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void arc_to_bezier(double cx, double cy, double rx, double ry,
double start_angle, double sweep_angle,
double* curve)
{
double x0 = cos(sweep_angle / 2.0);
double y0 = sin(sweep_angle / 2.0);
double tx = (1.0 - x0) * 4.0 / 3.0;
double ty = y0 - tx * x0 / y0;
double px[4];
double py[4];
px[0] = x0;
py[0] = -y0;
px[1] = x0 + tx;
py[1] = -ty;
px[2] = x0 + tx;
py[2] = ty;
px[3] = x0;
py[3] = y0;
double sn = sin(start_angle + sweep_angle / 2.0);
double cs = cos(start_angle + sweep_angle / 2.0);
unsigned i;
for(i = 0; i < 4; i++)
{
curve[i * 2] = cx + rx * (px[i] * cs - py[i] * sn);
curve[i * 2 + 1] = cy + ry * (px[i] * sn + py[i] * cs);
}
}