X Gesture Extension protocol - draft proposal v1
Chase Douglas
chase.douglas at canonical.com
Wed Aug 18 07:29:20 PDT 2010
On Wed, 2010-08-18 at 16:19 +1000, Peter Hutterer wrote:
> First of all, thanks for your efforts and for sending them for public
> review, it is much appreciated. Took me a while to go through, but I hope
> the below is useful. I've got a couple of higher-level comments, and a set
> of specific ones in-line. I've read the various blog posts but in the end
> what matters is the spec here so I concentrate on that only.
> I'm gonna answer some of the bits out of context too, just as a warning.
Thanks a lot for a quick response!
> * What's not clear is how exactly this incorporates with the XI2.1 extension
> proposal I sent the draft out for earlier. Is this supposed to be parallel
> or exclusive, i.e will a set of touchpoints that generate a gesture still be
> sent as touchpoints to the client? or will they be converted and the raw
> touchpoint data thus discarded? Or is it to replace the other draft
> completely (I don't think so, given section 1)
First, I see multitouch and gestures as two separate concepts, even
though they're closely linked. Multitouch is defined by the ability to
send raw data about each touch wherever they are on the screen, while
gestures are a grouping of multitouch touches as a higher order event.
There are times when an application wants only multitouch events, and
when an application only wants gesture events. For example, google maps
may just want pan and zoom gestures, while inkscape only wants
multitouch events for drawing.
The strict dependency here is the event stream to the GE that I didn't
define as fully as I should have. The GE receives MT events through XI
2.1 as its raw input stream to perform recognition. It only makes sense
to me to send these events using the XI 2.1 protocol instead of defining
a new protocol just to send events to the GE.
> * If this supplements the XI2.1 MT proposal, how does gesture recognition work
> when there's a grab active on any device?
Good question. I'll admit that I haven't fully worked out this issue. In
Maverick we are actually siphoning off events inside the evdev input
module, so gestures would supersede input device grabs. However, my goal
is to move the gesture event handling inside the server where we could
reverse the ordering. If I had to make an educated guess as to what
should happen, I would say that an active grab should override gesture
support for the grabbed device.
> * What kind of event stream is sent to the gesture engine (GE)? it just says
> "stream of events" but doesn't specify anything more.
XI 2.1 events. I'll be sure to make that more clear in the next
revision.
> * Why does the GE need to be registered by the X server? Kristian
> asked that already and I don't think Section 3 answers this sufficiently.
I like the idea of a pluggable gesture engine. Gesture recognition is a
new field IMHO, and to assume that we know the best mechanism for
recognition seems a little naive :). I want people to be able to play
with recognition to see if they can come up with better algorithms
and/or better gestures. We allow this in the protocol by letting an X
client be the gesture engine.
To me, this is similar to the kernel having multiple memory allocators
and I/O schedulers. The first implementation of each has been thrown
away by now, and we have a good selection of interest approaches right
now. Enabling the same capabilities in X seems appropriate to me, and
having them exist outside of the X process space enables capabilities
like threaded recognition.
That's not to say that we would oppose inclusion of a GE inside X
itself, but having the GE be a separate client also means GEs can have
different licensing and not be tied directly to the X release schedule.
> I'll be upfront again and re-state what I've said in the past - gestures do
> IMO not belong into the X server, so consider my view biased in this regard.
> The X protocol is not the ideal vehicle for gesture recognition and
> inter-client communication, especially given it's total lack of knowledge of
> user and/or session.
Believe me, we tried our hardest not to throw all this into the X
server :). It's not that gesture recognition needs to be inside the
server. The issue is correct event propagation. Gestures occur in
specific regions of the screen, and as such they must be propagated with
full knowledge of the X window environment.
Say you have one application with a parent window and a child window.
Both windows select for MT input events. The application wants to
receive gesture events on the parent window. I then make a gesture with
some fingers in the parent window and some in the child window. Without
the gesture recognition and propagation inside the X server, some of the
input events would be sent to the parent window and some to the child
window. It becomes very difficult to assimilate all the data properly
for gestures if the raw inputs are spread out among various child
windows.
