di2566.txt
;**************************************************************************************************
; LISTING 1 - HIT TEST FOR TRIANGULAR REGIONS
; 
; "Algorithm tests for point location," EDN, Aug 3, 2000, pg 126
;
; http://www.ednmag.com/ednmag/reg/2000/08032000/designideas.htm#16di4
;***************************************************************************************************

class TTriangle
{
	public:
		TTriangle (TPoint &PntA, TPoint &PntB, TPoint &PntC);
		BOOL Contains (TPoint &PntX);
		double Area (void) { return area; }
	private:
		TPoint PntA, PntB, PntC;	// vertices
		double area;
} ;
TTriangle::TTriangle(TPoint &pntA, TPoint &pntB, TPoint &pntC)
{
	PntA = pntA;
	PntB = pntB;
	PntC = pntC;
	area = 0.5*( ((PntB.x*PntC.y) - (PntC.x*PntB.y))
			     -((PntA.x*PntC.y) - (PntC.x*PntA.y))
			     +((PntA.x*PntB.y) - (PntB.x*PntA.y)));
	area = fabs(area);
}
BOOL TTriangle::Contains (TPoint &PntX)
{
	TTriangle ABX (PntA, PntB, PntX);
	TTriangle BCX (PntB, PntC, PntX);
	TTriangle CAX (PntC, PntA, PntX);
	double NewArea = ABX.Area()+BCX.Area()+CAX.Area();
	if (fabs(NewArea - area) > 1e-6)  return FALSE;
	return TRUE;
}
// an example
void main (void)
{
	TTriangle ABC (TPoint(1,1),TPoint(8,2), TPoint(5,9));
	TPoint X(4,4);
	if (ABC.Contains(X))   printf ("ABC contains X\n");
	else                                printf ("ABC does not contain X\n");
}