Category : C Source Code
Archive   : GRAFGEM1.ZIP
Filename : FIXEDTRI.C

/*
Fixed-Point Trigonometry with CORDIC Iterations
by Ken Turkowski
from "Graphics Gems", Academic Press, 1990
*/

#define COSCALE 0x22c2dd1c /* 0.271572 */
#define QUARTER ((int)(3.141592654 / 2.0 * (1 << 28)))
static long arctantab[32] = { /* MS 4 integral bits for radians */
297197971, 210828714, 124459457, 65760959, 33381290, 16755422,
8385879, 4193963, 2097109, 1048571, 524287, 262144, 131072,
65536, 32768, 16384, 8192, 4096, 2048, 1024, 512, 256, 128, 64,
32, 16, 8, 4, 2, 1, 0, 0,
};

CordicRotate(px, py, theta)
long *px, *py;
register long theta; /* Assume that abs(theta) <= pi */
{
register int i;
register long x = *px, y = *py, xtemp;
register long *arctanptr = arctantab;

/* The -1 may need to be pulled out and done as a left shift */
for (i = -1; i <= 28; i++) {
if (theta < 0) {
xtemp = x + (y >> i);
y = y - (x >> i);
x = xtemp;
theta += *arctanptr++;
} else {
xtemp = x - (y >> i);
y = y + (x >> i);
x = xtemp;
theta -= *arctanptr++;
}
}

*px = frmul(x, COSCALE); /* Compensate for CORDIC enlargement */
*py = frmul(y, COSCALE); /* frmul(a,b)=(a*b)>>31, high part */
/* of 64-bit product */
}




CordicPolarize(argx, argy)
long *argx, *argy; /* We assume these are already in the */
/* right half plane */
{
register long theta, yi, i;
register long x = *argx, y = *argy;
register long *arctanptr = arctantab;
for (i = -1; i <= 28; i++) {
if (y < 0) { /* Rotate positive */
yi = y + (x >> i);
x = x - (y >> i);
y = yi;
theta -= *arctanptr++;
} else { /* Rotate negative */
yi = y - (x >> i);
x = x + (y >> i);
y = yi;
theta += *arctanptr++;
}
}

*argx = frmul(x, COSCALE);
*argy = theta;
}