/*-----------------------------------------------
 *
 * In-place 2D Convolution, zero-padded border (Host SUN4/40)
 * requires symmetric co-efficients
 */

#include<stdlib.h>

#ifndef IMAGE
#define IMAGE   4096            /* 1024x1024 or 4096x4096 */
#endif  /* IMAGE */
#ifndef KERNEL_SIZE
#define KERNEL_SIZE     3       /* NxN convolution */
#endif  /* KERNEL_SIZE */
#define LOW     (- KERNEL_SIZE/2)
#define HIGH    (KERNEL_SIZE+LOW-1)
#define KERNEL_OFFSET   (-LOW)
#define WRITE_OFFSET    (KERNEL_OFFSET+1)       /* large enough for up
                                                 * and down */

typedef unsigned char contype;

static contype image[IMAGE + WRITE_OFFSET][ IMAGE],
               kernel[KERNEL_SIZE][ KERNEL_SIZE];

inline contype
       Image(int x, int y)
{
    if (x < 0 || x >= IMAGE || y < 0 || y >= IMAGE)
        return (0);
    else
        return image[x][ y];
}

void convolve()
{
    register contype result;
    register int x, y, j, k;

    for (x = IMAGE - 1; x >= 0; x--)
        for (y = 0; y < IMAGE; y++)
        {
            result = 0;
            for (j = LOW; j <= HIGH; j++)
                for (k = LOW; k <= HIGH; k++)
                    result += Image(x + j, y + k) *
                          kernel[j + KERNEL_OFFSET][ k + KERNEL_OFFSET];

            image[x + WRITE_OFFSET][ y] = result;
        }

}