v10 = x1-x0;
v11 = y1-y0;
v12 = z1-z0;
v20 = x2-x0;
v21 = y2-y0;
v22 = z2-z0;
vx = v11*v22 - v12*v21;
vy = v12*v20 - v10*v22;
vz = v10*v21 - v11*v20;
d = vx*vx + vy*vy + vz*vz;
if( d ) d = sqrt( d );
if( d ) {
vx /= d;
vy /= d;
vz /= d;
}