  In this work we address the challenging problem of 3D human pose estimation
from single images. Recent approaches learn deep neural networks to regress 3D
pose directly from images. One major challenge for such methods, however, is
the collection of training data. Specifically, collecting large amounts of
training data containing unconstrained images annotated with accurate 3D poses
is infeasible. We therefore propose to use two independent training sources.
The first source consists of accurate 3D motion capture data, and the second
source consists of unconstrained images with annotated 2D poses. To integrate
both sources, we propose a dual-source approach that combines 2D pose
estimation with efficient 3D pose retrieval. To this end, we first convert the
motion capture data into a normalized 2D pose space, and separately learn a 2D
pose estimation model from the image data. During inference, we estimate the 2D
pose and efficiently retrieve the nearest 3D poses. We then jointly estimate a
mapping from the 3D pose space to the image and reconstruct the 3D pose. We
provide a comprehensive evaluation of the proposed method and experimentally
demonstrate the effectiveness of our approach, even when the skeleton
structures of the two sources differ substantially.
