Direct Prediction of 3D Body Poses from Motion Compensated Sequences
We propose an efficient approach to exploiting motion information from consecutive frames of a video sequence to recover the 3D pose of people. Previous approaches typically compute candidate poses in individual frames and then link them in a post-processing step to resolve ambiguities. By contrast, we directly regress from a spatio-temporal volume of bounding boxes to a 3D pose in the central frame.
We further show that, for this approach to achieve its full potential, it is essential to compensate for the motion in consecutive frames so that the subject remains centered. This then allows us to effectively overcome ambiguities and improve upon the state-of-the-art by a large margin on the Human3.6m, HumanEva, and KTH Multiview Football 3D human pose estimation benchmarks.
In recent years, impressive motion capture results have been demonstrated using depth cameras, but 3D body pose recovery from ordinary monocular video sequences remains extremely challenging. Nevertheless, there is great interest in doing so, both because cameras are becoming ever cheaper and more prevalent and because there are many potential applications. These include athletic training, surveillance, and entertainment.
Early approaches to monocular 3D pose tracking involved recursive frame-to-frame tracking and were found to be brittle, due to distractions and occlusions from other people or objects in the scene [Urtasun05b]. Since then, the focus has shifted to “tracking by detection,” which involves detecting human pose more or less independently in every frame followed by linking the poses across the frames [Andriluka10, Ramanan05], which is much more robust to algorithmic failures in isolated frames. More recently, an effective single-frame approach to learning a regressor from a kernel embedding of 2D HOG features to 3D poses has been proposed [Ionescu14a]. Excellent results have also been reported using a Convolutional Neural Net [Li14a].
However, inherent ambiguities of the projection from 3D to 2D, including self-occlusion and mirroring, can still confuse these state-of-the-art approaches. A linking procedure can correct for these ambiguities to a limited extent by exploiting motion information a posteriori to eliminate erroneous poses by selecting compatible candidates over consecutive frames. However, when such errors happen frequently for several frames in a row, enforcing temporal consistency afterwards is not enough.