wl_tablet specification draft
Jason Ekstrand
jason at jlekstrand.net
Wed Jun 25 20:22:21 PDT 2014
On 06/25/2014 06:05 PM, Peter Hutterer wrote:
> On Wed, Jun 25, 2014 at 03:19:02PM +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.
> good point, thanks!
>
>>> - 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.
Is there a reason we are using an array rather than sending different
events? It's tempting to try and save something by using an array, but
extra events don't really cost anything significant. Also, once you use
an enum value in a public protocol, you can NEVER change it. In
particular, you can't change WL_TABLET_AXIS_CNT and you are locked into
only ever using those axes and never extending the protocol to include
more. I don't think this is what you intended.
Also, is there a reason why you have one axis event rather than
wl_tablet.position, wl_tablet.distance, wl_tablet.tilt, etc? Not that
having one axis event and an enum is bad, but it's something to think about.
>>>
>>> - 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.
> It's possible, look at this RFC from last september.
> http://lists.freedesktop.org/archives/wayland-devel/2013-September/011173.html
>
> The gist of that was that the wl_seat::get_tablet_manager request that
> returned a tablet manager object. that object was then the one that sent
> wl_tablet_manager::device_added events with the new tablet object. And that
> object had the wl_tablet interface as above (without the IDs).
>
> Lyude and I talked about this and couldn't decide which approach was the
> correct-er one for wayland :)
I'll second what pekka said. We already have a way of handling IDs and
agressively recycling them; it's called object IDs. I'm guessing the
IDs thing came from the wl_touch interface. There we have a wl_touch
object for the physical device and IDs for the different touchpoints.
Here we have one pen per physical device (I think).
>> 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?
> tablets control the cursor, so we definitely need to couple them
> with the seats somehow.
Does it control THE cursor or A cursor? I question whether we really
want it to be that intertwined with wl_pointer.
This brings up another point: tablet focus. How do you usually tell the
compositor what window you want to receive tablet input? For a tablet
with a screen, you can treat it more-or-less like a single-point
touch-screen, but what about tablets that aren't attached to a screen?
In the past, tablets frequently existed as a more natural way to move
the mouse around with a little extra meta-data for an app if it wanted
it. Is that the way they SHOULD work? That's a different question.
>> 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.
> good point, thanks.
>
>> Would clients need some information to differentiate between multiple
>> tablets in a human-friendly way?
> yes, we'll at least need to add PID/VID. From then on the clients can look
> up all they need in libwacom. I don't think we need much more than that.
>
>> Or should all clients always subscribe to all tablets?
> Yes, I think that's the sensible option. in 99% of the cases there is only
> one tablet anyway, sometimes there are 2 (laptops have wacom tablets
> built-in or a Cintiq+Intuos combo), I haven't seen a real-world setup with 3
> tablet yet.
>
> Cheers,
> Peter
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