New approach to multitouch using DIDs and bitmasked events

[Chase Douglas]

Hi all,

Last week I posted some patches for xf86-input-evdev to enable
multitouch through array-indexed valuators. However, I wasn't quite
satisfied with the results. First, depending on how many touch
properties your device presents there may be only enough room in the
valuators for five or six simultaneous touches. There are products
readily available that support more than this amount. Second, it still
sent touches as valuators of a device tied to a pointer. This seems ok
for a device like the Magic Mouse, but for most multitouch devices we
need to separate touches from X pointers due to the X pointer
architecture. One other small issue was that we have a nice new
optimized slotted events protocol from the kernel that doesn't send data
for touches and properties that haven't changed. However, in the evdev
patches we essentially undid all of that and passed all the properties
for every touch in every event.

To remedy the pointer architecture issues I turned to the concept of
direct input devices (DIDs). Peter Hutterer brought up the idea of DIDs
in an email here:

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

I have implemented this DIDs approach by creating a new internal event
type: ET_DirectMotion. These events are mostly just like ET_Motion
events, except they cannot be turned into core or XI1 events.

I heuristically determine if a device is a DID by checking whether it
has valuators for ABS_MT_POSITION_X or ABS_MT_POSITION_Y, but none of
ABS_X, ABS_Y, REL_X, or REL_Y. This check is performed at valuator class
device initialization. Any motion events that a DID posts are sent as
ET_DirectMotion events. The ET_DirectMotion events are transformed into
normal motion events before they are sent to an XI2 client, so XI2
clients will see touches as normal pointer motion events. Further, a DID
starts off as a floating device, unattached to any master pointer
device. Any attempt to attach it to a master will fail.

To remedy the protocol issues I looked at the bitmask valuator event
support of XI2. XI2 events send axis values alongside a bitmask
describing what values are valid for the event. However, the current X
input module API provides only for valuator ranges for events. I
extended the API by adding xf86Post*EventsM() and Get*EventsM()
functions. These functions take a "uint8_t *mask" argument instead of
the "int first_valuator" argument. The mask always starts at the 0th
valuator, and num_valuators is used to provide the position of the last
valid valuator for the event.

I updated the xf86-input-evdev module to use the new bitmask APIs and to
send each touch as its own event. I also added a small patch to xinput
for the test-xi2 command so it prints out the valuator index along with
all the valuator values (otherwise you have no clue what values
correspond to what properties).

All of these changes are available in my git repos on
http://kernel.ubuntu.com/git. I will be sending pull requests for
xserver, xf86-input-evdev, and a patch for xinput so people can find and
comment on the code. Also, I have uploaded versions of these packages to
the Ubuntu Maverick xorg-edgers/multitouch PPA if anyone wants to test
them out:

https://launchpad.net/~xorg-edgers/+archive/multitouch?field.series_filter=maverick

(They're still building as I send this...)

Thanks,

-- Chase