## Tuesday, June 13, 2017

### fp64 approximations for sin/cos for OpenGL

Minimax approximations for missing fp64 functions in GLSL, using remez exchange toolbox. For additional details see Double precision approximations for map projections in OpenGL.

//sin approximation, error < 5e-9
double sina_9(double x)
{
//minimax coefs for sin for 0..pi/2 range
const double a3 = -1.666665709650470145824129400050267289858e-1LF;
const double a5 =  8.333017291562218127986291618761571373087e-3LF;
const double a7 = -1.980661520135080504411629636078917643846e-4LF;
const double a9 =  2.600054767890361277123254766503271638682e-6LF;

const double m_2_pi = 0.636619772367581343076LF;
const double m_pi_2 = 1.57079632679489661923LF;

double y = abs(x * m_2_pi);
double q = floor(y);

double t = (quadrant & 1) != 0 ? 1 - y + q : y - q;
t *= m_pi_2;

double t2 = t * t;
double r = fma(fma(fma(fma(a9, t2, a7), t2, a5), t2, a3), t2*t, t);

r = x < 0 ? -r : r;

return (quadrant & 2) != 0 ? -r : r;
}

//sin approximation, error < 2e-11
double sina_11(double x)
{
//minimax coefs for sin for 0..pi/2 range
const double a3 = -1.666666660646699151540776973346659104119e-1LF;
const double a5 =  8.333330495671426021718370503012583606364e-3LF;
const double a7 = -1.984080403919620610590106573736892971297e-4LF;
const double a9 =  2.752261885409148183683678902130857814965e-6LF;
const double ab = -2.384669400943475552559273983214582409441e-8LF;

const double m_2_pi = 0.636619772367581343076LF;
const double m_pi_2 = 1.57079632679489661923LF;

double y = abs(x * m_2_pi);
double q = floor(y);

double t = (quadrant & 1) != 0 ? 1 - y + q : y - q;
t *= m_pi_2;

double t2 = t * t;
double r = fma(fma(fma(fma(fma(ab, t2, a9), t2, a7), t2, a5), t2, a3),

t2*t, t);

r = x < 0 ? -r : r;

return (quadrant & 2) != 0 ? -r : r;
}

Cos can be just offset by π/2

//cos approximation, error < 5e-9
double cosa_9(double x)
{
//sin(x + PI/2) = cos(x)
return sina_9(x + DBL_LIT(1.57079632679489661923LF));
}

//cos approximation, error < 2e-11
double cosa_11(double x)
{
//sin(x + PI/2) = cos(x)
return sina_11(x + DBL_LIT(1.57079632679489661923LF));
}

Amara said...

nnice

Amara said...