wl_tablet specification draft

[Pekka Paalanen]

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.

> 
>         - 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?


Thanks,
pq