Posts in category: Google AI

Posted by Jimmy Tobin and Katrin Tomanek, Software Engineers, Google Research, AI for Social Good

Google’s AI for Social Good team consists of researchers, engineers, volunteers, and others with a shared focus on positive social impact. Our mission is to demonstrate AI’s societal benefit by enabling real-world value, with projects spanning work in public health, accessibility, crisis response, climate and energy, and nature and society. We believe that the best way to drive positive change in underserved communities is by partnering with change-makers and the organizations they serve.

In this blog post we discuss work done by Project Euphonia, a team within AI for Social Good, that aims to improve automatic speech recognition (ASR) for people with disordered speech. For people with typical speech, an ASR model’s word error rate (WER) can be less than 10%. But for people with disordered speech patterns, such as stuttering, dysarthria and apraxia, the WER could reach 50% or even 90% depending on the etiology and severity. To help address this problem, we worked with more than 1,000 participants to collect over 1,000 hours of disordered speech samples and used the data to show that ASR personalization is a viable avenue for bridging the performance gap for users with disordered speech. We’ve shown that personalization can be successful with as little as 3-4 minutes of training speech using layer freezing techniques.

This work led to the development of Project Relate for anyone with atypical speech who could benefit from a personalized speech model. Built in partnership with Google’s Speech team, Project Relate enables people who find it hard to be understood by other people and technology to train their own models. People can use these personalized models to communicate more effectively and gain more independence. To make ASR more accessible and usable, we describe how we fine-tuned Google’s Universal Speech Model (USM) to better understand disordered speech out of the box, without personalization, for use with digital assistant technologies, dictation apps, and in conversations.

Addressing the challenges

Working closely with Project Relate users, it became clear that personalized models can be very useful, but for many users, recording dozens or hundreds of examples can be challenging. In addition, the personalized models did not always perform well in freeform conversation.

To address these challenges, Euphonia’s research efforts have been focusing on speaker independent ASR (SI-ASR) to make models work better out of the box for people with disordered speech so that no additional training is necessary.

Prompted Speech dataset for SI-ASR

The first step in building a robust SI-ASR model was to create representative dataset splits. We created the Prompted Speech dataset by splitting the Euphonia corpus into train, validation and test portions, while ensuring that each split spanned a range of speech impairment severity and underlying etiology and that no speakers or phrases appeared in multiple splits. The training portion consists of over 950k speech utterances from over 1,000 speakers with disordered speech. The test set contains around 5,700 utterances from over 350 speakers. Speech-language pathologists manually reviewed all of the utterances in the test set for transcription accuracy and audio quality.

Real Conversation test set

Unprompted or conversational speech differs from prompted speech in several ways. In conversation, people speak faster and enunciate less. They repeat words, repair misspoken words, and use a more expansive vocabulary that is specific and personal to themselves and their community. To improve a model for this use case, we created the Real Conversation test set to benchmark performance.

The Real Conversation test set was created with the help of trusted testers who recorded themselves speaking during conversations. The audio was reviewed, any personally identifiable information (PII) was removed, and then that data was transcribed by speech-language pathologists. The Real Conversation test set contains over 1,500 utterances from 29 speakers.

Adapting USM to disordered speech

We then tuned USM on the training split of the Euphonia Prompted Speech set to improve its performance on disordered speech. Instead of fine-tuning the full model, our tuning was based on residual adapters, a parameter-efficient tuning approach that adds tunable bottleneck layers as residuals between the transformer layers. Only these layers are tuned, while the rest of the model weights are untouched. We have previously shown that this approach works very well to adapt ASR models to disordered speech. Residual adapters were only added to the encoder layers, and the bottleneck dimension was set to 64.

Results

To evaluate the adapted USM, we compared it to older ASR models using the two test sets described above. For each test, we compare adapted USM to the pre-USM model best suited to that task: (1) For short prompted speech, we compare to Google’s production ASR model optimized for short form ASR; (2) for longer Real Conversation speech, we compare to a model trained for long form ASR. USM improvements over pre-USM models can be explained by USM’s relative size increase, 120M to 2B parameters, and other improvements discussed in the USM blog post.

Model word error rates (WER) for each test set (lower is better).

