On 07/30/2010 11:41 PM, Éric Piel wrote: [...] >> 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. > Maybe it would be useful to add a paragraph to the mt-protocol document > about the relation with the single-touch events. Indeed. > Well as an anecdote, on my old laptop with a single-touch synaptics > hardware, when I press 2 fingers, the average point is reported. That's > all in hardware, though so it probably doesn't count as a typical driver > behaviour ;-) Those devices have no other information to report. >>> 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. > What do you mean? ABS_X/Y is still very useful, for example for gesture, > or to manage horizontal/vertical scrolling. In the xorg synaptics > driver, it's not used to move the cursor position, but it's definitely > not masked out! Maybe the idea sounds radical, but there is a rationale. The semantics of ABS_X/Y is a singular, well-defined point. In the trackpad case, the movement of that point is used to guide a pointer on screen, but there are other scenarios. In the presence of multiple touch points, both ABS_X/Y and the pointer are ill-defined. This simple fact has far-reaching implications; in Xorg land, there is a whole new input protocol cooking to deal with it. For all practical purposes, the traditional ABS_X/Y point has no meaning in the multitouch case. Instead of extending the semantics of ABS_X/Y, one could simply omit the event during a multi-finger touch. In practice, this would most likely confuse some applications. Sending the last known one-finger touch point should be less confusing. Not quite equivalent, but close enough. >>> 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. ;-) > : > I'm completely against not reporting the position with 2 fingers, > because definitely, users would lose some features. With 3 and 4 > fingers, we report the lower coordinates (the only ones provided by the > hardware). So it'd be extremely weird not to report any for 2 fingers! This sounds like a misunderstanding. To rephrase: the suggestion is to always send the last known one-finger touch point, and to update that point only when there is a single finger on the pad. > As a user, I enjoy better when the average is reported, because > whichever finger I move, I get a feedback, there is never a finger > "hidden" behind the other one. Nevertheless, if consistency matters, > let's just leave it as is. This behavior can be created in userland using the MT touch points, if so preferred. Thanks, Henrik -- To unsubscribe from this list: send the line "unsubscribe linux-input" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
