Messages in this thread | | | Date | Sun, 10 Jul 2005 14:04:25 +0200 | From | Vojtech Pavlik <> | Subject | Re: [PATCH] Apple USB Touchpad driver (new) |
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On Sun, Jul 10, 2005 at 12:48:30AM +0200, Peter Osterlund wrote: > Vojtech Pavlik <vojtech@suse.cz> writes: > > > Btw, what I don't completely understand is why you need linear > > regression, when you're not trying to detect motion or something like > > that. Basic floating average, or even simpler filtering like the input > > core uses for fuzz could work well enough I believe. > > Indeed, this function doesn't make much sense: > > +static inline int smooth_history(int x0, int x1, int x2, int x3) > +{ > + return x0 - ( x0 * 3 + x1 - x2 - x3 * 3 ) / 10; > +} > > In the X driver, a derivative estimate is computed from the last 4 > absolute positions, and in that case the least squares estimate is > given by the factors [.3 .1 -.1 -.3]. However, in this case you want > to compute an absolute position estimate from the last 4 absolute > positions, and in this case the least squares estimate is given by the > factors [.25 .25 .25 .25], ie a floating average. If the function is > changed to this: > > +static inline int smooth_history(int x0, int x1, int x2, int x3) > +{ > + return (x0 + x1 + x2 + x3) / 4; > +} > > the standard deviation of the noise will be reduced by a factor of 2 > compared to the unfiltered values. With the old smooth_history() > function, the noise reduction will only be a factor of 1.29.
Using a function like
return (x_old * 3 + x) / 4;
eliminates the need for a FIFO, and has similar (if not better) properties to floating average, because its coefficients are [ .25 .18 .14 .10 ... ].
Setting
absfuzz[ABS_X] = some number;
activates the abovementioned filtering (with additional cutoff and fast motion compensation) directly in input.c, which should eliminate a lot of the code in the appletouch driver.
-- Vojtech Pavlik SuSE Labs, SuSE CR - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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