximity, e.g. pen, airbrush, etc.
> > >
> > > - Name: serial
> > > Type: uint
> > > The serial number of the tool that came into proximity. On tablets
> > > where this isn't provided, this value will always be 0.
> > >
> > > - Name: x
> > > Type: fixed
> > > Surface relative x coordinate
> > >
> > > - Name: y
> > > Type: fixed
> > > Surface relative y coordinate
> > >
> > > - Name: surface
> > > Type: object
> > > Interface: wl_surface
> > > The current surface the tablet tool is over
> > >
> > > - Name: time
> > > Type: uint
> > > The time of the event with millisecond granularity.
> > >
> > > - Name: axes
> > > Type: array
> > > The current values of each of the tablet axes starting at
> > > WL_TABLET_AXIS_DISTANCE. The length of the array is equal to the
> > > number of axes that are reported. Any axes >= WL_TABLET_AXIS_CNT
> > > must be ignored. The size of the array remains fixed for the
> > > lifetime of the tablet.
> > >
> > > - proximity_out
> > > Send whenever the tool leaves the proximity of the tablet or moves out of
> > > the client surface. When the tool goes out of proximity, a set of button
> > > release events are sent before the initial proximity_out event for each
> > > button that was held down before the tablet tool left proximity. In
> > > addition, axis updates always come before a proximity-out event.
> > > Arguments:
> > > - Name: id
> > > Type: uint
> > > The id of the tablet sending this event.
> > >
> > > - Name: time
> > > Type: uint
> > > The time of the event with millisecond granularity.
> > >
> > > - axis
> > > Sent whenever an axis on the tool changes. This can include movement on the
> > > X and Y axis.
> > > Arguments:
> > > - Name: id
> > > Type: uint
> > > The id of the tablet sending this event.
> > >
> > > - Name: x
> > > Type: fixed
> > > Surface relative x coordinate
> > >
> > > - Name: y
> > > Type: fixed
> > > Surface relative y coordinate
> > >
> > > - Name: surface
> > > Type: object
> > > Interface: wl_surface
> > > The current surface the tablet tool is over
> >
> > How about using enter/leave events telling the client which wl_surface
> > the input device is targeting? That way you don't have to repeat the
> > wl_surface argument in every event
> I did originally have enter/leave events in the protocol but we ended up
> removing them because they end up being somewhat redundant. When you
> think about it, there's not much use in differentiating the two. Most
> clients are only really going to care about whether or not the tool is
> on their surface, not whether or not it's in proximity while it's on
> their surface. And of course when the tool is out of proximity on the
> surface, there's no useful data it can send to the tablet. In that case
> we might as well get rid of the proximity_(in/out) stuff and just use
> enter and leave to minimize the amount we have to add to the protocol. I
> also don't think including the surface in all of the enter/leave events
> is too big of a deal.
Making proximity_in/out work as enter/leave would be fine by me, if it
works for you. I'm just looking to reduce the amount of duplicated data
in the most heavily used events sent over the wire.
I only now realized that you can have several different tools on the
same tablet. You'd probably have per-tool enter/leave rather than
per-tablet, sort of depending on how (or if?) the tablet maps to the
screen.
Does the tablet map to the screen in a way that you can use the tablet
address any window on screen at any time like a touchscreen does, or
would it be more appropriate to have the whole tablet surface assigned
to "the current" window and let the client use the whole range of the
tablet instead of just the sub-region determined by its window on
screen? (That is probably a stupid question, but would have you
considered if the latter way had any solid benefits?)
> Of course though, if you know of uses where a client would want to know
> when the tool's gone out of proximity without leaving the surface and
> couldn't just settle with a single event for both, feel free to bring it
> up.
I have never used tablets, so I can't imagine real use cases. What
happens if you press a physical button on a tool that is out of
proximity? Should it be ignored completely, or should it still be
delivered to the app?
> > >
> > > - Name: time
> > > Type: uint
> > > The time of the event with millisecond granularity.
> > >
> > > - Name: changed_axes
> > > Type: bitfield
> > > Indicates which axes have changed.
