When Recurrent Models Don't Need to be Recurrent


An earlier version of this post was published on Off the Convex Path. It is reposted here with the author’s permission.

In the last few years, deep learning practitioners have proposed a litany of different sequence models. Although recurrent neural networks were once the tool of choice, now models like the autoregressive Wavenet or the Transformer are replacing RNNs on a diverse set of tasks. In this post, we explore the trade-offs between recurrent and feed-forward models. Feed-forward models can offer improvements in training stability and speed, while recurrent models are strictly more expressive. Intriguingly, this added expressivity does not seem to boost the performance of recurrent models. Several groups have shown feed-forward networks can match the results of the best recurrent models on benchmark sequence tasks. This phenomenon raises an interesting question for theoretical investigation:

When and why can feed-forward networks replace recurrent neural networks without a loss in performance?

We discuss several proposed answers to this question and highlight our recent work that offers an explanation in terms of a fundamental stability property.


One-Shot Imitation from Watching Videos


Learning a new skill by observing another individual, the ability to imitate, is a key part of intelligence in human and animals. Can we enable a robot to do the same, learning to manipulate a new object by simply watching a human manipulating the object just as in the video below?

The robot learns to place the peach into the red bowl after watching the human do so.


BDD100K Blog Update


We are excited by the interest and excitement generated by our BDD100K dataset. Our data release and blog post were covered in an unsolicited article by the UC Berkeley newspaper, the Daily Cal, which was then picked up by other news services without our prompting or intervention. The paper describing this dataset is under review at the ECCV 2018 conference, and we followed the rules of that conference (as communicated to us by the Program Chairs in prompt email response when we asked for clarification following the reporter’s request; the ECCV PC’s replied that ECCV follows CVPR’s long-standing policy). We thus declined to speak to the reporters after they reached out to us. We did not, and have not, communicated with any media outlets regarding this story.

While the Daily Cal article was accurate; unfortunately, other media outlets who followed in reporting the story made claims that were attributed to us incorrectly, and which do not represent our view. In particular, several media outlets attributed to us a claim that the BDD100K dataset was “800 times” bigger than other industrial datasets, specifically mentioning Baidu’s ApolloScape. While it is true our dataset does contain more raw images than other datasets, including Baidu’s, the stated claim is misleading and we did not put that line or anything like it in a paper, blog post, or spoken comment to anyone. It appears that some reporters(s) viewed the data in tables in our paper and came up with this conclusory comment themselves as it made an exciting headline, yet attributed it to us. In fact, it is inappropriate in our view to summarize the difference between our dataset and Baidu’s in a single comment that ours is 800x larger. Comparing the number of raw images directly is not the most appropriate way to compare these types of datasets.

Importantly, different datasets focus on different aspects of the autonomous driving challenge. Our dataset is crowd-sourced, and covers a very large area and diverse visual phenomena (indeed significantly more diverse than previous efforts, in our view), but it is very clearly limited to monocular RGB image data and associated mobile device metadata. Other dataset collection efforts are complementary in our view. Baidu’s, KITTI, and CityScapes each contain important additional sensing modalities and are collected with fully calibrated apparatus including actuation channels. (The dataset from Mapillary is also notable, and similar to ours in being diverse, crowd-sourced, and densely annotated, but differs in that we include video and dynamic metadata relevant to driving control.) We look forward to projects at Berkeley and elsewhere that leverage both BDD100K and these other datasets as the research community brings the potential of autonomous driving to reality.


BDD100K: A Large-scale Diverse Driving Video Database


Update 06/18/2018: please also check our follow-up blog post after reading this.

TL;DR, we released the largest and most diverse driving video dataset with rich annotations called BDD100K. You can access the data for research now at http://bdd-data.berkeley.edu. We have recently released an arXiv report on it. And there is still time to participate in our CVPR 2018 challenges!


Delayed Impact of Fair Machine Learning


Machine learning systems trained to minimize prediction error may often exhibit discriminatory behavior based on sensitive characteristics such as race and gender. One reason could be due to historical bias in the data. In various application domains including lending, hiring, criminal justice, and advertising, machine learning has been criticized for its potential to harm historically underrepresented or disadvantaged groups.

In this post, we talk about our recent work on aligning decisions made by machine learning with long term social welfare goals. Commonly, machine learning models produce a score that summarizes information about an individual in order to make decisions about them. For example, a credit score summarizes an individual’s credit history and financial activities in a way that informs the bank about their creditworthiness. Let us continue to use the lending setting as a running example.


TDM: From Model-Free to Model-Based Deep Reinforcement Learning


You’ve decided that you want to bike from your house by UC Berkeley to the Golden Gate Bridge. It’s a nice 20 mile ride, but there’s a problem: you’ve never ridden a bike before! To make matters worse, you are new to the Bay Area, and all you have is a good ol’ fashion map to guide you. How do you get started?

Let’s first figure out how to ride a bike. One strategy would be to do a lot of studying and planning. Read books on how to ride bicycles. Study physics and anatomy. Plan out all the different muscle movements that you’ll make in response to each perturbation. This approach is noble, but for anyone who’s ever learned to ride a bike, they know that this strategy is doomed to fail. There’s only one way to learn how to ride a bike: trial and error. Some tasks like riding a bike are just too complicated to plan out in your head.

Once you’ve learned how to ride your bike, how would you get to the Golden Gate Bridge? You could reuse your trial-and-error strategy. Take a few random turns and see if you end up at the Golden Gate Bridge. Unfortunately, this strategy would take a very, very long time. For this sort of problem, planning is a much faster strategy, and requires considerably less real-world experience and trial-and-error. In reinforcement learning terms, it is more sample-efficient.