> I think the same goal could be achieved by having a daemon that communicates
> out-of-band with the applications to interpret gestures as needed, in the
> spirit of e.g. ibus.
I'm having a hard time finding any documentation on how ibus interacts
with X and applications for input. Can you provide any details on how it
works?
> In your reply to Kristian, you said :
> > Also, we think that there's a case to be made for environmental gestures
> > that should override gestures recognized by clients. This is provided by
> > the mutual exclusion flag when selecting for events. Again, this
> > wouldn't be possible without integrating it into the X propagation
> > mechanism.
> http://lists.freedesktop.org/archives/xorg-devel/2010-August/012045.html
>
> Unless I screwed up, the passive grab mechanism for touches allows you to
> intercept touchpoints and interpret them as gestures as required. The mutex
> mask described below is like a synchronised passive grab mask.
> The major difference here is that unlike a truly synchronised grab, the
> events are being continuously sent to the client until the replay is
> requested (called "flushing" here in this document). I think this
> requirement can be folded into the XI2.1 draft.
I think your idea would be useful for sending events to the gesture
engine and then replaying input events if no gestures are found. That
could simplify some of the GE part of this spec.
However, my reply to Kristian is more about a "passive grab" mechanism
for gestures themselves. This partly comes back to the fact that you
can't easily do proper gesture propagation outside of X, so you have to
be able to define a passive grab mechanism for gestures.
> > Gesture primitives comprise events such as pans, pinches, and rotatation.
>
> question: what are some more gestures that are not pan/swipe (horiz or
> vert), pinch/zoom and rotation? Can you think of any?
Tapping, which we support as well, but I don't think that's your real
point :).
> the problem with gestures is that there's a million of them, but at the same
> time there's only a handful that are "obvious". beyond that you're in
> unchartered territory and _highly_ context dependent (at this point I'd like
> to thank Daniel Wigdor for a highly interesting talk I recently attended
> that outlined these issues)
Yes, I agree. I have yet to think of any further gesture types that
would be obvious. However, I don't want to exclude the possibility of
recognizing other primitives at a later time.
If your question is geared more towards why we call these primitives
instead of just gestures, it comes from an idea we have that primitives
may be strung together at a high level (maybe a toolkit?) to have some
predefined meaning. For example, a DJ application may define a gesture
sequence as two finger down, release one finger, drag one finger, then
tap the second finger again. This gesture may be defined to have a
specific meaning when it occurs over a mixer level control. This is very
much a new idea though, it's not been implemented and tested yet.
> One example (though gestures are notoriously difficult to explain via
> email):
> I have a map application like google maps. I put two fingers down and move
> them apart. What does the app do? Zoom? No, what I'd like it to do is to
> move the two waypoints I touched to two different locations. But only the
> app that controls these objects can know that. The app needs to decide
> whether this particular event is to be a gesture or not.
>
> The current proposal allows for an all-or-nothing approach, so the app
> above cannot support this particular type of interaction or not supporting
> the X-provided gestures. A more flexible approach would be to have the app
> hand events to the GE, thus making gestures a last resort if no other
> meaningful interaction can be performed.
> (Exactly the inverse of the AllowEvents approach which only flushes the
> events once no gesture has been recognised)
I think the easiest answer to this question is to give an overview of
how this would work when the application is running inside the Unity
window manager:
Our overall idea is that three and four finger initiated gestures are
window manager/environment gestures, and two finger initiated gestures
are application gestures. The Unity window manager will listen for three
and four finger initiated gestures, and it will use these gestures for
window management and other high level environment use cases. Firefox
will listen for two finger initiated events.
Firefox will listen likely for two finger scroll on the entire page for
normal pages. When a web page requests for multitouch input in a given
region, like a google map, FF would create a new subwindow for it and
cut out the subwindow's input region from the web page parent window
using X Shape. The subwindow would then be free to listen to all one or
two finger XI 2.1 input events, or more fingers as long as the touches
are initiated with one or two fingers.
Of course, if FF only listened to XI 2.1 events instead of using X
Gesture, then this simplifies greatly.
To answer the last part of your thought, having the client hand events
to the gesture engine breaks down when gestures occur over multiple
windows that select for input events.