We see that the USM adapted with disordered speech significantly outperforms the other models. The adapted USM’s WER on Real Conversation is 37% better than the pre-USM model, and on the Prompted Speech test set, the adapted USM performs 53% better.

These findings suggest that the adapted USM is significantly more usable for an end user with disordered speech. We can demonstrate this improvement by looking at transcripts of Real Conversation test set recordings from a trusted tester of Euphonia and Project Relate (see below).

A comparison of the Pre-USM and adapted USM transcripts revealed some key advantages:

• The first example shows that Adapted USM is better at recognizing disordered speech patterns. The baseline misses key words like “XBox” and “controller” that are important for a listener to understand what they are trying to say.
• The second example is a good example of how deletions are a primary issue with ASR models that are not trained with disordered speech. Though the baseline model did transcribe a portion correctly, a large part of the utterance was not transcribed, losing the speaker’s intended message.

Conclusion

We believe that this work is an important step towards making speech recognition more accessible to people with disordered speech. We are continuing to work on improving the performance of our models. With the rapid advancements in ASR, we aim to ensure people with disordered speech benefit as well.

Acknowledgements

Key contributors to this project include Fadi Biadsy, Michael Brenner, Julie Cattiau, Richard Cave, Amy Chung-Yu Chou, Dotan Emanuel, Jordan Green, Rus Heywood, Pan-Pan Jiang, Anton Kast, Marilyn Ladewig, Bob MacDonald, Philip Nelson, Katie Seaver, Joel Shor, Jimmy Tobin, Katrin Tomanek, and Subhashini Venugopalan. We gratefully acknowledge the support Project Euphonia received from members of the USM research team including Yu Zhang, Wei Han, Nanxin Chen, and many others. Most importantly, we wanted to say a huge thank you to the 2,200+ participants who recorded speech samples and the many advocacy groups who helped us connect with these participants.

¹Audio volume has been adjusted for ease of listening, but the original files would be more consistent with those used in training and would have pauses, silences, variable volume, etc. ^↩

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Posted by Trond Andersen and Yuri Lensky, Research Scientists, Google Quantum AI Team

Imagine you’re shown two identical objects and then asked to close your eyes. When you open your eyes, you see the same two objects in the same position. How can you determine if they have been swapped back and forth? Intuition and the laws of quantum mechanics agree: If the objects are truly identical, there is no way to tell.

While this sounds like common sense, it only applies to our familiar three-dimensional world. Researchers have predicted that for a special type of particle, called an anyon, that is restricted to move only in a two-dimensional (2D) plane, quantum mechanics allows for something quite different. Anyons are indistinguishable from one another and some, non-Abelian anyons, have a special property that causes observable differences in the shared quantum state under exchange, making it possible to tell when they have been exchanged, despite being fully indistinguishable from one another. While researchers have managed to detect their relatives, Abelian anyons, whose change under exchange is more subtle and impossible to directly detect, realizing “non-Abelian exchange behavior” has proven more difficult due to challenges with both control and detection.

In “Non-Abelian braiding of graph vertices in a superconducting processor”, published in Nature, we report the observation of this non-Abelian exchange behavior for the first time. Non-Abelian anyons could open a new avenue for quantum computation, in which quantum operations are achieved by swapping particles around one another like strings are swapped around one another to create braids. Realizing this new exchange behavior on our superconducting quantum processor could be an alternate route to so-called topological quantum computation, which benefits from being robust against environmental noise.

Exchange statistics and non-Abelian anyons

In order to understand how this strange non-Abelian behavior can occur, it’s helpful to consider an analogy with the braiding of two strings. Take two identical strings and lay them parallel next to one another. Swap their ends to form a double-helix shape. The strings are identical, but because they wrap around one another when the ends are exchanged, it is very clear when the two ends are swapped.

The exchange of non-Abelian anyons can be visualized in a similar way, where the strings are made from extending the particles’ positions into the time dimension to form “world-lines.” Imagine plotting two particles’ locations vs. time. If the particles stay put, the plot would simply be two parallel lines, representing their constant locations. But if we exchange the locations of the particles, the world lines wrap around one another. Exchange them a second time, and you’ve made a knot.

