int
draw_vert_line (int x)
{
	unsigned char buf[SCROLL_Y_DIM];	/* buffer for graphical image of line */
	unsigned char* addr;							/* address of first pixel in build    */
																		/*     buffer (without plane offset)  */
	
	int p_off;												/* offset of plane of first pixel     */
	int i;														/* loop index over pixels             */
	
	/* Check whether requested line falls in the logical view window. */
	if (x < 0 || x >= SCROLL_X_DIM)
		return -1;
	
	/* Adjust x to the logical col value. */
	x += show_x;
	
	/* Get the image of the line. */
	(*vert_line_fn) (x, show_y, buf);
	
	/* Calculate starting address in build buffer. */
	addr = img3 + show_y * SCROLL_X_WIDTH + (x >> 2);
	
	/* Calculate plane offset of first pixel. */
	p_off = (3 - (show_x & 3));
	
	/* Copy image data into appropriate planes in build buffer. */
	for (i = 0; i < SCROLL_Y_DIM; i++) {
		addr[p_off * SCROLL_SIZE] = buf[i];
		if (--p_off < 0) {
		  p_off = 3;
		  addr++;
		}
	}
	
	/* Return success. */
	return 0;
}


/*
 * draw_horiz_line
 *   DESCRIPTION: Draw a horizontal map line into the build buffer.  The 
 *                line should be offset from the top of the logical view 
 *                window screen by the given number of pixels.  
 *   INPUTS: y -- the 0-based pixel row number of the line to be drawn
 *                within the logical view window (equivalent to the number
 *                of pixels from the top pixel to the line to be drawn)
 *   OUTPUTS: none
 *   RETURN VALUE: Returns 0 on success.  If y is outside of the valid 
 *                 SCROLL range, the function returns -1.  
 *   SIDE EFFECTS: draws into the build buffer
 */   
int
draw_horiz_line (int y)
{
    unsigned char buf[SCROLL_X_DIM]; /* buffer for graphical image of line */
    unsigned char* addr;             /* address of first pixel in build    */
   				     /*     buffer (without plane offset)  */
    int p_off;                       /* offset of plane of first pixel     */
    int i;			     /* loop index over pixels             */

    /* Check whether requested line falls in the logical view window. */
    if (y < 0 || y >= SCROLL_Y_DIM)
	return -1;

    /* Adjust y to the logical row value. */
    y += show_y;

    /* Get the image of the line. */
    (*horiz_line_fn) (show_x, y, buf);

    /* Calculate starting address in build buffer. */
    addr = img3 + (show_x >> 2) + y * SCROLL_X_WIDTH;

    /* Calculate plane offset of first pixel. */
    p_off = (3 - (show_x & 3));

    /* Copy image data into appropriate planes in build buffer. */
    for (i = 0; i < SCROLL_X_DIM; i++) {
        addr[p_off * SCROLL_SIZE] = buf[i];
	if (--p_off < 0) {
	    p_off = 3;
	    addr++;
	}
    }

    /* Return success. */
    return 0;
}