> > When a gesture engine recognizes a primitive that occurs within a window
> > with a client that selected to receive events for the primitive, a gesture
> > event is sent to the client.
>
> What happens if a group of touches represent a gesture but another group of
> touches present are not represented as part of a gesture? Use-case: I pinch
> to zoom while trying to rotate with another finger in the same app.
Multiple gestures can be performed at the same time, whether in one
window or many windows. The gesture engine can separate out input events
into separate groupings as it sees fit before performing gesture
recognition. If one grouping is a gesture but another isn't recognized,
then there may be gesture events and XI 2.1 events interleaved.
> > It is necessary for a gesture engine to recognize mutually exclusive gesture
> > primitives for each set of primitives defined for a given number of touches.
>
> In light of the often-used pinch and rotate gestures - how do you plan to
> handle this then? Require the user to lift the fingers before rotating the
> photo they just zoomed? I doubt so, but I don't quite know how this is
> supposed to work given the details here.
If possible, I would suggest giving what we have in Maverick a spin to
see how this works.
Say you have a picture on screen that you can drag, rotate, and resize
at will. You would listen for pan, pinch, and zoom gestures. When you
start touching the screen, you begin a series of gesture primitives. If
you pinch and then rotate, you'll see a pinch primitive start, update,
then end at the same time a rotate primitive starts. If you are
listening to all three of these primitives, you end up seeing a lot of
gesture primitive transitions over the lifetime of a gesture.
Perhaps I should give such an example in the protocol document?
> On Mon, Aug 16, 2010 at 11:13:20AM -0400, Chase Douglas wrote:
> >
> > The X Gesture Extension
> > Version 1.0
>
> 1.0 is optimistic if you haven't had any exposure yet, I recommend changing
> this to something pre 1.0, especially given the massive draft proposal
> warning just below.
My idea was that this protocol as proposed can change, but once people
agree to it and an implementation is available, it then becomes version
1.0 and no longer is a draft proposal. If it makes more sense to
yourself or others to call it 0.9.x until it's really implemented, then
we can switch to that scheme.
> > ********************************************************************************
> > ********************************************************************************
> > ************************** ***************************
> > ************************** DRAFT PROPOSAL (v1) ***************************
> > ************************** ***************************
> > ********************************************************************************
> > ********************************************************************************
> >
> >
> > 1. Introduction
> >
> > The X Gesture Extension is a mechanism to provide the following:
> > - Interface for X clients to register and receive gesture primitive events
> > - Interface for an X client to act as a gesture engine
> >
> > Gestures may be seen as logical groupings of multitouch input events. Thus,
> > this extension is dependent on the X Input Extension version 2.1, which
> > implements multitouch input support.
> >
> > ❧❧❧❧❧❧❧❧❧❧❧
> >
> > 2. Notations used in this document
>
> <skip>
>
> >
> > ❧❧❧❧❧❧❧❧❧❧❧
> > 3. Data types
> >
> > DEVICE { DEVICEID, AllDevices }
> > A DEVICE specifies either an X Input DEVICEID or AllDevices.
>
> AllMasterDevices is missing here, and in the other mentions below.
This is intentional. Gestures are tied to absolute input devices, and
properties are given in screen coordinates. Thus, you should only be
listening for input on individual devices themselves, not the aggregates
that are master devices.
I'm not 100% convinced of this approach though, it just feels right to
me. I'd be happy to add in master devices if it makes sense.
In Maverick, since we're siphoning events off inside the input module,
master input devices will never have gesture events.
> > GESTUREID { CARD16 }
> > A GESTUREID is a numerical ID for an X Gesture type currently available
> > in the server. The server may re-use a gesture ID if available gesture
> > types change.
> >
> > GESTUREFLAG { MUTEX }
> > A flag telling the server to not propagate gestures to child clients
> > when a gesture type in the associated mask is set. The gesture will only
> > be sent to the registering client.
> > When registering for gestures, a client using this flag may not listen
> > for gesture IDs that any other client has registered for with the MUTEX
> > flag.
