Calculates the closest point to a segment source code

This snippet submitted by Adri�n Deccico on 2005-03-06. It has been viewed 30146 times.
Rating of 5.3 with 206 votes
 //--------------------------------------------------------------------------------------------------------
/*
Description: calculate the more near point belonging to a given segment.
Parameters:
       nSx1, nSy1 =  First point of the segment
       nSx2, int nSy2 = Second point of the segment
       nPx, int nPy = The point to analize
       *nCercaX,  *nCercaY = The closest point to the given point (nPx,nPy) that belongs to the segment
Returns: The distance between the point and the founded point of the segment
*/
double ElPuntoMasCercanoAUnSegmento(double nSx1, double nSy1, double nSx2,
                                    double nSy2, double nPx, double nPy,
                                    double *nCercaX,  double *nCercaY)
{

       //caracterizo la recta del segmento dado Y=m*X + b
       double m, b;

       bool bEsHorizontalOVertical = false;

       //el segmento es vertical
       if (nSx2 == nSx1){
               *nCercaX = nSx1;
               *nCercaY = nPy;
               bEsHorizontalOVertical = true;
       }

       //el segmento es horizontal
       if (nSy2 == nSy1){
               *nCercaY = nSy1;
               *nCercaX = nPx;
               bEsHorizontalOVertical = true;
       }

       //el segmento est� caracterizado por una recta \"AX + BY + C = 0\" con
       // A y B distintos de cero
       if (!bEsHorizontalOVertical)
       {
               m= (nSy2 - nSy1) / (nSx2 - nSx1);
               b = nSy1 - (nSx1 * m);

               //caracterizo la recta que une al punto dado con el punto m�s
               // cercano dentro del segmento Y=m1*X + b1
               double m1, b1;

               m1 = -1 / m;
               b1 = -m1 * nPx + nPy;

               //encuentro el punto de intersecci�n
               *nCercaX =  (b1 - b) / (m - m1);
               *nCercaY =  m * *nCercaX + b;
       }

       //verificar que el punto encontrado est� dentro del segmento
       if (*nCercaX < min(nSx1,nSx2)
               ||  *nCercaX > max(nSx1,nSx2)
               ||  *nCercaY < min(nSy1,nSy2)
               ||  *nCercaY > max(nSy1,nSy2) )
       {
               //el punto m�s cercano ser� uno de los dos extremos (el m�s cercano al punto)
               if (dDistanciaEntrePuntos(nSx1, nSy1, nPx, nPy)
               <= dDistanciaEntrePuntos(nSx2, nSy2, nPx, nPy))
               {
                       *nCercaX = nSx1;
                       *nCercaY = nSy1;
               }
               else
               {
                       *nCercaX = nSx2;
                       *nCercaY = nSy2;
               }
       }

       return dDistanciaEntrePuntos(nPx, nPy, *nCercaX , *nCercaY);

}
//--------------------------------------------------------------------------------------------



//------------------------------------------------------------------------
//return the distance between thwo points in a x,y plane.
double dDistanciaEntrePuntos(double lOldX, double lOldY,
                             double lNewX, double lNewY)
{
       double x = lNewX - lOldX;
       double y = lNewY - lOldY;

       return sqrt(pow(x,2) + pow(y,2));
}
//------------------------------------------------------------------------
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