#define setbit(bitmap, n)       (bitmap[n/8] |= (0x80 >> (n % 8)))
#define clearbit(bitmap, n)     (bitmap[n/8] &= (~(0x80 >> (n % 8))))
/***************************************************************************
 * Func: dither                                                            *
 *                                                                         *
 * Desc: dithers a pgm file, writes out a pbm file                         *
 *                                                                         *
 * Params: image - pointer to image in memory                              *
 *         rows - number of rows in the image                              *
 *         cols - number of columns in the image                           *
 *         matrix - dither matrix stored as a one-dimensional array        *
 *         m - height of dither matrix                                     *
 *         n - width of dither matrix                                      *
 *         filename - name of output file to create                        *
 *                                                                         *
 * Returns: nothing                                                        *
 ***************************************************************************/

void dither(image_ptr image, int rows, int cols,
	    unsigned char *matrix, int m, int n, char *filename)
    {
    FILE *fp;                     /* file pointer for output file */
    unsigned long index;          /* address of current input pixel */
    int row_size;                 /* size of row in bytes */
    unsigned long i, j;           /* loop variable */
    unsigned char line_buff[1024];/* storage for 1 line */

    /* open new output file */
    if((fp=fopen(filename, "wb")) == NULL)
	{
	printf("Unable to open %s for output\n",filename);
	exit(1);
	}

    /* print out the portable bitmap header */
    fprintf(fp, "P4\n%d %d\n", cols, rows);

    /* calcute the number of bytes in each row */
    row_size = cols/8 + (1 * ((cols % 8)) != 0) ;

    for(i=0; i<rows; i++)
	{
	for(j=0; j<cols; j++)
	    {
	    index = i * cols + j;
	    if(image[index] > (matrix[(i % m) * m + (j % n)]))
		clearbit(line_buff, j);   /* this function can be swapped */
	    else
		setbit(line_buff, j);     /* with this one if necessary */
	    }
	fwrite(line_buff, 1, row_size, fp); /* write out line */
	}

    fclose(fp);
    }