> > >
> > > - Name: axes
> > > Type: array
> > > The current values of each of the tablet axes starting at
> > > WL_TABLET_AXIS_DISTANCE. The length of the array is equal to the
> > > number of axes that are reported. Any axes >= WL_TABLET_AXIS_CNT
> > > must be ignored.
> > >
> > > - button
> > > Sent whenever a button on the tool is pressed or released.
> > > Arguments:
> > > - Name: id
> > > Type: uint
> > > The id of the tablet sending this event.
> > >
> > > - Name: button
> > > Type: uint
> > > The button whose state has changed
> > >
> > > - Name: state
> > > Type: uint
> > > Whether the button was pressed or released
> > >
> > > - Name: time
> > > Type: uint
> > > The time of the event with millisecond granularity.
> > >
> > > - added
> > > Sent when a tablet device is added.
> > > Arguments:
> > > - Name: id
> > > Type: uint
> > > The id of the tablet sending this event.
> > >
> > > - removed
> > > Sent when the tablet device has been removed.
> > > Arguments:
> > > - Name: id
> > > Type: uint
> > > The id of the tablet sending this event.
> > >
> > > How tablet IDs are generated:
> > > Tablet IDs are aggressively recycled, e.g. we always try to get the lowest
> > > possible tablet ID. This means that the client can always assume that the ID
> > > number for a new tablet will never be greater then the number of tablets
> > > currently connected.
> >
> > Hi,
> >
> > using tablet IDs this way seems like this is a raw wire protocol, and
> > not object oriented like Wayland usually is.
> >
> > Could this be wrapped into an object like the following?
> >
> > Define interface "wl_tablet" which represents one device (I assume
> > aggregating makes no sense, just like with gamepads). Then you can drop
> > the "id" argument from all events and requests, and just use the
> > protocol object.
> >
> > I'm not sure how you want to advertise and create the wl_tablet objects.
> > How do tablet devices relate to wl_seat protocol objects?
> >
> > While the protocol is experimental, you could have your own (temporary)
> > global interface for advertising tablets, so that you don't need to
> > modify the Wayland core protocol (wl_seat). Or you could advertise each
> > tablet device as a separate global wl_tablet that a client can bind to.
> > The proper approach finally depends on the relation to wl_seat once you
> > are ready to set the protocol in stone.
> >
> > Would clients need some information to differentiate between multiple
> > tablets in a human-friendly way? Or should all clients always subscribe
> > to all tablets?
>
> This was brought up between Peter and I. Personally I'm in favor of the
> raw IDs, all the client really needs to be able to do is tell one tablet
> from another. And we already use IDs in a few different places, like
> with multitouch devices. His original paper had a manager for wl_tablets
> though that could be used in the way you're describing. Which brings up
> the question, what advantages do we get by using objects here as opposed
> to IDs? This is something I was hoping to get input on from the mailing
> list anyway.
In my other reply today, I understood the situation is a bit more
complex than I thought first. A tablet could be a protocol object, and a
tool could be described by an ID (if not also a protocol object!). Refer
to that email for more discussion there.
When you use protocol objects, the Wayland protocol library deals with
the ID allocations for you, and you do not need an extra argument in
the events/requests for your own ID. A protocol object can be associated
to "context" at the time it is created, like wl_tablet being part of
wl_seat, or a tool being part of wl_tablet if that makes sense. That way
you need to communicate only the object ID which Wayland does for you
already, and the association is implicit and known on both sides.
> As for being able to differentiate between tablets, I think the only
> real use case for that is debugger applications that need to filter out
> everything but a single tablet. So being able to do that isn't a huge
> concern IMO, most clients will either not want tablet events at all, or
> want them from all connected tablets.
Differentiating between tablets in the protocol (which is what you seem
to think about) happens preferably with object IDs. I was more thinking
about human-friendly information, like "this instance of wl_tablet is
your Blaa Blah device from Vendor", or any other metadata. Otherwise
there is no way for apps to meaningfully discriminate multiple wl_tablet
instances.
But I see Peter mentioned something about PID/VID and then getting the
metadata from another place based on that, so that's probably the thing
here.
Thanks,
pq
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