  Although current deep learning-based face forgery detectors achieve
impressive performance in constrained scenarios, they are vulnerable to samples
created by unseen manipulation methods. Some recent works show improvements in
generalisation but rely on cues that are easily corrupted by common
post-processing operations such as compression. In this paper, we propose
LipForensics, a detection approach capable of both generalising to novel
manipulations and withstanding various distortions. LipForensics targets
high-level semantic irregularities in mouth movements, which are common in many
generated videos. It consists in first pretraining a spatio-temporal network to
perform visual speech recognition (lipreading), thus learning rich internal
representations related to natural mouth motion. A temporal network is
subsequently finetuned on fixed mouth embeddings of real and forged data in
order to detect fake videos based on mouth movements without overfitting to
low-level, manipulation-specific artefacts. Extensive experiments show that
this simple approach significantly surpasses the state-of-the-art in terms of
generalisation to unseen manipulations and robustness to perturbations, as well
as shed light on the factors responsible for its performance. Code is available
on GitHub.
