Learning Acrobatics by Watching YouTube


Simulated characters imitating skills from YouTube videos.

Whether it’s everyday tasks like washing our hands or stunning feats of
acrobatic prowess, humans are able to learn an incredible array of skills by
watching other humans. With the proliferation of publicly available video data
from sources like YouTube, it is now easier than ever to find video clips of
whatever skills we are interested in. A staggering 300 hours of videos are
uploaded to YouTube every minute. Unfortunately, it is still very challenging
for our machines to learn skills from this vast volume of visual data. Most
imitation learning approaches require concise representations, such as those
recorded from motion capture (mocap). But getting mocap data can be quite a
hassle, often requiring heavy instrumentation. Mocap systems also tend to be
restricted to indoor environments with minimal occlusion, which can limit the
types of skills that can be recorded. So wouldn’t it be nice if our agents can
also learn skills by watching video clips?

In this work, we present a framework for learning skills from videos (SFV). By
combining state-of-the-art techniques in computer vision and reinforcement
learning
, our system enables simulated characters to learn a diverse
repertoire of skills from video clips. Given a single monocular video of an
actor performing some skill, such as a cartwheel or a backflip, our characters
are able to learn policies that reproduce that skill in a physics simulation,
without requiring any manual pose annotations.

[youtube https://www.youtube.com/watch?v=4Qg5I5vhX7Q?rel=0]

The problem of learning full-body motion skills from videos has received some
attention in computer graphics. Previous techniques
often rely on manually-crafted control structures that impose strong
restrictions on the behaviours that can be produced. Therefore, these methods
tend to be limited in the types of skills that can be learned, and the resulting
motions can look fairly unnatural. More recently, deep learning techniques have
demonstrated promising results for visual imitation on domains such as Atari and fairly simple robotics tasks. But these tasks
often only have modest domain shifts between the demonstrations and the agent’s
environment, and results on continuous control have largely been on tasks with
relatively simple dynamics.

Framework

Our framework is structured as a pipeline, consisting of three stages: pose
estimation, motion reconstruction, and motion imitation. The input video is
first processed by the pose estimation stage, which predicts the pose of the
actor in each frame. Next, the motion reconstruction stage consolidates the pose
predictions into a reference motion and fixes artifacts that might have been
introduced by the pose predictions. Finally, the reference motion is passed to
the motion imitation stage, where a simulated character is trained to imitate
the motion using reinforcement learning.


The pipeline consists of three stages: pose estimation, motion reconstruction,
and motion imitation. It receives as input, a video clip of an actor performing
a particular skill and a simulated character model, and learns a control policy
that enables the character to reproduce the skill in a physics simulation.

Pose Estimation

Given a video clip, we use a vision-based pose estimator to predict the actor’s
pose $hatq_t$ in each frame. The pose estimator is built on the work from human mesh recovery, which uses a
weakly-supervised adversarial approach to train a pose estimator to predict
poses from monocular images. While pose annotations are required to train the
pose estimator, once trained, the pose estimator can be applied to new images
without any annotations.


A vision-based pose estimator is used to predict the pose of the actor in each video frame.

Motion Reconstruction

Since the pose estimator predicts the pose of the actor independently for each
video frame, the predictions between frames can be inconsistent, resulting in
jittery artifacts. Furthermore, while vision-based pose estimators have improved
substantially in recent years, they can still occasionally make some pretty big
mistakes, which can result in peculiar poses popping up every now and then.
These artifacts can produce motions that are physically impossible to imitate.
Therefore, the role of the motion reconstruction stage is to mitigate these
artifacts in order to produce a more physically-plausible reference motion that
will be easier for the simulated character to imitate. To do this, we optimize a
new reference motion hatQ = hatq_0, hatq_1, ldots, hatq_t to
satisfy the following objective:

min_hatQ ; ; w_p l_p(hatQ) + w_sm l_sm(hatQ)

where $l_p(hatQ)$ encourages the reference motion to be similar to the
original pose predictions, and $l_sm(hatQ)$ encourages the poses in
adjacent frames to be similar in order to produce a smoother motion. In
addition, $w_p$ and $w_sm$ are the weights for the different losses.

This procedure can substantially improve the quality of the reference motion,
and can fix a lot of the artifacts from the original pose predictions.


Comparison of reference motions before and after motion reconstruction. Motion
reconstruction mitigates many of the artifacts and produces a smoother reference
motion.

Motion Imitation

Once we have the reference motion \hatq_0, hatq_1, ldots,
hatq_T, we can then proceed to training a simulated character to imitate
the skill. The motion imitation stage uses a similar RL approach to the one we
previously proposed for imitating mocap
data
. The reward function simply encourage the policy to minimize the
difference between the pose of the simulated character and the pose of the
reconstructed reference motion $hatq_t$ at each frame $t$,

r_t = exp Big(-2 |hatq_t-q_t|^2 Big).

Again, this simple approach ends up working surprisingly well, and our
characters are able to learn a diverse repertoire of challenging acrobatic
skills, where each skill is learned from a single video demonstration.


Simulated humanoids learn to perform a diverse array of skills by imitating video clips.

Results

In total, our characters are able to learn over 20 different skills from various
video clips collected from YouTube.


Our framework can learn a large repertoire of skills from video demonstrations.

Even though the morphology of our characters are often quite different from the
actors in the videos, the policies are still able to closely reproduce many of
the skills. As an example of more extreme morphological differences, we can also
train a simulated Atlas robot to imitate video clips of humans.


Simulated humanoid learns to perform a diverse array of skills by imitating video clips.

One of the advantages of having a simulated character is that we can leverage
the simulation to generalize the behaviours to new environments. Here we have
simulated characters that learn to adapt motions to irregular terrain, where the
original video clips were recorded from actors on flat ground.


Motions can be adapted to irregular environments.

Even though the environments are quite different from those in the original
videos, the learning algorithm still develops fairly plausible strategies for
handling these new environments.

All in all, our framework is really just taking the most obvious approach that
anyone can think of when tackling the problem of video imitation. The key is in
decomposing the problem into more manageable components, picking the right
methods for those components, and integrating them together effectively.
However, imitating skills from videos is still an extremely challenging problem,
and there are plenty of video clips that we are not yet able to reproduce:


Nimble dance steps, such as this Gangnam style clip, can still be difficult to imitate.

But it is encouraging to see that just by integrating together existing
techniques, we can already get pretty far on this challenging problem. We still
have all of our work ahead of us, and we hope that this work will help inspire
future techniques that will enable agents to take advantage of the massive
volume of publicly available video data to acquire a truly staggering array of
skills.

To learn more, check out our paper and the project webpage.

We would like to thank the co-authors of this work, without whom none of this
would have been possible: Jitendra Malik, Pieter Abbeel, and Sergey Levine. This
research was funded by NSERC, UC Berkeley, BAIR, and AWS.