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NVIDIA researchers will present their paper “An Unbiased Ray-Marching Transmittance Estimator” at SIGGRAPH 2021, August 9-13, showing a new way to compute visibility in scenes with complex volumetric effects.

NVIDIA researchers will present their paper “An Unbiased Ray-Marching Transmittance Estimator” at SIGGRAPH 2021, August 9-13, showing a new way to compute visibility in scenes with complex volumetric effects.

When Monte Carlo sampling is used to compute lighting effects such as soft shadows and global illumination, shadow rays are used to query the visibility between lights and surfaces.  In many scenes, visibility is a simple binary answer that is efficiently queried using the RTX RT Cores.  However, for volumetric effects such as clouds, smoke and explosions, visibility is a fractional quantity ranging from 0.0 to 1.0, and computing this quantity efficiently and accurately between any two points in a scene is an essential part of photorealistic real-time rendering.  Visibility that accounts for volumetric effects is also called transmittance.

We present a new algorithm, which we call unbiased ray marching, for evaluating this transmittance in general scenes. Our key result is a new statistically-unbiased Monte Carlo method that randomly queries the volume densities along a ray and computes a visibility estimate from these values.  For high-performance rendering, the goal is to compute an accurate low-noise estimate while minimizing the number of times the volume data needs to be  accessed.  

The research paper provides a new efficiency analysis of the transmittance estimation problem. This yields a number of key insights about when and how various transmittance estimators achieve the optimal balance of low noise and low cost.  Building on these insights, and leveraging previous work from graphics, physics and statistics, we derive a new transmittance estimator that is universally optimal relative to 50 years of prior art, and often ten times more efficient than earlier methods.  Technically, the end result is based on a power-series expansion of the exponential function that appears in the transmittance equation, and 90% of the time our new estimator simply evaluates the first term in this expansion. This first term corresponds to the traditional biased ray-marching algorithm. The subsequent terms of the expansion, which our estimator evaluates 10% of the time, correct for the bias.  This key insight allows us to benefit from the efficiency of ray marching without being plagued by its bias – occasional light-leaking artifacts seen as, for example, overly bright clouds.

The new method helps to reduce the noise of shadows, such as the one cast on the floor by the smoke plume in the above figure where we see a dramatic reduction in noise at the same render time.

Putting a new twist on an old method, unbiased ray marching virtually eliminates a common source of noise in path-traced renderings of scenes with rich volumetric effects.

Learn more: Check out the project website.

In this month’s technical digest we’re highlighting this powerful capability and offer a collection of resources to help ROS users familiarize with the power of Jetson, ISAAC SDK, ISAAC Sim and a success story from NVIDIA Inception Member, Aerobotics.

While you are likely already familiar with Jetson as the NVIDIA AI platform for edge AI, you might be surprised to learn that the power of Jetson isn’t limited to solutions built entirely on Jetson platforms. Jetpack on Jetson and Isaac SDK, our robotics platform, adds AI capabilities to existing autonomous machines and bridges dependencies for diverse hardware and software solutions.

In this month’s technical digest, we’re highlighting this powerful capability and offering a collection of resources to help Robotic Operating System (ROS) users get familiar with the power of Jetson, Isaac SDK, Isaac Sim, and a success story on a startup in the NVIDIA Inception program.

Jetson and ROS tutorials

Accelerating AI Modules for ROS and ROS 2 on NVIDIA Jetson Platform
A showcase of support for ROS and ROS 2 on NVIDIA Jetson developer kits.
Read now >

Building Robotics Applications Using ROS and NVIDIA Isaac SDK
Learn how to use a ROS application stack with NVIDIA Isaac and ISAAC Sim.
Read now >  

Implementing Robotics Applications with ROS 2 and AI on the NVIDIA Jetson Platform
Learn how to perform classification, object detection and pose estimation with ROS 2 on Jetson.
Read now >

Top 5 Jetson resources

Meet Jetson, the platform for AI at the edge
NVIDIA Jetson is used by professional developers to create breakthrough AI products across all industries, and by students and enthusiasts for hands-on AI learning and making amazing projects.
Meet Jetson >

Getting started with Jetson Developer Kits
Jetson developer kits are used by professionals to develop and test software for products based on Jetson modules, and by students and enthusiasts for projects and learning. Each developer kit includes a non-production specification Jetson module attached to a reference carrier board with standard hardware interfaces for flexible development and rapid prototyping.
Learn more >

Free 8-hour class on getting started with AI on Jetson Nano
In this course, you’ll use Jupyter iPython notebooks on your own Jetson Nano to build a deep learning classification project with computer vision models. Upon completion, you receive a certificate.
Start learning hands-on today >

Jetson and Embedded Systems developer forums
The Jetson and Embedded Systems community is active, vibrant, and continually monitored by the Jetson Product and Engineering teams.
Meet the community >

Jetson community projects
Explore and learn from Jetson projects created by us and our community.
Explore community projects >

Recommended hardware

Inception spotlight

Bilberry: Using Jetson for Computer Vision for Crop Surveying
In 2016, the former dormmates at École Nationale Supérieure d’Arts et Métiers in Paris, founded Bilberry. The startup developed a solution to recognize weeds powered by the NVIDIA Jetson edge AI platform for precision application of herbicides at corn and wheat farms, offering as much as a 92% reduction in herbicide usage.
Learn more >

Do you have a startup? Join NVIDIA Inception’s global network of over 7,500 AI and data science startups.

