ref: f4f19cdf1a8f0d2aa5f9fce10c896201528b5347
dir: /sys/src/cmd/plot/libplot/rarc.c/
#include "mplot.h"
/* arc plotting routine */
/* from x1,y1 to x2,y2 */
/* with center xc,yc and radius rr */
/* integrates difference equation */
/* negative rr draws counterclockwise */
#define PI4 0.7854
void rarc(double x1, double y1, double x2, double y2, double xc, double yc, double rr){
register double dx, dy, a, b;
double ph, dph, rd, xnext;
register int n;
dx = x1 - xc;
dy = y1 - yc;
rd = sqrt(dx * dx + dy * dy);
if (rd / e1->quantum < 1.0) {
move(xc, yc);
vec(xc, yc);
return;
}
dph = acos(1.0 - (e1->quantum / rd));
if (dph > PI4)
dph = PI4;
ph=atan2((y2-yc),(x2 - xc)) - atan2(dy, dx);
if (ph < 0)
ph += 6.2832;
if (rr < 0)
ph = 6.2832 - ph;
if (ph < dph)
plotline(x1, y1, x2, y2);
else {
n = ph / dph;
a = cos(dph);
b = sin(dph);
if (rr < 0)
b = -b;
move(x1, y1);
while ((n--) >= 0) {
xnext = dx * a - dy * b;
dy = dx * b + dy * a;
dx = xnext;
vec(dx + xc, dy + yc);
}
}
}