> >
> > GESTUREMASK
> > A GESTUREMASK is a binary mask defined as (1 << gesture ID). A
> > SETofGESTUREMASK is a binary OR of zero or more GESTUREMASK.
> >
> > GESTUREPROP { property: ATOM
> > property_type: ATOM }
> > A GESTUREPROP is the definition of a single property of a gesture. The
> > property field specifies a label for the gesture. The property_type
> > field specifies the data type of the property. For example, the property
> > type may be the atom representing "FLOAT" for an IEEE 754 32-bit
> > representation of a floating point number. Where applicable, both the
> > property and the type should conform to the standard gesture
> > definitions.
> >
> > EVENTRANGE { start: CARD16
> > end: CARD16 }
> > An EVENTRANGE specifies a range of event sequence numbers, inclusively.
> >
> > ❧❧❧❧❧❧❧❧❧❧❧
> > 3. Gesture Primitives and Events
>
> btw, two sections 3 here.
Heh, whoops :).
> > The XInput protocol through version 2.0 supports single touch input devices.
> > Beginning with version 2.1, XInput will support multitouch input devices. The
> > protocol extension provides a mechansim for selection and propagation of
> > multitouch events to windows they occur within. The selection and propagation
> > is performed per touch; if a user touches a parent window and a child window
> > with separate touches simultaneously, and both windows have input events
> > selected for, the touch events will be sent to their respective windows. This
> > functionality is useful for general multitouch events, but is ill-suited for
> > user interface paradigms that require groupings of touches to be interpreted as
> > a single unit.
> >
> > The X Gesture extension aims to provide for selection and propagation of gesture
> > primitives. Gesture primitives can be best thought of as fundamental multitouch
> > input groupings that convey specific meaning. Gesture primitives comprise events
> > such as pans, pinches, and rotatation.
> ^ typo, "rotation"
Yep, Tiago caught that yesterday :)
> > Primitives are also defined by the number
> > of touches that comprise them: two finger pans and three finger pans are separate
> > primitives.
> >
> > 3.1 Gesture Event Selection and Propagation
> >
> > The mechanism for gesture selection and propagation follows the typical X window
> > event model. An X client requests for gesture events for primitives it selects
> > for on a given window. When a gesture engine recognizes a primitive that occurs
> > within a window with a client that selected to receive events for the primitive,
> > a gesture event is sent to the client.
> >
> > Proper selection and propagation of new gesture events depends on a few
> > variables: where each touch event occurs, which clients requested for a
> > recognized primitive, and if any clients hold primitive mutual exclusion for the
> > window. There are two gesture primitive selections a client can make: initiation
> > and continuation selections.
> >
> > 3.1.1 Gesture Life
> >
> > The lifetime of a gesture is comprised of its beginning, continuation, and
> > ending. During this time, a gesture may change primitive forms. For example, a
> > user may initiate a gesture with a three finger pan, but then continue the same
> > gesture by lifting one of the fingers. When a client makes a gesture event
> > selection request, the initiation selection is used to determine which clients
> > may begin receiving a newly recognized gesture. The continuation selection is
> > used to determine whether a gesture continues or ends when the primitive form
> > changes. Thus, a gesture may continue even if the number of fingers or the type
> > of gesture primitve changes.
> >
> > 3.1.2 Gesture Mutual Exclusion
> >
> > Clients can request to receive gesture events on any windows, including windows
> > it does not own. This level of control provides for many use cases, but it
> > limits the ability of gestures to be controlled in an environmental manner.
>
> I think I know what you mean here but "gestures that apply to the whole
> desktop" may be a better way to put it. the term "environmental gestures" is
> confusing, to me anyway.
That's probably a better wording. I'll fix it up before the next
revision.
> > For example, a window manager may wish to define gestures that should
> > always be available to a user, regardless of what other clients select
> > for.
>
> > The X Gesture extension provides a mechanism for mutual exclusion of gestures.
