var mvdist,dvx,dvy;
mvdist=point_distance(xprevious,yprevious,x,y);
if (mvdist)  {                   // this precaution needs to be taken, in order to avoid a possible "DIVIDE BY ZERO" numeric exception
  dvx=(x-xprevious)/mvdist;
  dvy=(y-yprevious)/mvdist;
}
else  {dvx=0; dvy=0}