  Remote photoplethysmography (rPPG), which aims at measuring heart activities
and physiological signals from facial video without any contact, has great
potential in many applications (e.g., remote healthcare and affective
computing). Recent deep learning approaches focus on mining subtle rPPG clues
using convolutional neural networks with limited spatio-temporal receptive
fields, which neglect the long-range spatio-temporal perception and interaction
for rPPG modeling. In this paper, we propose two end-to-end video transformer
based architectures, namely PhysFormer and PhysFormer++, to adaptively
aggregate both local and global spatio-temporal features for rPPG
representation enhancement. As key modules in PhysFormer, the temporal
difference transformers first enhance the quasi-periodic rPPG features with
temporal difference guided global attention, and then refine the local
spatio-temporal representation against interference. To better exploit the
temporal contextual and periodic rPPG clues, we also extend the PhysFormer to
the two-pathway SlowFast based PhysFormer++ with temporal difference periodic
and cross-attention transformers. Furthermore, we propose the label
distribution learning and a curriculum learning inspired dynamic constraint in
frequency domain, which provide elaborate supervisions for PhysFormer and
PhysFormer++ and alleviate overfitting. Comprehensive experiments are performed
on four benchmark datasets to show our superior performance on both intra- and
cross-dataset testings. Unlike most transformer networks needed pretraining
from large-scale datasets, the proposed PhysFormer family can be easily trained
from scratch on rPPG datasets, which makes it promising as a novel transformer
baseline for the rPPG community.
