  Scaling nature of absorbing critical phenomena is well understood for the
directed percolation (DP) and the directed Ising (DI) systems. However, a full
analysis of the crossover behavior is still lacking, which is of our interest
in this study. There are three different routes from the DI to the DP classes
by introducing a symmetry breaking field (SB), breaking a modulo 2 conservation
(CB), or making channels connecting two equivalent absorbing states (CC). Each
route can be characterized by a crossover exponent, which is found numerically
as $\phi=2.1\pm 0.1$ (SB), $4.6\pm 0.2$ (CB), and $2.9\pm 0.1$ (CC),
respectively. The difference between the SB and CB crossover can be understood
easily in the domain wall language, while the CC crossover involves an
additional critical singularity in the auxiliary field density with the memory
effect to identify itself independent.
