VTBUF_MAX_HEIGHT
((vb)->vb_rows[((vb)->vb_roffset + (r)) % VTBUF_MAX_HEIGHT(vb)])
((vb)->vb_rows[((vb)->vb_roffset + (r)) % VTBUF_MAX_HEIGHT(vb)][(c)])
((vb)->vb_rows[((vb)->vb_curroffset + (r)) % VTBUF_MAX_HEIGHT(vb)][(c)])
VTBUF_MAX_HEIGHT(vb)];
VTBUF_MAX_HEIGHT(vb)];