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Re: [PATCH] input: wacom: Use touch size for ABS_MT_TOUCH_MAJOR

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On Thu, Mar 15, 2012 at 8:22 PM, Chris Bagwell <chris@xxxxxxxxxxxxxx> wrote:
> On Thu, Mar 15, 2012 at 7:10 PM, Jason Gerecke <killertofu@xxxxxxxxx> wrote:
>> On Mon, Mar 12, 2012 at 3:41 PM, Chris Bagwell <chris@xxxxxxxxxxxxxx> wrote:
>>> On Fri, Mar 9, 2012 at 5:26 PM, Jason Gerecke <killertofu@xxxxxxxxx> wrote:
>>>> On Fri, Mar 9, 2012 at 2:30 PM, Chris Bagwell <chris@xxxxxxxxxxxxxx> wrote:
>>>>> On Wed, Mar 7, 2012 at 7:39 PM, Jason Gerecke <killertofu@xxxxxxxxx> wrote:
>>>>>> 3rd-gen Bamboo devices report both "amplitude" and "size" data
>>>>>> in their touch packets. This patch changes the source for
>>>>>> ABS_MT_TOUCH_MAJOR to be the latter rather than the former.
>>>>>> ---
>>>>>>  drivers/input/tablet/wacom_wac.c |    2 +-
>>>>>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/input/tablet/wacom_wac.c b/drivers/input/tablet/wacom_wac.c
>>>>>> index d0b0fc4..5daf11d 100644
>>>>>> --- a/drivers/input/tablet/wacom_wac.c
>>>>>> +++ b/drivers/input/tablet/wacom_wac.c
>>>>>> @@ -916,7 +916,7 @@ static void wacom_bpt3_touch_msg(struct wacom_wac *wacom, unsigned char *data)
>>>>>>        if (touch) {
>>>>>>                int x = (data[2] << 4) | (data[4] >> 4);
>>>>>>                int y = (data[3] << 4) | (data[4] & 0x0f);
>>>>>> -               int w = data[6];
>>>>>> +               int w = data[5];
>>>>>>
>>>>>>                input_report_abs(input, ABS_MT_POSITION_X, x);
>>>>>>                input_report_abs(input, ABS_MT_POSITION_Y, y);
>>>>>> --
>>>>>> 1.7.9.1
>>>>>>
>>>>>
>>>>> I found time to test this patch and used "mtview" so I could get some
>>>>> visual feedback.
>>>>>
>>>>> The lines because extremely thin with this change.  Part of reason I
>>>>> quickly traced to we are setting range as 0-255 but I couldn't get
>>>>> data[5] to go above a value of 18.  So I changed range to 0-16 and it
>>>>> works better but the lines still never gets very thick (this could
>>>>> well be that mtview doesn't scale values and prefers 0-255.  I didn't
>>>>> look).
>>>> Looks like its a combination of two problems, one mine one mtview's...
>>>>
>>>> I did some reading, and the MT protocol docs specify that
>>>> ABS_MT_TOUCH_MAJOR are in terms of surface units. That means a value
>>>> of e.g. "20" should represent a touch that spans 20 units of X. The
>>>> touch sensor has a resolution of ~0.1mm, which would imply the touch
>>>> was only 2 mm wide... There's going to have to be a correction factor
>>>> to transform the value to something more appropriate.
>>>>
>>>> As for mtview, it looks like it doesn't scale the major/minor axis
>>>> values based on the output size. At the very least it should scale
>>>> everything by roughly (output_max_x / sensor_max_x) so that a touch
>>>> taking up e.g. 10% of the tablet would take up 10% of the output.
>>>
>>> I see now that I got lucky that I had a 0-255 value available and
>>> didn't look into what I needed to report close enough.
>>>
>>>>
>>>>>
>>>>> Amplitude feels better to me using test apps but your in a better
>>>>> position to say if we should really change this.  Please adjust
>>>>> declared range though if you still think we should change this.
>>>>>
>>>>> Chris
>>>>
>>>> My main reasons for switching away from amplitude were that amplitude
>>>> is a very crude number (I think I can get three or four unique values
>>>> out of it), and that the tablet reports a size already. Worst case --
>>>> assuming a correction factor isn't sufficient to get data[5] working
>>>> as MAJOR -- we should at least send data[5] through PRESSURE (which
>>>> the MT docs state may be used instead of MAJOR/MINOR if the units are
>>>> arbitrary).
>>>>
>>>
>>> I've not found time to dig deeply to this (to compute real scale
>>> value) but I'll offer this.  I compared behaviour of original data[6]
>>> to this:
>>>
>>> int w = (data[5] > 31) ? 255 : data[5] << 3;
>>>
>>> and with mtview the size change is much more responsive and
>>> consistent.  So I'm on board with changing to data[5] in some form.
>>>
>>> Chris
>>
>> I did a little more testing, and it looks like data[5] is proportional
>> to the area (not linear dimension) of the touch. Given the intended
>> semantics (and ignoring the huge blobs it produces in mtview),
>> int_sqrt(data[5] << 13) seems to work well. We'd probably have to
>> increase the maximum to 512 or 1024 though since I've been able to get
>> my palm to produce values of ~600...
>>
>> Also, I don't yet know how these values scale between tablet sizes --
>> I imagine smaller tablets produce larger values (since their sensors
>> are denser), but haven't been able to get my hands on other hardware
>> yet.
>>
>
> You've already given it much more thought than I but I made the
> following assumption a little bit related.  The sensors report X/Y
> maximums of 4096 even though its a rectangle tablet. Thats why I made
> sure resolution was reported proportionally.  Here is from Xorg log of
> my medium size tablet:
>
>  (--) Wacom Wireless Receiver Finger pad: Wacom Unknown USB tablet
> maxX=4096 maxY=4096 maxZ=0 resX=18000 resY=29000
>
> I suspect that same ratio also applies to width reporting as well.
>
> I'm also guessing the value of size is really defining an ellipse with
> fixed orientation.  But we can't really expose that to user as
> WIDTH_MINOR because it will be incorrectly interrupted as a pressure
> estimate.
>
> But I suspect the above resolution calculation will help push the
> scaling issue to user land and we can concentrate on reporting
> something like the int_sqrt() values you've come up with.
>
> Chris

Whoops. Looks like this fell off my radar.

So, if I were to adjust the uncorrected width by the aspect ratio to
get values for ABS_MT_TOUCH_MAJOR and ABS_MT_TOUCH_MINOR, would that
be correct? For example, should placing a quarter (diameter of
24.26mm) on my Intuos5 Medium (logical size: 4096 x 4096, physical
size: 223mm x 139mm) ideally report a contact with a MAJOR axis of 714
and MINOR axis of 445?

Jason

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