[PATCH weston 1/3] Introduce pointer lock interface

Jason Ekstrand jason at jlekstrand.net
Wed Sep 24 07:41:30 PDT 2014

On Wed, Sep 24, 2014 at 1:46 AM, Matthieu Gautier <dev at mgautier.fr> wrote:

>  Hi Jason,
> Well, I'll try to be more concise.
> In fact, there are two unrelated parts in my proposition :
> # First part : getting relative events
> Passing through a wl_pointer that may not exists to get relative events
> seems a bit odd to me.
> My main idea is to have a new object (I've call wl_relative) *not related*
> to wl_pointer that send to client the relative events.
> If a client want relative events, it open this object and receive events
> from it. Simply.
> No need to look the wl_pointer or anything else.
> The wl_pointer still continue to work as usual. There is strictly no
> change in that part.
> The only relation we may find between wl_relative and wl_pointer is that
> relative events are sent only when the wl_surface as the focus.
> In a first approximation, we can say that the wl_relative focus is the
> same of wl_pointer focus and that relative events are send only when
> wl_pointer.motion are send (ie, between wl_pointer.enter and
> wl_pointer.leave)
> But a special compositor could follow its own rules.

What is a relative input device?  How does the client know if/how it's
related to the wl_pointer?

The first question is the hard one.  A pointer is a relatively well-dfined
thing.  We've had mice for ~30 years now and everyone knows what to expect
from one.  They give you motion (sort-of relative), they have buttons (at
least one, but usually at least 3), they frequently have at least one axis
of relative motion that we call "scroll".  A mouse is a well-defined input
device and we know what to do with one.  We also know that, classically,
they move this little cursor around on the screen in a way that's defined
by the window system and is usually in absolute coordinates as far as
applications are concerned.

What is a relative input device?  It gives relative motion.  How many axes
does it have?  How should each axis be interpreted by applications?  What
kind of pseudo-physical model should we use to understand it.  (a mouse
cursor "moves" on the screen).  Does it have buttons?  How many?  What do
they do?  In order for the protocol to be useful for applications, all of
those questions need well-defined answers.  If it's just "gives relative
events" then we're right back to the horrific "remap your joystick for
every game" problem we had in the '90s only now you have to remap your
mouse too.  In fact, one of the things about mice that's great for game
devs is precisely that they always know what they're getting.

Along those lines, there was a protocol proposed for gamepads some time
ago.  (you could go dig it up on the list and read about it).  The basic
idea there was that most gamepad designs have more-or-less coalesced into
the XBox/PS form-factor of two sticks, a d-pad, 4 buttons, 2 or 4 triggers,
and a "start" buton.  You can go online and find dozens of different
companies shipping gamepads that all have this same basic layout.  The idea
was to then create a protocol based on that layout so that game developers
could have a common base to build on.  The reason I bring this up is that
the entire reason why the protocol was feasible at all is that the concept
of a gamepad has become reasonably well-defined to the point where you
could write a common protocol that works for most combinations of game and

As a small note, a classic mouse isn't really relative because the position
on screen sort-of corresponds to the position on the mouse pad.  Sure, it's
sloppy and subject to acceleration, but it's not truly relative like a
trackball.  Not that this really matters, but it's worth pointing out.