> > When a client registers for a selection of gestures and uses the MUTEX flag in
> > the request, it may be granted mutual exclusion for the selected gestures on the
> > window and all its children. At any given moment, only one client may select for
> > mutual exclusion on each window. Any gesture occurring within the window and its
> > children that matches a mutually exclusive client gesture initiation selection
> > will only be sent to the mutually exclusive client. Further, mutually exclusive
> > gesture initiation selections of parent windows are given priority over their
> > child windows. Note that mutually exclusive gesture propagation is in opposite
> > priority ordering from non-mutually exclusive gesture propagation.
>
> If I understand this correctly, this is the same principle we use for
> passive grabs, is this correct? If so, I recommend chosing the same wording
> for consistency with other specs.
At first I didn't think it was. Henrik Rydberg questioned me on it
yesterday too, and after thinking a bit I agree with both of you :).
I'll fix this up for the next revision.
<skip>
> > 5.1 Gesture Engine Operation
> >
> > Once a gesture engine is registered, it will begin receiving a stream of events
> > from the X server. The events are buffered inside the server until a request by
> > the engine is received with instructions for how to handle the events.
> >
> > When the engine recognizes a gesture primitive, it sends a gesture event to the
> > server with the set of input event sequence numbers that comprise the gesture
> > primitive. The server then selects and propagates the gesture event to clients.
> > If clients were selected for propagation, the input events comprising the
> > gesture primitive are discarded. Otherwise, the input events are released to
> > propagate through to clients as normal XInput events.
> >
> > When the engine does not recognize any gesture primitive for a set of input
> > events, it sends a request to the server to release the input events to
> > propagate through to clients as normal XInput events.
> >
> > The server may set a timeout for receiving requests from the gesture engine. If
> > no request from the engine is received within the timeout period, the server may
> > release input events to propagate through to clients as normal XInput events.
> > The timeout value is implementation specific.
>
> any guesses as to what this timeout may be? do we have any data on how long
> the average gesture takes to be recognised?
Currently we do all the event processing in the same thread as the rest
of X, and I think there's minimal latency such that it doesn't impact
performance. The gesture recognition code is only a few hundred lines.
Since we haven't split it out into a separate client yet, we haven't had
to deal with acceptable GE latency. I think there's some research that
says any UI latency above 100 ms can become an issue, so perhaps a
timeout value based on that would be a good starting point?
> As I read this at the moment, this approach means that _any_ touch event is
> delayed until the server gets the ok from the GE. The passive grab approach
> for gesture recognition also means that any event is delayed if there is at
> least one client that wants gestures on the root window. What's the impact
> on the UI here?
I don't think I understand :). The gesture engine does its recognition
and then hands gesture events off to the server or tells the server to
allow XI 2.1 events it's queued up. At this point, it's just a matter of
propagation and selection. Maybe your argument is that you have to check
the full lineage from the child window to the root window to find if
anyone is listening for the gesture event, but that shouldn't take very
long.
> > ❧❧❧❧❧❧❧❧❧❧❧
> > 6. Errors
> >
> > Errors are sent using core X error reports.
> >
> > GestureEngineRegistered
> > A client attempts to register a gesture engine while an engine is
> > already registered.
>
> This error isn't really necessary, you might as well return BadMatch from
> that one request that may generate the error. It's the semantics that count
> anyway.
Yes, we can probably get away without defining any gesture extension
specific errors. I'll fix this up for the next revision.
> > ❧❧❧❧❧❧❧❧❧❧❧
> > 7. Requests:
> <skip>
>
> > 7.1 General Client Requests
> >
> > ┌───
> > GestureQueryVersion
>
> <skip>
>
> > ┌───
> > GestureQueryAvailableGestures
>
> ListGestures would be more explanatory
Agreed. I'll fix this up for the next revision.
> > ▶
> > num_gestures: CARD16
> > gestures: LISTofGESTUREMAP
> > └───
> >
> > GESTUREMAP { gesture_id: GESTUREID
> > gesture_type: ATOM }
> >
> > GestureQueryAvailableGestures details information about the gesture types
> > that are recognizeable by the gesture recognizer registered with the server.
> >
> > Each gesture is detailed as follows:
> > gesture_id
> > The unique ID of the gesture. Gesture IDs may be re-used when the
> > available gestures to be recognized changes.
> > gesture_type
> > An ATOM specifying the type of gesture. The gesture type should conform
> > to the list of standard gesture types if applicable.
