  The rise of variational autoencoders for image and video compression has
opened the door to many elaborate coding techniques. One example here is the
possibility of conditional interframe coding. Here, instead of transmitting the
residual between the original frame and the predicted frame (often obtained by
motion compensation), the current frame is transmitted under the condition of
knowing the prediction signal. In practice, conditional coding can be
straightforwardly implemented using a conditional autoencoder, which has also
shown good results in recent works. In this paper, we provide an information
theoretical analysis of conditional coding for inter frames and show in which
cases gains compared to traditional residual coding can be expected. We also
show the effect of information bottlenecks which can occur in practical video
coders in the prediction signal path due to the network structure, as a
consequence of the data-processing theorem or due to quantization. We
demonstrate that conditional coding has theoretical benefits over residual
coding but that there are cases in which the benefits are quickly canceled by
small information bottlenecks of the prediction signal.
