fasjittr.c

Graphics Gems 源码 a collection of algorithms, programs, and mathematical techniques for the computer

/*
 * 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.
/* */

#define			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	(x,y,s,xj,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	(x,y,s,xj,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());
}