www.pudn.com > 2D.rar > dt4.c

#include	
#define	NMAX	1001
main()
{
	int	x[NMAX],y[NMAX],z[NMAX];/* input points xy,z=x^2+y^2 */
	int	n;			/* number of input points */
	int	i, j, k, m;		/* indices of four points */
	int	xn, yn, zn;		/* outward vector normal to (i,j,k) */
	int	flag;			/* t if m above of (i,j,k) */
	int	F = 0;			/* # of lower faces */

	/* Input points and compute z = x^2 + y^2. */
	scanf("%d", &n);
	for ( i = 0; i < n; i++ ) {
		scanf("%d %d", &x[i], &y[i]);
		z[i] = x[i] * x[i] + y[i] * y[i];
	}

	/* For each triple (i,j,k) */
	for ( i = 0; i < n - 2; i++ )
	for ( j = i + 1; j < n; j++ )
	for ( k = i + 1; k < n; k++ ) 
	if ( j != k ) {
		/* Compute normal to triangle (i,j,k). */
		xn = (y[j]-y[i])*(z[k]-z[i]) - (y[k]-y[i])*(z[j]-z[i]);
		yn = (x[k]-x[i])*(z[j]-z[i]) - (x[j]-x[i])*(z[k]-z[i]);
		zn = (x[j]-x[i])*(y[k]-y[i]) - (x[k]-x[i])*(y[j]-y[i]);

		/* Only examine faces on bottom of paraboloid: zn < 0. */
		if ( flag = (zn < 0) )
			/* For each other point m */
			for (m = 0; m < n; m++)
				/* Check if m above (i,j,k). */
				flag = flag && 
					((x[m]-x[i])*xn +
					 (y[m]-y[i])*yn +
					 (z[m]-z[i])*zn <= 0);
		if (flag) {
	  	   printf("z=%10d; lower face indices: %d, %d, %d\n", 
		     zn, i, j, k);
		   F++;
		}
	}
	printf("A total of %d lower faces found.\n", F);
}