>
> Why do we need id and type? Can there be more ID's of one type? The
> QueryProperties request doesn't allow for per-id selection, so the ID seems
> superfluous, it might as well be just the type, right.
Type should be a universally global identifier. Any gesture engine that
can recognize a two-finger pinch should use the same type, "2-Pinch"
perhaps. A thought of mine is to publish a standard for gesture types
which could be extended as more useful primitives are defined, but maybe
there really are only four "obvious" gestures in this world :).
When a GE registers with the server, the type is assigned to an ID,
which is also used as the bitmask position for selecting events. I think
I can make that clearer in a second revision.
> >
> > ┌───
> > GestureListenForGestureChanges
> > listen: CARD8
> > └───
>
> tbh I got confused by the description below and I'm not sure what it
> actually does. though if I understand it correctly, this request should be
> folded into the SelectEvents request.
This could be folded into the SelectEvents request. I'll fix that up for
the next revision.
> > GestureListenForGestureChanges registers the client to be sent a
> > GesturesChanged event.
> >
> > listen
> > Inequal to 0 if the client wants to receive GestureChanged events
>
> I don't quite see why this is necessary. If a client doesn't care, let it
> not register the event mask, otherwise just send the event and let the
> client ignore it. A simple bit in SelectEvents is enough here.
I want a client to be able to connect while a GE is unavailable, but
then receive an event when a GE registers so it knows it can begin
listening for gestures. One way we could do this is to define the
GestureChanged event as one of the events in the bitmask as you
suggested.
> >
> > On receipt of a GestureChanged event, the client may send the
> > GestureQueryAvailableGestures request to receive the new list of available
> > gestures.
> > Note that all clients listening for any gesture events on any window will
> > receive a GestureChanged event regardless of whether they have called this
> > request with any value for listen. However, the server will honor the last
> > listen value sent in this request whenever the client is not listening for
> > gesture events.
>
> I'm confused...
Hopefully my comments above help sort out the issue :).
> > ┌───
> > GestureQueryGestureProperties
> > gesture_type: ATOM
> > ▶
> > num_properties: CARD16
> > properties: ListofGESTUREPROP
> > └───
> >
> > GestureQueryGestureProperties details properties of the requested gesture
> > type.
> >
> > ┌───
> > GestureSelectEvents
> > window: Window
> > device_id: CARD16
> > flags: SETofGESTUREFLAG
> > mask_len: CARD16
> > init_mask: GESTUREMASK
> > cont_mask: GESTUREMASK
> > └───
> >
> > window
> > The window to select the events on.
> > device_id
> > Numerical device ID, or AllDevices.
> > flags
> > Flags that may affect gesture recognition, selection, or propagation.
> > mask_len
> > Length of mask in 4 byte units.
> > init_mask
> > Gesture mask for initiation. A gesture mask for an event type T is
> > defined as (1 << T).
>
> Don't do this (1 << Y) thing, this wasn't one of the smarter decisions in
> XI2. Simply define the masks as they are, don't bind them to event types.
> Though it hasn't become a problem yet, I already ran into a few proposals
> where this would either be too inflexible or would create holes in the mask
> sets (latter not really a problem, but...).
This was lazy copy and paste from me :). The proposal should read:
init_mask
Gesture mask for initiation. A gesture mask for an event ID I is
defined as (1 << I).
Does the distinction between types and IDs resolve your issue, or are
you referring to some other issue?
> > cont_mask
> > Gesture mask for continuation. A gesture mask for an event type T is
> > defined as (1 << T).
> >
> > GestureSelectEvents selects for gesture events on window.
> >
> > The mask sets the (and overwrites a previous) gesture event mask for the
> > DEVICE specified through device_id. The device AllDevices is treated as a
> > separate device by server. A client's gesture mask is the union of
> > AllDevices and the per-device gesture mask.
>
> I'd add a reference to the XI2 definition of AllDevices and AllMasterDevices
> event mask handling here to avoid duplicating (and possibly accidentally
> changing) the definition
Alright. I'll fix it up for the next revision.
