New approach to multitouch using DIDs and bitmasked events

[Chase Douglas]

On Tue, 2010-07-06 at 11:33 +1000, Peter Hutterer wrote:
> On Fri, Jul 02, 2010 at 09:09:48PM -0400, Chase Douglas wrote:
> > > This is a good start. Eventually, we should have DIDs exposed as a new 
> > > device class on the protocol though. Likewise, I don't think ET_DM 
> > > should not be converted to normal XIDeviceEvents but rather to their own 
> > > type that can be expanded in the future to accommodate for additional 
> > > features (e.g. the touch area).
> > 
> > I hacked some more today, and I'm not sure we need a separate XI2 event
> > type. I'm going to send out another RFC "pull request" with the work
> > soon, but as an overview I wanted to work on the "magicmouse" scenario.
> > The result is the following:
> > 
> > MT Touchscreen:
> >   - Device is marked as a DID
> >   - Starts out floating, can't be attached
> >   - MT touch coordinates are the first two valuators, XIDeviceEvents are
> >     in screen coordinates. However, an app can grab XIRawEvents for
> >     device coordinates. *
> >   - Since it's a DID, button events are also blocked for core, XI1
> > 
> > Magic Mouse:
> >   - Device is *not* marked as a DID
> >   - Starts out attached to virtual core master pointer
> >   - First two axes are REL_X, REL_Y
> >   - Device is marked as relative (for core and XI1 events)
> >   - Abs MT axes are marked as absolute
> >     * Can only be sent through XI2 since XI can't handle mixed mode
> >   - Since Abs MT touches are not the first two axes, they are not
> >     translated to screen coordinates
> >   - Abs MT axes sent through XI2 are also sent through attached master
> >   - Button events are not blocked for core, XI1
> > 
> > I am not aware of any use case that is not covered by the above scheme
> > and requires a new XI2 event type. If you know of any, please let me
> > know.
> 
> How will a client know if a device is a DID? XIQueryDevice must return this
> information.
> 
> One of the things I do also expect from DIDs is that the _first_ touchpoint
> is converted into core events. Thus, even on a multitouch device you can
> still interact in a traditional way with apps that won't support touch
> events  Hoping for Tk and others to be updated to support a new device type
> is a bit too optimistic. Worse, it introduces a hard transition between what
> we have now and what we can, i.e. you force everyone to update or otherwise
> they lose what they have. That's not good IMO, we need a transitional
> solution.
> 
> Anyway. My idea of DIDs was the following:
> the driver creates one master pointer device for a physical touchscreen.
> This master pointer device is controlled by the first touchpoint on the
> screen. At the same time, the driver creates DIDs on-demand as it is being
> used. Each DID stands for one touchpoint and sends DirectEvents or whatever
> they're called. Obviously, that gives you a duplication but the device
> hierarchy should reference which device is a DID device, thus MT aware apps
> can just ignore pointer events from that master (or not, in the case of the
> magic mouse, possibly need some flag here).
> 
> DIDs are short-lived and don't show up in the hierarchy, only the parent
> device does, indicating that DIDs may be present. Button or key events are
> all routed through the master pointer.
> 
> This is the approach that I think could work. though we won't know until we
> write down the spec. Something I recommend for your approach as well, sit
> down with XI2proto.txt and write the additions and how clients can use them.
> More often than not, you'll likely notice some issues and have to go back to
> the code.

I started to think about this, which led to the following:

1. Instead of allocating devices one at a time each time a touch begins
and then deallocating when the touch ends, lets just have a pool of
devices.
2. If we have a pool of devices and we have MTDev, why not index the
devices in the pool by ABS_MT_SLOT. If we encounter a higher slot than
we have allocated for, just allocate a new one on the spot.
3. If we have a pool of slotted devices, then what's the point of DIDs?
I was keying all my work off of the concepts Peter pointed out here:

   http://lists.x.org/archives/xorg-devel/2010-June/009757.html

   The arguments boiled down mostly to: input devices can't be transient
due to grab issues. But our touch devices aren't transient anymore.

I took a look at Carlos' evdev driver (mentioned in the above email),
and it matches rather closely to my thinking. The difference is that he
preallocates a defined amount of touch devices instead of using MTDev.

I also wonder why we can't just create a master pointer for each touch
point device? I can see where there could be some funkiness if the user
sees cursors for touch pointers (slots change between touches, so if you
dragged one cursor to a window, ended the touch, then touched again you
may see a different cursor be controlled). However, disabling the cursor
sprite for touch pointers would solve that issue. Otherwise, everything
should just work as well as MPX does today. (This could also be an evdev
option if people didn't want touches as MPX master pointers).

The main difference I see between the DIDs approach and Carlos' approach
is that the DIDs would explicitly disappear after the touch ends and a
new one would be created when a touch begins. Is there a necessity for
this behavior?

-- Chase