  We revisit residual algorithms in both model-free and model-based
reinforcement learning settings. We propose the bidirectional target network
technique to stabilize residual algorithms, yielding a residual version of DDPG
that significantly outperforms vanilla DDPG in the DeepMind Control Suite
benchmark. Moreover, we find the residual algorithm an effective approach to
the distribution mismatch problem in model-based planning. Compared with the
existing TD($k$) method, our residual-based method makes weaker assumptions
about the model and yields a greater performance boost.
