点到直线的距离算法来源于mapserver(目前所知效率最高的) 推荐*****

double msDistancePointToSegment(pointObj *p, pointObj *a, pointObj *b)
{//计算点到线段(a,b)的距离  
double l; /* length of line ab */ 
double r,s; 
l = msDistancePointToPoint(a,b);   
if(l == 0.0) /* a = b */   
 return( msDistancePointToPoint(a,p)); 

r = ((a->y - p->y)*(a->y - b->y) - (a->x - p->x)*(b->x - a->x))/(l*l);   

if(r > 1) /* perpendicular projection of P is on the forward extention of AB */    
return(MS_MIN(msDistancePointToPoint(p, b),msDistancePointToPoint(p, a)));  

if(r < 0) /* perpendicular projection of P is on the backward extention of AB */   
 return(MS_MIN(msDistancePointToPoint(p, b),msDistancePointToPoint(p, a)));  
 
s = ((a->y - p->y)*(b->x - a->x) - (a->x - p->x)*(b->y - a->y))/(l*l);   

return(fabs(s*l));
}

double msDistancePointToPoint(pointObj *a, pointObj *b)
{
  double d;
  double dx, dy;
 
  dx = a->x - b->x;
  dy = a->y - b->y;
  d = sqrt(dx*dx + dy*dy);
  return(d);
}