Category : C Source Code
Archive   : GRAFGEM1.ZIP
Filename : FASTJITT.C

/*
* 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());
}