Może przez wypełnienie konturu ?

int FillContour(double dXseed, double dYseed, unsigned char color,
unsigned char _data[])
{
/*
fills contour with black border using seed point inside contour
and horizontal lines
it starts from seed point, saves max right( iXmaxLocal) and max
left ( iXminLocal) interior points of horizontal line,
in new line ( iY+1 or iY-1) it computes new interior point :
iXmidLocal=iXminLocal + (iXmaxLocal-iXminLocal)/2;
result is stored in _data array : 1D array of 1-bit colors (
shades of gray)
it does not check if index of _data array is good so memory error
is possible
*/

int iXseed = (int)((dXseed - ZxMin)/PixelWidth);
int iYseed = (int)((dYseed - ZyMin)/PixelHeight);

int iX, /* seed integer coordinate */
iY=iYseed,
/* most interior point of line iY */
iXmidLocal=iXseed,
/* min and max of interior points of horizontal line iY */
iXminLocal,
iXmaxLocal;
int i ; /* index of _data array */;


/* --------- move up --------------- */
do{
iX=iXmidLocal;
i =f(iX,iY); /* index of _data array */;

/* move to right */
while (_data[i]==iInterior)
{ _data[i]=color;
iX+=1;
i=f(iX,iY);
}
iXmaxLocal=iX-1;

/* move to left */
iX=iXmidLocal-1;
i=f(iX,iY);
while (_data[i]==iInterior)
{ _data[i]=color;
iX-=1;
i=f(iX,iY);
}
iXminLocal=iX+1;


iY+=1; /* move up */
iXmidLocal=iXminLocal + (iXmaxLocal-iXminLocal)/2; /* new iX inside
contour */
i=f(iXmidLocal,iY); /* index of _data array */;
if ( _data[i]==iJulia) break; /* it should not cross the border */

} while (iY<iYmax);


/* ------ move down ----------------- */
iXmidLocal=iXseed;
iY=iYseed-1;


do{
iX=iXmidLocal;
i =f(iX,iY); /* index of _data array */;

/* move to right */
while (_data[i]==iInterior) /* */
{ _data[i]=color;
iX+=1;
i=f(iX,iY);
}
iXmaxLocal=iX-1;

/* move to left */
iX=iXmidLocal-1;
i=f(iX,iY);
while (_data[i]==iInterior) /* */
{ _data[i]=color;
iX-=1; /* move to right */
i=f(iX,iY);
}
iXminLocal=iX+1;

iY-=1; /* move down */
iXmidLocal=iXminLocal + (iXmaxLocal-iXminLocal)/2; /* new iX inside
contour */
i=f(iXmidLocal,iY); /* index of _data array */;
if ( _data[i]==iJulia) break; /* it should not cross the border */
} while (0<iY);



return 0;
}

HTH

Adam