/*
 * Efficient Generation of Sampling Jitter Using Look-up Tables
 * by Joseph M. Cychosz
 * from "Graphics Gems", Academic Press, 1990
 */

/*  	Jitter.c - Sampling jitter generation routines.  	    
 *
 *	Description:
 *	    Jitter.c contains the routines for generation of sampling
 *	    jitter using look-up tables.
 *
 *	Contents:
 *	    Jitter1	Generate random jitter function 1.
 *	    Jitter2	Generate random jitter function 2.
 *	    JitterInit	Initialize look-up tables.
 * */

enum {
NRAN =	1024	/* Random number table length	*/
};

static	double	URANX[NRAN],		/* Random number tables		*/
				URANY[NRAN];
static	int		IRAND[NRAN];	/* Shuffle table	*/
static	int		MASK = NRAN-1;	/* Mask for jitter mod function */
extern	double	xranf();		/* Random number generator pro- */
					/* ducing uniform numbers 0 to 1 */

/*  	Jitter1 - Generate random jitter. 	*/

void	Jitter1	(int x, int y, int s, double *xj, double *yj)
/* int	x, y;		Pixel location	 */
/* int	s;		Sample number for the pixel */
/* double	*xj, *yj;	Jitter (x,y)	*/
{
		*xj = URANX[ (x + (y<<2) + IRAND[(x+s)&MASK]) & MASK ];
		*yj = URANY[ (y + (x<<2) + IRAND[(y+s)&MASK]) & MASK ];
}



/*  	Jitter2 - Generate random jitter.  	*/

void	Jitter2	(int x, int y, int s, double *xj, double *yj)
/* int	x, y;			Pixel location  */
/* int	s;			Sample number for the pixel */
/* double	*xj, *yj;	Jitter (x,y)	*/
{
		*xj = URANX[ ((x | (y<<2)) + IRAND[(x+s)&MASK]) & MASK ];
		*yj = URANY[ ((y | (x<<2)) + IRAND[(y+s)&MASK]) & MASK ];
}


/*  	JitterInit - Initialize look-up tables.  	*/

void	JitterInit	()
{
		int	i;

		for ( i = 0 ; i < NRAN ; i++ ) URANX[i] = xranf();
		for ( i = 0 ; i < NRAN ; i++ ) URANY[i] = xranf();
		for ( i = 0 ; i < NRAN ; i++ ) IRAND[i] = (int) (NRAN *
						xranf());
}