> > The removal of a device from the server unsets the gesture masks for the
> > device. If a gesture mask is set for AllDevices, the gesture mask is not
> > cleared on device removal and affects all future devices.
> >
> > If mask_len is 0, the gesture mask for the given device is cleared. However,
> > a client requesting for mutual exclusion may register for any valid mask_len
> > length of mask with all bits set to 0. This allows a mutual exclusion client
> > to prohibit any other client from gaining exclusive privilege.
> >
> > ┌───
> > GestureGetSelectedEvents
>
> <skip>
>
> > ┌───
> > GestureGetAllSelectedEvents
> > window: Window
> > ▶
> > num_masks: CARD8
> > masks: LISTofCLIENTEVENTMASK
> > └───
> >
> > CLIENTEVENTMASK { client_id: CLIENT,
> > device_id: DEVICE,
> > mask_len: CARD8,
> > init_mask: GESTUREMASK
> > cont_mask: GESTUREMASK }
> >
> <skip>
> >
> > GestureGetAllSelectedEvents retrieves the gesture selections for all clients
> > on the given window.
>
> Is there a specific need for this request?
One idea is to make a legacy translation application. It is a client of
the server that listens for gestures such as pinch and pan to do zooming
and scrolling. It would then translate the gesture events into key and
button events and send them to clients using XTest. Note that this is
useful only until toolkits start listening to gestures to interpret them
properly.
In order to do this correctly, the application needs to only listen on
windows that another client is not already listening for gestures on.
This request provides a way to determine if any other clients are
listening already.
I tried to find out how a client could get the XI 2 event selection mask
of all clients, but I couldn't find any mechanism. If there is a
mechanism, let me know and I'll take a look to see if it would work here
as well.
> > 7.2 Gesture engine client requests
> >
> > ┌───
> > GestureEngineRegister
> > num_gestures: CARD16
> > gestures: LISTofGESTUREINFO
> > └───
> >
> > GESTUREINFO { gesture_type: ATOM,
> > num_properties: CARD16,
> > properties: LISTofGESTUREPROP }
> >
> > GestureEngineRegister is the mechanism by which a gesture engine registers
> > with the X Gesture extension to be able to process gesture events. Only one
> > gesture engine may be registered to the server at any given time. Further
> > registration requests will cause a GestureEngineRegistered error.
> > When the gesture engine is registered successfully, a GesturesChanged event
> > is sent to all clients registered to listen for the event. The clients may
> > then request the new list of available gestures from the server.
> >
> > ┌───
> > GestureEngineUnregister
> > └───
> >
> > GestureEngineUnregister unregisters the gesture engine from the server. If
> > the client has not registered a gesture engine successfully through the
> > GestureEngineRegister request, a BadValue error will result. Otherwise, a
> > GesturesChanged event will be sent to all clients registered to listen for
> > the event.
> >
> > ┌───
> > GestureAllowInputEvents
> > num_ranges: CARD16
> > ranges: LISTofEVENTRANGE
> > └───
> >
> > GestureAllowInputEvents instructs the server to flush the input events to
> > clients unmodified. This is used when no gestures are recognized from
> > sequences of input events.
> > If any of the EVENTRANGE values are invalid, the BadValue error is reported
> > and no input events are flushed.
> >
> > ┌───
> > GestureRecognized
> > num_ranges: CARD16
> > ranges: LISTofEVENTRANGE
>
> sequence numbers are not suited for this type of range. the sequence number
> set in the EVENTHEADER only increments if additional requests are processed.
> For clients that purely listen to events and e.g. dump them into a file, the
> sequence number does not change after the first set of requests.
Hmmm. I guess some other mechanism will be needed. I'll have to think
about this. Hopefully your suggestion of using active grabbing and
replaying events could work instead, making this issue moot.
> > gesture_id: CARD16
> > gesture_instance: CARD16
> > device_id: CARD16
> > root_x: Float
> > root_y: Float
> > event_x: Float
> > event_y: Float
>
> probably better to use the same type as in the XI2 spec.
I want to provide a protocol using XCB, and I couldn't figure out an
easy way to do so with an FP1616 type. If there's a way, then that would
be fine with me. If not, which would be easier? Fixing up XCB to provide
a way or just using IEEE 754 floats instead?
