Message-ID: <CAGS+omDzg-xADYxKQmWy1KRhJ=n9pn9XR_HMc01xqCSDSLa5UA@mail.gmail.com>
Date:	Thu, 7 Jul 2011 00:41:26 +0800
From:	Daniel Kurtz <djkurtz@...omium.org>
To:	Henrik Rydberg <rydberg@...omail.se>
Cc:	dmitry.torokhov@...il.com, chase.douglas@...onical.com,
	rubini@...l.unipv.it, linux-input@...r.kernel.org,
	linux-kernel@...r.kernel.org, derek.foreman@...labora.co.uk,
	daniel.stone@...labora.co.uk, olofj@...omium.org
Subject: Re: [PATCH 09/12] Input: synaptics - add image sensor support

On Wed, Jul 6, 2011 at 3:27 AM, Henrik Rydberg <rydberg@...omail.se> wrote:
>> ...
>> This implementation addresses the following issues:
>>
>>   (1) Improves handling of the 2-finger case.  Image sensors due allow
>> true MT-B for two finger case.  This, for example, allows detecting
>> whether the click in a click+drag is happening in bottom right or
>> bottom left quadrant of the pad.
>
> In principle, this could be done within the semi-mt realm by adding a
> binary value to the protocol, distinguishing the diagonals of the
> bounding box. It would be ugly too, but at least it would be
> compatible with the current semi-mt effort in userspace.

(A) I think the "semi-mt" protocol works like this:
  (1) When two or more fingers are on the pad, report two MT-B slots, where:
    (a) slot[0] contains (x_min, y_min) of the two fingers
    (b) slot[1] contains (x_max, y_max) of the two fingers
    (c) Total number of fingers on pad is indicated by EV_KEY/BTN_TOOL_* is set.
  (2) If one of the two fingers is removed, then the first slot is
used to report
      the remaining finger, whichever it is, without changing the
slot's tracking_id.
  (3) Advertise this behavior with the SEMI_MT device property

(B) This is what the proposed T5R2 driver does:
  (1) When N fingers are on the pad, report N MT-B slots, where:
    (a) The lowest indexed active slot (tracking_id != -1) contains
valid finger position data.
    (b) The highest indexed active slot (tracking_id != -1) contains
valid finger position data.
    (c) Any active slots (tracking_id != -1) between these does not
contain valid finger position data, these 'hidden' slots are just
active to indicate that there are that many fingers on the pad.
    (d) Total number of fingers on the pad is always the number of active slots.
  (2) When fingers are removed from the pad:
    (a) keep reporting the fingers that are left in the same slots,
with the same tracking_ids, if possible.
    (b) If the identities of the fingers that remain is ambiguous,
invalidate all slots,then re-assign the fingers that remain to new
slots, with new tracking ids.
  (3) Advertise this behavior with the T5R2 device property

(C) A third, compromise implementation might be to do something like this:
  (1) When >=2 fingers are on the pad, report 2 MT-B slots, where:
    (a) slot[0] reports finger 1
    (b) slot[1] reports finger 2
    (c) Total number of fingers on pad is indicated by EV_KEY/BTN_TOOL_* is set.
  (2) If 1 finger is removed, the tracking_id for its slot is invalidated;
      but, the finger that remains stays in its same slot with the
same tracking_id.
  (3) Whenever the number of fingers changes from/to 3 or more
fingers, both slots are always invalidated.
      New tracking_ids are assigned to both slots which contain the
two fingers now being reported.
  (4) No special driver property is required for this, not even "semi-mt".
      It should be as pure MT-B as we can get (plus BTN_TOOL_* hack).


IMHO, (C) is an improvement over (A) for two major reasons:
  (1) tracking the individual touches allows distinguishing a clicking
finger in bottom right versus bottom left, for example.
  (2) It makes it possible to track the touch that remains when one of
the two touches is released.
      The touch that remains will stay in the same slot, with the same
tracking id.
      The tracking id of the touch that is removed will be set to -1.
        => This eliminates unnecessary gaps in the event stream,
without introducing position jumps.
      This behavior occurs often when switching between 1-finger
motion and 2f-scroll, or when performing 2f-click+drag gestures.

Note: Tracking which fingers are left on the pad gets much more
complicated with 3, 4, or 5 fingers.
This is addressed more fully by the T5R2 solution, but (C), while
slightly less optimal, should at least not do the wrong thing.

Sorry, I find it a bit difficult to explain this clearly in writing.
I guess I'll just have to submit a new patchset that shows what I mean.


>> A bare minimum patchset might do:
>>    (1) Do true MT-B for two fingers
>>    (2) Improved number-of-finger tracking for just the 2-finger case.
>> (Keep tracking finger 1 when finger 2 is removed, and vice versa)
>
> In what way is this different from 1)?
>
>>    (3) Properly parse, but don't use, AGM_CONTACT packets

The image sensor AGM mode acts different than the profile sensor.  For
the profile sensor, there is always one AGM for every SGM when 2 or
more fingers are present.  With the image sensor, there may be more 1
AGM per SGM if the number of fingers is changing - especially when
there are more than 3 fingers present.  This is what I meant by
properly parse these packets.  At least it is a form of documentation
for the next poor guy who comes along and tries to figure out what in
the world his trackpad is sending him.

Thanks,
-Daniel

>
> I am tempted to write something about Schrödinger's cat here... ;-)
>
> Thanks,
> Henrik
>
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