/************************************************************************/
/* Transfer source block of dimension DX x DY from XS,YS                */
/* to XD, YD                                                            */
/************************************************************************/

slow_bitblt(xs, ys, xd, yd, dx, dy, fn)
int     xs, ys;                         /* Source upper left corner     */
int     xd, yd;                         /* Destination upper left       */
int     dx, dy;                         /* Dimensions of the block      */
int     fn;                             /* Logical operation            */
        {
        #define COPY    0
        #define CLEAR   1
        #define OR      2
        #define XOR     3
        #define AND     4
        int     x_incr, y_incr, x_max, y_max, x, y, i, j, value;

        /****************************************************************/
        /* Compute starting points and directions of transfer           */
        /****************************************************************/

        x = 0;                          /* Assume no overlap or ++      */
        y = 0;                          /* i.e. xs > xd && ys > yd      */
        x_incr = 1;
        y_incr = 1;

        if (xs <= xd && xs + dx >= xd)  /* Reverse x direction if source*/
                {                       /* overlaps left half of dest.  */
                x = dx - 1;
                x_incr = -1;
                }

        if (ys > yd && ys <= yd + dy)   /* Revers y direction if source */
                {                       /* overlaps bottom half of dest */
                y = dy - 1;
                y_incr = -1;
                }

        /****************************************************************/
        /* Transfer the block                                           */
        /****************************************************************/

        for (j = 0; j < dy; j++)                /* Loop over rasters    */
                {
                for (i = 0; i < dx; i++)        /* Loop over pixels     */
                        {
                        switch (fn)             /* Select logical op    */
                                {
                                case COPY:
                                        value = pixel_read(xs+x,ys+y);
                                        pixel_write(xd+x,yd+y,value);
                                        break;
                                case CLEAR:
                                        value = 0;
                                        pixel_write(xd+x,yd+y,value);
                                        break;
                                case OR:
                                        value = pixel_read(xs+x,ys+y) |
                                                pixel_read(xd+x,yd+x);
                                        pixel_write(xd+x,yd+y,value);
                                        break;
                                case XOR:
                                        value = pixel_read(xs+x,ys+y) ^
                                                pixel_read(xd+x,yd+x);
                                        pixel_write(xd+x,yd+y,value);
                                        break;
                                case AND:
                                        value = pixel_read(xs+x,ys+y) &
                                                pixel_read(xd+x,yd+x);
                                        pixel_write(xd+x,yd+y,value);
                                        break;
                                default:
                                        break;
                                }
                        x += x_incr;            /* Update pixel pointer */
                        }
                y += y_incr;                    /* Update raster pointer*/
                x -= dx * x_incr;               /* Reset pixel pointer  */
                }
        }