  Simple tree models for articulated objects prevails in the last decade.
However, it is also believed that these simple tree models are not capable of
capturing large variations in many scenarios, such as human pose estimation.
This paper attempts to address three questions: 1) are simple tree models
sufficient? more specifically, 2) how to use tree models effectively in human
pose estimation? and 3) how shall we use combined parts together with single
parts efficiently?
  Assuming we have a set of single parts and combined parts, and the goal is to
estimate a joint distribution of their locations. We surprisingly find that no
latent variables are introduced in the Leeds Sport Dataset (LSP) during
learning latent trees for deformable model, which aims at approximating the
joint distributions of body part locations using minimal tree structure. This
suggests one can straightforwardly use a mixed representation of single and
combined parts to approximate their joint distribution in a simple tree model.
As such, one only needs to build Visual Categories of the combined parts, and
then perform inference on the learned latent tree. Our method outperformed the
state of the art on the LSP, both in the scenarios when the training images are
from the same dataset and from the PARSE dataset. Experiments on animal images
from the VOC challenge further support our findings.