While a bit difficult to visualize, knots in four dimensions (three spatial plus one time dimension) can always easily be undone. They are trivial — like a shoelace, simply pull one end and it unravels. But when the particles are restricted to two spatial dimensions, the knots are in three total dimensions and — as we know from our everyday 3D lives — cannot always be easily untied. The braiding of the non-Abelian anyons’ world lines can be used as quantum computing operations to transform the state of the particles.

A key aspect of non-Abelian anyons is “degeneracy”: the full state of several separated anyons is not completely specified by local information, allowing the same anyon configuration to represent superpositions of several quantum states. Winding non-Abelian anyons about each other can change the encoded state.

How to make a non-Abelian anyon

So how do we realize non-Abelian braiding with one of Google’s quantum processors? We start with the familiar surface code, which we recently used to achieve a milestone in quantum error correction, where qubits are arranged on the vertices of a checkerboard pattern. Each color square of the checkerboard represents one of two possible joint measurements that can be made of the qubits on the four corners of the square. These so-called “stabilizer measurements” can return a value of either + or – 1. The latter is referred to as a plaquette violation, and can be created and moved diagonally — just like bishops in chess — by applying single-qubit X- and Z-gates. Recently, we showed that these bishop-like plaquette violations are Abelian anyons. In contrast to non-Abelian anyons, the state of Abelian anyons changes only subtly when they are swapped — so subtly that it is impossible to directly detect. While Abelian anyons are interesting, they do not hold the same promise for topological quantum computing that non-Abelian anyons do.

To produce non-Abelian anyons, we need to control the degeneracy (i.e., the number of wavefunctions that causes all stabilizer measurements to be +1). Since a stabilizer measurement returns two possible values, each stabilizer cuts the degeneracy of the system in half, and with sufficiently many stabilizers, only one wave function satisfies the criterion. Hence, a simple way to increase the degeneracy is to merge two stabilizers together. In the process of doing so, we remove one edge in the stabilizer grid, giving rise to two points where only three edges intersect. These points, referred to as “degree-3 vertices” (D3Vs), are predicted to be non-Abelian anyons.

In order to braid the D3Vs, we have to move them, meaning that we have to stretch and squash the stabilizers into new shapes. We accomplish this by implementing two-qubit gates between the anyons and their neighbors (middle and right panels shown below).

Non-Abelian anyons in stabilizer codes. a: Example of a knot made by braiding two anyons’ world lines. b: Single-qubit gates can be used to create and move stabilizers with a value of –1 (red squares). Like bishops in chess, these can only move diagonally and are therefore constrained to one sublattice in the regular surface code. This constraint is broken when D3Vs (yellow triangles) are introduced. c: Process to form and move D3Vs (predicted to be non-Abelian anyons). We start with the surface code, where each square corresponds to a joint measurement of the four qubits on its corners (left panel). We remove an edge separating two neighboring squares, such that there is now a single joint measurement of all six qubits (middle panel). This creates two D3Vs, which are non-Abelian anyons. We move the D3Vs by applying two-qubit gates between neighboring sites (right panel).

Now that we have a way to create and move the non-Abelian anyons, we need to verify their anyonic behavior. For this we examine three characteristics that would be expected of non-Abelian anyons:

1. The “fusion rules” — What happens when non-Abelian anyons collide with each other?
2. Exchange statistics — What happens when they are braided around one another?
3. Topological quantum computing primitives — Can we encode qubits in the non-Abelian anyons and use braiding to perform two-qubit entangling operations?

The fusion rules of non-Abelian anyons

We investigate fusion rules by studying how a pair of D3Vs interact with the bishop-like plaquette violations introduced above. In particular, we create a pair of these and bring one of them around a D3V by applying single-qubit gates.

While the rules of bishops in chess dictate that the plaquette violations can never meet, the dislocation in the checkerboard lattice allows them to break this rule, meet its partner and annihilate with it. The plaquette violations have now disappeared! But bring the non-Abelian anyons back in contact with one another, and the anyons suddenly morph into the missing plaquette violations. As weird as this behavior seems, it is a manifestation of exactly the fusion rules that we expect these entities to obey. This establishes confidence that the D3Vs are, indeed, non-Abelian anyons.

