  One-shot talking head generation has no explicit head movement reference,
thus it is difficult to generate talking heads with head motions. Some existing
works only edit the mouth area and generate still talking heads, leading to
unreal talking head performance. Other works construct one-to-one mapping
between audio signal and head motion sequences, introducing ambiguity
correspondences into the mapping since people can behave differently in head
motions when speaking the same content. This unreasonable mapping form fails to
model the diversity and produces either nearly static or even exaggerated head
motions, which are unnatural and strange. Therefore, the one-shot talking head
generation task is actually a one-to-many ill-posed problem and people present
diverse head motions when speaking. Based on the above observation, we propose
OSM-Net, a \textit{one-to-many} one-shot talking head generation network with
natural head motions. OSM-Net constructs a motion space that contains rich and
various clip-level head motion features. Each basis of the space represents a
feature of meaningful head motion in a clip rather than just a frame, thus
providing more coherent and natural motion changes in talking heads. The
driving audio is mapped into the motion space, around which various motion
features can be sampled within a reasonable range to achieve the one-to-many
mapping. Besides, the landmark constraint and time window feature input improve
the accurate expression feature extraction and video generation. Extensive
experiments show that OSM-Net generates more natural realistic head motions
under reasonable one-to-many mapping paradigm compared with other methods.
