ABS_MT_POSITION_X and Y counted more than once when multi-touching

[Peter Hutterer]

Hi Leslie,

On Sat, Jul 27, 2013 at 11:16:53AM +0800, Leslie Zhai wrote:
> Hi xf86-input-evdev developers,
> 
> I use mtdev "read_mtdev.cpp" in test.tar.bz2
> https://www.dropbox.com/s/43jp0yl0avf9i3p/test.tar.bz2 to get
> ABS_MT_POSITION_X and ABS_MT_POSITION_Y sample "2s_zoom_in_sample.txt"
> in the same tarball for each ABS_MT_SLOT when multi-touching.

apologies, but I don't think you'll find many on this list who'll download
some random tarball and start debugging your program (mainly a time issue
too). So don't expect too much here.

> You can use scons http://www.scons.org to build the test case.
> "sample_slot.cpp" use Vtk http://www.vtk.org to render plot chart, you
> might need to install libvtk.

.. especially if it requires more than just a simple compile step.

 
> But ABS_MT_POSITION_X and Y counted more than once for each slot
> "sample_plot.cpp", the plot chart shown as "2slots_zoom_in.png".
> 
> When ./sample_plot 2s_zoom_in_sample.txt 0
> to only show slot[0] as "slot0_zoom_in.png", there are three blue lines,
> but instead should be only one.
> 
> When ./sample_plot 2s_zoom_in_sample.txt 1
> to only show slot[1] as "slot1_zoom_in.png", there are also three red
> lines, but instead should be only one too.
> 
> My question is whether or not I process the ABS_MT_POSITION_X and Y for
> EV_ABS motion event type in "m_process_absolute_motion_event" uncorrectly?
> I read the source code "xf86-input-evdev"
> http://cgit.freedesktop.org/xorg/driver/xf86-input-evdev/tree/src/evdev.c
> It mentioned that
> 
> |MT axes are counted twice - once as ABS_X (which the kernel keeps for
> backwards compatibility), once as ABS_MT_POSITION_X. So we need to keep a
> mapping of those axes to make sure we only count them once|

this is a X-specific issue. a device can only have one x and one y axis in
X, with the MT support it may have two (ABS_X and ABS_MT_POSITION_X). We
need to map them accordingly so that events from both are sent through the
same X axis (though we ignore ABS_X for MT devices).

it most certainly won't apply to your program - whatever it does. you can
just ignore the ABS_X/Y coordinates and got for the MT ones only. other
than that you most likely don't get more than one axis per slot, unless your
device is broken. note that once the slot is set, future events for this
slot won't be preceded by an ABS_MT_SLOT event. maybe that's what you're
seeing as duplicate events?

Cheers,
   Peter

 
> It means I have to filter or remove some _X and _Y from the sample ?
> HOWTO : other issue;
> 
> You can use scons http://www.scons.org to build the test case.
> "sample_slot.cpp" use Vtk http://www.vtk.org to render plot chart, you
> might need to install libvtk.
> 
> Many thanks!