Hi,
This code uses a lookup table and is accurate to +/-1 degree (-180 < X
<= +180 degrees). I am sure you can tweak it to improve the accuracy.
Regards
Luke
const short int ATAN_LUT[65] =
{
0, 1, 2, 3, 4, 5, 6, 7, 8, 8, 9, 10, 11, 12, 13, 14, 14, 15, 16, 17,
18,
18, 19, 20, 21, 22, 22, 23, 24, 25, 25, 26, 27, 27, 28, 29, 30, 30,
31,
32, 32, 33, 33, 34, 35, 35, 36, 36, 37, 37, 38, 39, 39, 40, 40, 41,
41,
42, 42, 43, 43, 44, 44, 45, 45
};
const int SCALE_UP_BITS = 6; // scale up 6 bits (i.e., 64)
//----------------------------------------------------------------------
----
// Description : calculate the arctangent (i.e.,
atan(y_number/x_number))
// Parameters : y_number = y coordinate of the point
// x_number = x coordinate of the point
// Assumptions : short int = 16 bits
// Return : the arctangent in degree between 180 and -180,
// excluding -180.
// If y_number and x_number are both 0, 0 will be return.
// The resolution is +/- 1 degree.
//----------------------------------------------------------------------
----
short int
atan2(short int y_number, short int x_number)
{
// check if (y_number == 0)
if (0 == y_number)
{
return (x_number < 0 ? 180 : 0);
}
// get the absolute value of x_number and y_number
int x_number_abs = x_number > 0 ? x_number : 0 - x_number;
int y_number_abs = y_number > 0 ? y_number : 0 - y_number;
// get the arctangent in the first quadrant
//
// do y_number_abs/x_number_abs if y_number_abs < x_number_abs
// else do x_number_abs/y_number_abs
// because ATAN LUT is from 0 degree to 45 degree
short int result;
if(y_number_abs < x_number_abs)
{
result = ATAN_LUT[(y_number_abs << SCALE_UP_BITS) /
x_number_abs];
}
else
{
result = 90 -
ATAN_LUT[(x_number_abs << SCALE_UP_BITS) /
y_number_abs];
}
// calculate the final arctangent
if (x_number > 0)
{
if (y_number < 0)
{
return (0 - result);
}
return result;
}
if (y_number < 0)
{
return (-180 + result);
}
return (180 - result);
}
-----Original Message-----
From: Dmitry [mailto:di...@eis.ru]
Sent: Monday, 17 March 2003 4:28 PM
To: mspgcc-users@lists.sourceforge.net
Subject: Re: [Mspgcc-users] who knows the best atan() approximation?
I need about 0.5 degree tolerance within [0,45] degrees range.
However, need more precise results at about 45 degrees point.
~d
On Monday 17 March 2003 10:56, Garst R. Reese wrote:
> Dmitry wrote:
> > Fellows,
> > who knows the best atan() approx for argument within [0,1[ (return
angles
> > from 0 to 45 degrees)?
>
> How much error can you tolerate?
>
> x / (1 + .28*x*x) |e(x)| <= 5e-3
>
> Garst
>
>
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