z_prev
info->z_start = z_prev(info, info->z_size << 1, 1);
start = z_prev(info, info->z_start,
z_prev(info, info->z_size << 1, 1);
center = z_prev(info, info->z_start, info->z_size); \
start = z_prev(info, info->z_start, (info->z_size << 1) - 1) * ch; \