Re: [PATCH 7/7] elantech: average the two coordinates when 2 fingers

[Henrik Rydberg <rydberg@euromail.se>]

On 07/30/2010 08:55 PM, Éric Piel wrote:

> On 21-07-10 05:36, Dmitry Torokhov wrote:
>> On Mon, Jun 21, 2010 at 11:06:28PM +0200, Éric Piel wrote:
>>> When 2 fingers are pressed, the low and high coordinates are given.
>>> Instead of reporting just the low coordinate as the normal position,
>>> report an average of both. This allow to have the cursor always move,
>>> whichever finger moves. In practice, this improves the support of
>>> two-fingers scolling with the synaptics X driver.
> :
>>> -        input_report_abs(dev, ABS_X, x1<<  2);
>>> -        input_report_abs(dev, ABS_Y, y1<<  2);
>>> +        input_report_abs(dev, ABS_X, (x1 + x2)<<  1);
>>> +        input_report_abs(dev, ABS_Y, (y1 + y2)<<  1);
>>
>> I am concerned what happens when you put one finger first and then add
>> another one. Would not that couse the cursor to jump?


I agree, I have seen this behavior before and it is not quite what you want. The
average point does jump.

>> I'd say we need to obey MT protocol and continue reporting the first
>> contact in non-MT protocol and rely on MT-aware drivers to do better job
>> of contact tracking. This is also consistent with how other drivers,
>> such as bcm5974, report coordinates.
> If the MT protocol says we should always report on the single-touch side
> the first contact, then I guess it's better to do so. However, I've just
> read the latest version of the mt-protocol document, and couldn't see
> anything saying that. Are you sure this is the official behavior? I'd
> tend to think that single-touch and multi-touch are never read together,
> so they can be different.


It is true that there are no specific section on how to handle the ABS_X/Y
events, and different drivers have slightly different strategies. However,
common to them all is that a single finger is consistently used to report the
events.

> More specifically, on this hardware, when there are two fingers, we get
> only the lower coordinates and the higher coordinates, but we don't know
> exactly where are the fingers. As there is no tracking, the lower
> coordinate might be the second finger applied, so even if we report just
> the lower coordinates, there is 75% chance that the cursor will jump.


Perhaps it is better to not report ABS_X/Y at all in the case of two fingers on
this trackpad. With any reasonable userspace driver, the ABS_X/Y will be masked
out anyways.

> That's actually the reason I prefer the average: it gives a 100%
> expectable behavior (because the average is always correct). In
> practice, when using the xorg synaptics driver, with the default
> 2-fingers-scrolling mode, the cursor never jumps because as soon as the
> second finger is applied, X and Y are not used to move the cursor.


It is predictable, but it is still bad behavior. ;-)

> So, I think this patch is an improvement for the user, but if you think
> that it would not respect the MT protocol, just drop it.


Either dropping the patch or muting ABS_X/Y in case of multiple fingers is what
I would recommend.

Cheers,
Henrik
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