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Re: atan2 and sqrt --> in driver

On Tue, Dec 09, 2008 at 02:42:45PM -0500, der Mouse wrote:
> >> The driver is for the wii-remote, so get 1 byte for x-, y- and
> >> z-axis.  Based on this values I will calculate the
> >> pitch = atan2(y, sqrt(x^2+z^2)) and yaw = atan2(z, sqrt(x^2+y^2))
> Why do these have to be done in the driver, rather than userland?

Well, I think our wsmouse interface allows only absolute coordinates
right now.

> > Alternatively, just provide a data table, indexed by 0..255 x 0..255,
> > with the second operand shifted and the sqrt computed in integer
> > math.  Search wikipedia for easy algorithms to compute isqrt.
> Since sqrt isn't being used in full generality, but just for hypot(),
> you could use a lookup table for that too.  Even with cache miss
> effects I suspect it may be faster than integer square root code
> (though I'd want to benchmark before saying anything definite).

I wasn't thinking much about speed, just size. Computing integer square
root is simple, atan2 likely is more involved.


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