PointPillars: Fast Encoders for Object Detection from Point Clouds

point pillars network.png
point pillars results.png

I’m excited to finally be able to share some of the stuff I have been working on since joining nuTonomy: an Aptiv company. We recently released our paper on PointPillars (with code), a cutting edge method for object detection using point clouds.

First, what problem are we trying to solve? Our company goal is to create the software stack to run a self driving taxi. Specifically, the machine learning team’s charter is to tackle the problems that are too tough to model explicitly. Therefore, our method of choice is deep learning, and we usually work closely with the raw sensor data. Our cars have a 360 degree coverage through multiple lidars, cameras, and radars (check out nuScenes for our actual data!), but of these lidar is the most important sensor. Lidar is a laser ranging sensor that provides sparse, yet accurate, points in the 3D world. These point clouds are the key inputs for 3D object detection since they allow precise localization in the real world.

The ideal deep learning model would incorporate all sensor modalities (lidar, cameras, and radar), but a first step is to separately model each sensor. Images are relatively easy since a multitude of methods exist in the literature, so our research has focused on how to do lidar and radar. I’m going to keep focusing on lidar since it is the main sensor, but everything that follows about PointPillars could equally well be used on radar after a few minor changes as I’ll explain later.

So, what was the state of the art for lidar only object detection when we started our research? There were two main schools of thought that are best represented by PIXOR and VoxelNet. The fundamental difference is how to represent the sparse lidar point cloud. One school of thought (PIXOR, MV3D, …) is to create a set of fixed, hand crafted features. The other school (PointNet, Frustum PointNet, VoxelNet, SECOND) believes in end to end learning and just lets the network learn directly from the point cloud. From a performance and engineering perspective, end to end learning is always better because (1) the network should always be able to match (and usually far exceed) fixed encodings and (2) we let the network do the hard work of finding the encoder, rather than having to devote engineer’s time to discover the right encoding. So we should all do end to end learning!

But there is always a catch. The issue with VoxelNet is that it is too slow to run in realtime. The central problem is that they chose to do end to end learning on voxels. This forces them to use 3D convolutions which are extremely slow. In contrast, PIXOR can just use 2D convolutions which are well optimized for GPU computing. If only there was a way to blend the performance of end to end learning with the speed of fixed encoders.

It turns out, we found a method to do so: PointPillars. The fundamental realization (courtesy of Oscar Beijbom) was that pillars are the best representation. A pillar is a vertical column that can extend infinitely up and down. By learning end to end on pillars, we achieved state of the art detection performance on the KITTI leaderboard at blazing fast speeds (60 to >100 Hz), for a 2-4 fold improvement in runtime.

A few more details on why we are so fast. First, by using pillars, we eliminate 3D convolutions since we immediately learn a 2D representation. Second, we sped up the network by eliminating parameters in the encoder and network. Third, while our initial model for training is in PyTorch, we convert that model to a NVIDIA TensorRT planfile which allows additional optimizations for GPUs.

So where do we go from here? The next step is to work on sensor fusion. First, we need a radar network, which at first glance looks like it might require more work. But it turns out, radar is also a sparse point cloud of range returns. While lidar points return the x, y, z position and reflectance of an object, radar returns the radial range, angular velocity, and a host of other features. So we can just plug in radar point clouds to PointPillar and go! Since the radar returns have worse spatial localization than lidar, it turns out the radar only network doesn’t give great performance. Finally, now that we have separate networks for lidar, images, and radar, it is time to fuse them together! We are actively working on this now and hopefully I can share some of our tricks soon.



QFT Resources

Quantum Field Theory is a notoriously difficult subject to learn, but I found the following resources to be extremely helpful when I took the course a few years ago. I just learned about a few resources that I wish I had then, so here are my current tips for learning QFT. 
Tony Zee’s book QFT in a Nutshell provides a great intuition into what QFT is all about. If you actually want to do calculations, then Peskin and Schroeder’s book is a nice compliment. These two books were the heart of my studies into QFT.
David Tong’s Notes:
Great set of lecture notes that provides a different perspective.
Sidney Coleman’s Lectures:
Apparently, all modern QFT books are based on Coleman (since all the authors learned QFT from him or his students), and you can still see the original videos.  For years there was a set of hand-written notes that served as a transcript of the video but this was recently LaTeXed and shared on the ArXiv.