Left: some skills you learn by trial and error. Right: other times, planning ahead is better.

While simple, this thought experiment highlights some important aspects of human intelligence. For some tasks, we use a trial-and-error approach, and for others we use a planning approach. A similar phenomena seems to have emerged in reinforcement learning (RL). In the parlance of RL, empirical results show that some tasks are better suited for model-free (trial-and-error) approaches, and others are better suited for model-based (planning) approaches.

However, the biking analogy also highlights that the two systems are not completely independent. In particularly, to say that learning to ride a bike is just trial-and-error is an oversimplification. In fact, when learning to bike by trial-and-error, you’ll employ a bit of planning. Perhaps your plan will initially be, “Don’t fall over.” As you improve, you’ll make more ambitious plans, such as, “Bike forwards for two meters without falling over.” Eventually, your bike-riding skills will be so proficient that you can start to plan in very abstract terms (“Bike to the end of the road.”) to the point that all there is left to do is planning and you no longer need to worry about the nitty-gritty details of riding a bike. We see that there is a gradual transition from the model-free (trial-and-error) strategy to a model-based (planning) strategy. If we could develop artificial intelligence algorithms--and specifically RL algorithms--that mimic this behavior, it could result in an algorithm that both performs well (by using trial-and-error methods early on) and is sample efficient (by later switching to a planning approach to achieve more abstract goals).

This post covers temporal difference model (TDM), which is a RL algorithm that captures this smooth transition between model-free and model-based RL. Before describing TDMs, we start by first describing how a typical model-based RL algorithm works.


Shared Autonomy via Deep Reinforcement Learning


A blind, autonomous pilot (left), suboptimal human pilot (center), and combined human-machine team (right) play the Lunar Lander game.

Imagine a drone pilot remotely flying a quadrotor, using an onboard camera to navigate and land. Unfamiliar flight dynamics, terrain, and network latency can make this system challenging for a human to control. One approach to this problem is to train an autonomous agent to perform tasks like patrolling and mapping without human intervention. This strategy works well when the task is clearly specified and the agent can observe all the information it needs to succeed. Unfortunately, many real-world applications that involve human users do not satisfy these conditions: the user's intent is often private information that the agent cannot directly access, and the task may be too complicated for the user to precisely define. For example, the pilot may want to track a set of moving objects (e.g., a herd of animals) and change object priorities on the fly (e.g., focus on individuals who unexpectedly appear injured). Shared autonomy addresses this problem by combining user input with automated assistance; in other words, augmenting human control instead of replacing it.


Towards a Virtual Stuntman


Simulated humanoid performing a variety of highly dynamic and acrobatic skills.

Motion control problems have become standard benchmarks for reinforcement learning, and deep RL methods have been shown to be effective for a diverse suite of tasks ranging from manipulation to locomotion. However, characters trained with deep RL often exhibit unnatural behaviours, bearing artifacts such as jittering, asymmetric gaits, and excessive movement of limbs. Can we train our characters to produce more natural behaviours?


Transfer Your Font Style with GANs


Left: Given movie poster, Right: New movie title generated by MC-GAN.

Text is a prominent visual element of 2D design. Artists invest significant time into designing glyphs that are visually compatible with other elements in their shape and texture. This process is labor intensive and artists often design only the subset of glyphs that are necessary for a title or an annotation, which makes it difficult to alter the text after the design is created, or to transfer an observed instance of a font to your own project.

Early research on glyph synthesis focused on geometric modeling of outlines, which is limited to particular glyph topology (e.g., cannot be applied to decorative or hand-written glyphs) and cannot be used with image input. With the rise of deep neural networks, researchers have looked at modeling glyphs from images. On the other hand, synthesizing data consistent with partial observations is an interesting problem in computer vision and graphics such as multi-view image generation, completing missing regions in images, and generating 3D shapes. Font data is an example that provides a clean factorization of style and content.

Recent advances in conditional generative adversarial networks (cGANS) [1] have been successful in many generative applications. However, they do best only with fairly specialized domains and not with general or multi-domain style transfer. Similarly, when directly used to generate fonts, cGAN models produce significant artifacts. For instance, given the following five letters,

a conditional GAN model is not successful in generating all 26 letters with the same style:


Learning Robot Objectives from Physical Human Interaction


Humans physically interact with each other every day – from grabbing someone’s hand when they are about to spill their drink, to giving your friend a nudge to steer them in the right direction, physical interaction is an intuitive way to convey information about personal preferences and how to perform a task correctly.

So why aren’t we physically interacting with current robots the way we do with each other? Seamless physical interaction between a human and a robot requires a lot: lightweight robot designs, reliable torque or force sensors, safe and reactive control schemes, the ability to predict the intentions of human collaborators, and more! Luckily, robotics has made many advances in the design of personal robots specifically developed with humans in mind.

However, consider the example from the beginning where you grab your friend’s hand as they are about to spill their drink. Instead of your friend who is spilling, imagine it was a robot. Because state-of-the-art robot planning and control algorithms typically assume human physical interventions are disturbances, once you let go of the robot, it will resume its erroneous trajectory and continue spilling the drink. The key to this gap comes from how robots reason about physical interaction: instead of thinking about why the human physically intervened and replanning in accordance with what the human wants, most robots simply resume their original behavior after the interaction ends.

We argue that robots should treat physical human interaction as useful information about how they should be doing the task. We formalize reacting to physical interaction as an objective (or reward) learning problem and propose a solution that enables robots to change their behaviors while they are performing a task according to the information gained during these interactions.