inconvexe.c
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	#include <grille.h>

	/*
	* Parameters :
	* - P point that is inside or outside the contour
	* - np number of points of the contour
	* - I[NPOINTs] polygonal contour
	*
	* Return 1 if P is inside the contour, otherwise 0
	*/

	int inconvexe(struct point P, int np, struct point I[NPOINT])
	{
	register int k;
	double xi, xj, yi, yj, theta = 0.;

	I[np].x = I[0].x;
	I[np].y = I[0].y;
	for (k = 0; k < np; k++)
	{
	xi = I[k].x - P.x;
	xj = I[k+1].x - P.x;
	yi = I[k].y - P.y;
	yj = I[k+1].y - P.y;
	/* in a triangle OIJ, (OI,OJ)=atan(Det(OI,OJ)/Pscal(OI,OJ) */
	theta += atan2((xj * yi - xi * yj), (xi * xj + yi * yj));
	};

	if (fabs(theta) > PI) /* if P is on the edge of the polygon theta=PI */
	return(1);
	else /theta is close to 0/
	return(0);
	}