Built with Enterprise IT Partners and Deployed First at Equinix, NVIDIA AI LaunchPad Includes End-to-End NVIDIA Hardware and Software Stack to Accelerate AI from Hybrid Cloud to EdgeSANTA …

SaaS Platform Helps Leading Device Makers, Factories, Warehouses and Retailers Deploy AI Products and ServicesSANTA CLARA, Calif., June 22, 2021 (GLOBE NEWSWIRE) — NVIDIA today announced …

Tackling one of the largest computing challenges of this lifetime requires larger than life computing. At CVPR this week, Andrej Karpathy, senior director of AI at Tesla, unveiled the in-house supercomputer the automaker is using to train deep neural networks for Autopilot and self-driving capabilities. The cluster uses 720 nodes of 8x NVIDIA A100 Tensor Read article >

The post Tesla Unveils Top AV Training Supercomputer Powered by NVIDIA A100 GPUs appeared first on The Official NVIDIA Blog.

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In NVIDIA Clara Train 4.0, we added homomorphic encryption (HE) tools for federated learning (FL). HE enables you to compute data while the data is still encrypted. In Clara Train 3.1, all clients used certified SSL channels to communicate their local model updates with the server. The SSL certificates are needed to establish trusted communication … Continued

In NVIDIA Clara Train 4.0, we added homomorphic encryption (HE) tools for federated learning (FL). HE enables you to compute data while the data is still encrypted.

In Clara Train 3.1, all clients used certified SSL channels to communicate their local model updates with the server. The SSL certificates are needed to establish trusted communication channels and are provided through a third party that runs the provisioning tool and securely distributes them to the hospitals. This secures the communication to the server, but the server can still see the raw model (unencrypted) updates to do aggregation.

With Clara Train 4.0, the communication channels are still established using SSL certificates and the provisioning tool. However, each client optionally also receives additional keys to homomorphically encrypt their model updates before sending them to the server. The server doesn’t own a key and only sees the encrypted model updates.

With HE, the server can aggregate these encrypted weights and then send the updated model back to the client. The clients can decrypt the model weights because they have the keys and can then continue with the next round of training (Figure 1).

HE ensures that each client’s changes to the global model stays hidden by preventing the server from reverse-engineering the submitted weights and discovering any training data. This added security comes at a computational cost on the server. However, it can play an important role in healthcare in making sure that patient data stays secure at each hospital while still benefiting from using federated learning with other institutions.

HE implementation in Clara Train 4.0

We implemented secure aggregation during FL with HE using the TenSEAL library by OpenMined, a convenient Python wrapper around Microsoft SEAL. Both libraries are available as open-source and provide an implementation of Homomorphic encryption for arithmetic of approximate numbers, also known as the CKKS scheme, which was proposed as a solution for encrypted machine learning.

Our default uses the following HE setting, specified in Clara Train’s provisioning tool for FL:

# homomorphic encryption
he:
  lib: tenseal
  config:
    poly_modulus_degree: 8192
    coeff_mod_bit_sizes: [60, 40, 40]
    scale_bits: 40
    scheme: CKKS 

These settings are recommended and should work for most tasks but could be further optimized depending on your specific model architecture and machine learning problem. For more information about different settings, see this tutorial on the CKKS scheme and benchmarking.

HE benchmarking in FL

To compare the impact of HE to the overall training time and performance, we ran the following experiments. We chose SegResNet (a U-Net like architecture used to win the BraTS 2018 challenge) trained on the CT spleen segmentation task from the Medical Segmentation Decathlon.

Each federated learning run was trained for 100 rounds with each client training for 10 local epochs on their local data on an NVIDIA V100 (server and clients are running on localhost). In each run, half of the clients each used half of the training data (16/16) and half of the validation data (5/4), respectively. We recorded the total training time and best average validation dice score of the global model. We show the relative increase added by HE in Table 1.

There is a moderate increase in total training time of about 20% when encrypting the full model. This increase in training time is due to the added encryption and decryption steps and aggregation in homomorphically encrypted space. Our implementation enables you to reduce that extra time by only encrypting a subset of the model parameters, for example, all convolutional layers (“conv”). You could also encrypt just three of the key layers, such as the input, middle, and output layers.

The added training time is also due to increased message sizes needed to send the encrypted model gradient updates, requiring longer upload times. For SegResNet, we observe an increase from 19 MB to 283 MB using the HE setting mentioned earlier (~15x increase).

Next, we compare the performance of FL using up to 30 clients with the server running on AWS. For reference, we used an m5a.2xlarge with eight vCPUs, 32-GB memory, and up to 2,880 Gbps network bandwidth. We show the average encryption, decryption, and upload time, comparing raw compared to encrypted model gradients being uploaded in Figure 2 and Table 2. You can see the longer upload times due to the larger message sizes needed by HE.

Try it out

If you’re interested in learning more about how to set up FL with homomorphic encryption using Clara Train, we have a great Jupyter notebook on GitHub that walks you through the setup.

HE can reduce model inversion or data leakage risks if there is a malicious or compromised server. However, your final models might still contain or memorize privacy-relevant information. That’s where differential privacy methods can be a useful addition to HE. Clara Train SDK implements the sparse vector technique (SVT) and partial model sharing that can help preserve privacy. For more information, see Privacy-preserving Federated Brain Tumour Segmentation. Keep in mind that there is a tradeoff between model performance and privacy protection.

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