# Second part : pointer lock
> The pointer lock should not be related to relative events. This is two
> different parts.
> To acquire a pointer lock, a client just ask it.
> It can be by the request wl_pointer.get_lock(lock_type)    (I've called it
> set_mode in my past mail but get_lock is a better name)
> This request return a wl_callback used by the compositor to end the lock
> (either cause it refuse it or the lock normally ends)
> The lock can be released by the client using wl_pointer.release_lock()
> (former reset_mode)
> The lock_type can be :
> - lock : Do not move the pointer (What was in the Jonas' proposal)
> - confine : Confine the pointer into the surface.
> During a lock, all events are still send as usual :
> - Relative events from wl_relative are not impacted by the wl_pointer lock
> - wl_pointer.motion are still send in confine mode
> - wl_pointer button, axis events are still sent
> # Remarks and QA:
> The main difference compared to Jonas' proposal is that I totally separate
> the relative events problem from the pointer lock problem. (I maybe should
> have start by this :) )
> In Jonas' proposal you have to acquire the pointer lock to get relative
> events. In my proposition we can get one without having the other.
> I don't especially care if this is my proposal or Jonas' ones that is
> kept. But separate the two part in unrelated problems seems better to me.
> I've you give me times, I will send you a patch of wayland.xml to describe
> more precisely what I'm thinking about.
> It could be interesting that Jonas update its protocol xml with remarks
> from others to be sure that we compare up to date proposal and not miss
> anything.
> - Why not let the application open the device ?
>  => For security reasons. We probably don't want that any applications can
> open input devices and read from them all events.
>  => Scalability. Devices are not always mouses. Applications may not know
> how to read them.
>                        In my past job, I've made a android application
> that connect to the STB in Wifi and discuss directly with the compositor to
> send input events. There was no device to read from except a socket.
> - What my proposal can do that Jonas' one can't do ?
>  => The only thing I can find is the case where a seat doesn't have a
> wl_pointer but have relative events. For all the rest, Jonas' proposal
> seems to work.
>  => My proposition seems simpler to me. Less dependencies between
> elements, less questions about who create and release objects.  (But It's
> maybe also cause it's mine :) )
> Regards,
> Matthieu.
> Le 23/09/2014 23:42, Jason Ekstrand a écrit :
>  Hi Matthieu,
>  Could you please provide more explanation on what use-cases you are
> considering and why the current proposal fails to accomplish them?  All I
> was able to get from your post was the example of a TV remote.  It's all
> well and good to completely disagree with the proposed approach, but
> without a good reason as to why it doesn't work, it's kind of moot.
>  It's also worth noting the scope of the proposal.  The primary objective
> here is to provide a mechanism to take a device that could be providing
> relative events but which the compositor has turned into a pointer and get
> the relative events out of it.  It's not intended to be a generic system
> for getting relative events.
>  On that note, if you want to get a generic relative motion events, can't
> you just open the device and read them?  Sure we could cobble together a
> specification for how to read a bunch of buttons and relative events and
> then create a cursor from them.  I'm pretty sure it exists in the form of
> the USB HID spec.  How does reimplementing that in Wayland help us?  There
> was, at one point, a proposal for gamepads, but nothing has happened there
> in a while.
> On Tue, Sep 23, 2014 at 9:01 AM, Matthieu Gautier <dev at mgautier.fr> wrote:
>> Hello,
>> I'm pretty new into wayland and the discussion is relatively long, so I
>> may have missed arguments/constraints.
>> However I would like to share my point of view.
>> It seems to me that we are taking the problem the wrong way.
>> Relative motions exist as soon as there is a device generating them.
>> wl_pointer is just a particular interpretation of those events.
>> In fact, we may have a system where we have relative motion events but no
>> wl_pointer. Think about a smart tv with a remote control with
>> accelerator/gyroscope detectors.
>> This remote may behave as a mouse, generating relative motion events. But
>> the main interface of the TV may have no pointer. The interface should been
>> a set of icons and user move between them with the remote buttons.
>  If you're just pressing buttons to move between icons, then "pointer" is
> probably the wrong way to look at it anyway.  It's more "arrow keys" than a
> pointer.
> Exactly, there is no pointer in this example. But there are relative
> events applications may want to have.
>> In the same way, we may want that special applications still have access
>> to motion events:
>> - A web browser that will display itself the pointer (or activate
>> wl_pointer in the compositor)
>> - A video game
>> - Any application that want make gesture recognition.
>> In this context, wl_pointer is a special use-case of a shell and having a
>> mouse device doesn't imply having a pointer.
>> Relative motions should be always available (if there is a device) and
>> wl_pointer should be created on top of relative motions.
>> Trying to reduce the wl_pointer behavior to have the raw events seems to
>> me the contrary of what we have to do.
>> What I propose is :
>> - Having a way to get "relative input object" (lets call it wl_relative
>> for now) from wl_seat.
>> - Having a way to get a wl_pointer from the wl_seat at it is already the
>> case.
>> Relative events a sent to client if it is active (It is to the compositor
>> to decide this, as usual) whatever there is a wl_pointer or not.
>> The pointer lock interface will become some kind of "deactivate/configure
>> wl_pointer".
>> # Functionally :
>> A combination of :
>> - Hide the cursor (already available with wl_pointer.set_cursor)
>> - Don't not update wl_pointer position from relative events.
>> - Confine the pointer position into my wl_surface.
>> - Set wl_pointer at this position.
>> - A fps game will hide the cursor and deactivate update of wl_pointer
>> position.
>> - A strategy game will just confine the pointer.
>> - A application with a 3D view that what to rotate it when user drag the
>> mouse will just deactivate update of pointer position between button_down
>> and button_up.
>> - A application that just want relative motion events do nothing.
>> At any time, relative motion events are sent to client through the
>> wl_relative object. Regardless of the state of wl_pointer.
>> It is up to the client to handle events from wl_pointer or wl_relative
>> depending of which kind of information it wants.
>> # Interface :
>> The wl_pointer could gain two (four?) more requests :
>> - set_mode(mode, callback)
>> - reset_mode()
>> ( - has_mode
>> - get_mode )
>> The default mode is the mode we have for now (no special constraints)
>> A client can change the mode of a wl_pointer. It gets a callback.
>> When compositor stops the special mode (or refuse it) the done event of
>> the callback is sent.
>> When the client has finished with special mode, it sends the reset_mode
>> request.
>> The wl_pointer.leave event may or not be sent to client when the done
>> event is sent (The pointer may still being inside the wl_surface when
>> special mode ends)
>> However a wl_pointer.leave event implies a done event. (We cannot have a
>> special mode if we don't have the pointer focus)
>> On the interface to get the wl_relative object from seat, it depends :
>> Is there a possibility to have several cursor on one seat ?
>  One cursor per seat.  If there are multiple physical mice, then you get
> an aggregate of all the mice.  You can, however, have multiple seats in
> which case you will get multiple pointers.
>> (Two mouses moving two cursors)
>> Does wl_seat.get_cursor return always a proxy to the same object ?
>  Yes, all pointers created from the same seat are identical.
>> - If there is only one cursor, we can simply add a get_relative request
>> to wl_seat.
>> - If not, we should get a object from the other.
>>   . Ideally, get the wl_pointer from wl_relative. (and wl_relative from
>> wl_seat)
>>   . Practically, cause of the existent, get wl_relative from wl_pointer.
>> Regards,
>> Matthieu Gautier.
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