Admittedly, I didn't spend a large amount of time looking for an FP1616
solution in XCB since I don't understand the appeal of FP1616 :).
> > status: STATUS
> > num_touches: CARD8
> > num_properties: CARD8
> > touches: TOUCHES
> > properties: LISTofCARD32
> > └───
> >
> > See the GestureNotify event definition below for gesture data definitions.
> >
> > GestureRecognized instructs the server to perform propagation and selection
> > for a potential GestureNotify event to be sent to clients. If clients are
> > selected to receive events, the input events specified in the event ranges
> > must be discarded by the server. Otherwise, the input events must be
> > propagated to clients as defined by the XInput protocol.
> >
> > 8. Events:
>
> <skip>
>
> > The following event types are available in X Gesture.
> >
> > GesturesChanged
> > GestureNotify
> >
> > All events have a set of common fields specified as EVENTHEADER.
> >
> > EVENTHEADER { type: BYTE
> > extension: BYTE
> > sequenceNumber: CARD16
> > length: CARD32
> > evtype: CARD16 }
> >
> > type
> > Always GenericEvent.
> > extension
> > Always the X Gesture extension offset.
> > sequenceNumber
> > Sequence number of last request processed by the server.
> > length
> > Length in 4-byte units after the initial 32 bytes.
> > evtype
> > X Gesture-specific event type.
> >
> >
> > ┌───
> > GesturesChanged:
> > EVENTHEADER
> > └───
> >
> > A GesturesChanged event is sent whenever the gestures available from the
> > server have changed. A client receiving this event may choose to request the
> > new list of available gestures from the server.
> >
> > ┌───
> > GestureNotify:
> > EVENTHEADER
> > gesture_id: CARD16
> > gesture_instance: CARD16
> > device_id: CARD16
> > time: Time
> > root: Window
> > event: Window
> > child: Window
> > root_x: Float
> > root_y: Float
> > event_x: Float
> > event_y: Float
> > status: STATUS
> > num_touches: CARD8
> > num_properties: CARD8
> > touches: TOUCHES
> > properties: LISTofCARD32
> > └───
> >
> > STATUS { Begin, Update, Continue }
> >
> > TOUCHES { x: FLOAT
> > y: FLOAT
> > valuators_len CARD16
> > valuators LISTofFP3232 }
> >
> > x
> > y
> > Location of touch in screen coordinates.
> > valuators_len
> > Number of values in axisvalues.
> > axisvalues
>
> need to pick one of axisvalues or valuators.
Yep, I'll fix that in the next revision.
> > Values of all valuators for the touch. Valuators are defined in the
> > XInput protocol specification. The specific meaning of each valuator is
> > specific to the input device.
> >
> > A GestureNotify event is generated whenever a gesture occurs in a window for
> > which the client has requested the gesture ID.
> >
> > gesture_id
> > Gesture ID of the gesture type.
> > gesture_instance
> > Unique ID of this gesture instance. The ID will be maintained as the
> > gesture progresses from start to end as signified in the status field.
> > This value is monotonically increased by the server for every gesture
> > causing events to be sent to a client. The ID will only be reused once
> > a 32-bit wrap occurs.
> > device_id
> > X Input device ID of the slave device generating this gesture.
>
> why the slave device? why not use device_id and source_id?
For the same reason I didn't provide for all master devices when
selecting for events. If there's a good reason for all master device
selection, then the device_id and source_id would be warranted.
> I think that's all I have for now, but I guess more will come up in the
> discussion. Thanks again for this draft!
I really appreciate the time and effort you put into this review. Thanks
a lot!
As you said earlier, you're biased against putting gesture stuff inside
X. I admit that I had the exact same bias at first too. However, I
couldn't find any mechanism to handle event propagation and selection
properly without embedding the logic within X itself. If you can come up
with some scheme that fulfills the needs outlined here and sits outside
of X, we'd be quite happy to scrap this proposal and move on :).
If it would help, perhaps we should start a new thread just on the
question of whether gestures should propagate through X or if there's
some better solution outside of X.
Thanks,
-- Chase
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