Deep Learning Seminar Course

This semester Terry Sejnowski is teaching a graduate seminar course that is focused on Deep Learning. The course meets weekly for two hours to discuss papers. Here I’ll just outline the course and in later posts I’ll add some thoughts on each specific week.

Week 1: Perceptrons

Week 2: Hopfield Nets and Boltzmann Machines

Week 3: Backprop

Week 4: Independent Component Analysis (ICA)

Week 5: Convolutional Neural Networks (CNN)

Week 6: Recurrent Neural Networks (RNN)

Week 7: Reinforcement Learning

Week 8: Information and Control Theory

Deep Learning in Python

So maybe after reading some of my past posts, you are fired up to start programming a deep neural network in Python. How should you get started?

If you want to be able to run anything but the simplest neural networks on easy problems, you will find that since pure Python is an interpreted language, it is too slow. Does that mean we have to give up and write our own C++ code? Luckily GPUs and other programmers come to your rescue by offering between 5-100X speedup (I would estimate my average speedup at 10X, but it varies for specific tasks).

There are two main Python packages, Theano and TensorFlow, that are designed to let you write Python code that can either run on a CPU or a GPU. In essence, they are each their own mini-language with the following changes from standard Python:

  • Tensors (generalizations of matrices) are the primary variable type and treated as abstract mathematical objects (don’t need to specify actual values immediately).
  • Computational graphs are utilized to organize operations on the tensors.
  • When one wants to actually evaluate the graph on some data, it is stored in a shared variable that when possible gets sent to the GPU. This data is then processed by the graph (in place of the original tensor placeholders).
  • Automatic differentiation (ie it understands derivatives symbolically).
  • Built in numerical optimizations.

So to get started you will want to install either Theano (pip install theano), TensorFlow (details here), or both. I personally have only used Theano, but if Google keeps up the developmental progress of TensorFlow, I may end up switching to it.

At the end of the day, that means that if one wants to actually implement neural networks in Theano or TensorFlow, you essentially will learn another language. However, people have built various libraries that are abstractions on top of these mini-languages. Lasagne is one example that basically organizes Theano code so that you have to interact less with Theano, but you will still need to understand Theano. I initially started with Theano and Lasagne, but I am now a convert to Keras.

Instead, I advocate for Keras (pip install keras) for two major reasons:

  1.  High level abstraction. You can write standard Python code and get a deep neural network up and running very quickly.
  2. Back-end agnostic. Keras can run on either Theano or TensorFlow.

So it seems like a slam dunk right? Unfortunately life is never that simple, instead there are two catches:

  1. Mediocre documentation (using Numpy as a gold standard, or even comparing to Lasagne). You can get the standard things up and running based on theirs docs. But if you want to do anything advanced, you will find yourself looking into their source code on GitHub, which has some hidden, but useful, comments.
  2. Back-end agnostic. This means if you do want to introduce a modification to the back-end, and you want it to always work in Keras, you need to implement it in both Theano and TensorFlow. In practice this isn’t too bad since Keras has done a good job of implementing low-end operations.

Fortunately, the pros definitely outweigh the cons for Keras and I highly endorse it. Here are a few tips I have learned from my experience with Keras:

  • Become familiar with the Keras documentation.
  • I recommend only using the functional API which allows you to implement more complicated networks. The sequential API allows you to write simple models in fewer lines of code, but you lose flexibility (for example, you can’t access intermediate layers) and the code won’t generalize to complex models. So just embrace the functional API.
  • Explore the examples (here and here).
  • Check out the Keras GitHub.
  • Names for layers are optional keywords, but definitely use them! It will significantly help you when you are debugging.

Now start coding your own deep neural networks!