Demonstration of anyonic fusion rules (starting with panel I, in the lower left). We form and separate two D3Vs (yellow triangles), then form two adjacent plaquette violations (red squares) and pass one between the D3Vs. The D3Vs deformation of the “chessboard” changes the bishop rules of the plaquette violations. While they used to lie on adjacent squares, they are now able to move along the same diagonals and collide (as shown by the red lines). When they do collide, they annihilate one another. The D3Vs are brought back together and surprisingly morph into the missing adjacent red plaquette violations.

Observation of non-Abelian exchange statistics

After establishing the fusion rules, we want to see the real smoking gun of non-Abelian anyons: non-Abelian exchange statistics. We create two pairs of non-Abelian anyons, then braid them by wrapping one from each pair around each other (shown below). When we fuse the two pairs back together, two pairs of plaquette violations appear. The simple act of braiding the anyons around one another changed the observables of our system. In other words, if you closed your eyes while the non-Abelian anyons were being exchanged, you would still be able to tell that they had been exchanged once you opened your eyes. This is the hallmark of non-Abelian statistics.

Braiding non-Abelian anyons. We make two pairs of D3Vs (panel II), then bring one from each pair around each other (III-XI). When fusing the two pairs together again in panel XII, two pairs of plaquette violations appear! Braiding the non-Abelian anyons changed the observables of the system from panel I to panel XII; a direct manifestation of non-Abelian exchange statistics.

Topological quantum computing

Finally, after establishing their fusion rules and exchange statistics, we demonstrate how we can use these particles in quantum computations. The non-Abelian anyons can be used to encode information, represented by logical qubits, which should be distinguished from the actual physical qubits used in the experiment. The number of logical qubits encoded in N D3Vs can be shown to be N/2–1, so we use N=8 D3Vs to encode three logical qubits, and perform braiding to entangle them. By studying the resulting state, we find that the braiding has indeed led to the formation of the desired, well-known quantum entangled state called the Greenberger-Horne-Zeilinger (GHZ) state.

Using non-Abelian anyons as logical qubits. a, We braid the non-Abelian anyons to entangle three qubits encoded in eight D3Vs. b, Quantum state tomography allows for reconstructing the density matrix, which can be represented in a 3D bar plot and is found to be consistent with the desired highly entangled GHZ-state.

Conclusion

Our experiments show the first observation of non-Abelian exchange statistics, and that braiding of the D3Vs can be used to perform quantum computations. With future additions, including error correction during the braiding procedure, this could be a major step towards topological quantum computation, a long-sought method to endow qubits with intrinsic resilience against fluctuations and noise that would otherwise cause errors in computations.

Acknowledgements

We would like to thank Katie McCormick, our Quantum Science Communicator, for helping to write this blog post.

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Posted by Shaina Mehta, Program Manager, Google

This week marks the beginning of the premier annual Computer Vision and Pattern Recognition conference (CVPR 2023), held in-person in Vancouver, BC (with additional virtual content). As a leader in computer vision research and a Platinum Sponsor, Google Research will have a strong presence across CVPR 2023 with 90 papers being presented at the main conference and active involvement in over 40 conference workshops and tutorials.

If you are attending CVPR this year, please stop by our booth to chat with our researchers who are actively exploring the latest techniques for application to various areas of machine perception. Our researchers will also be available to talk about and demo several recent efforts, including on-device ML applications with MediaPipe, strategies for differential privacy, neural radiance field technologies and much more.

You can also learn more about our research being presented at CVPR 2023 in the list below (Google affiliations in bold).

Board and organizing committee

Panels

Best Paper Award candidates

Highlight papers

Papers

Workshops

Tutorials

* Work done while at Google

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Posted by Juhyun Lee and Raman Sarokin, Software Engineers, Core Systems & Experiences

The proliferation of large diffusion models for image generation has led to a significant increase in model size and inference workloads. On-device ML inference in mobile environments requires meticulous performance optimization and consideration of trade-offs due to resource constraints. Running inference of large diffusion models (LDMs) on-device, driven by the need for cost efficiency and user privacy, presents even greater challenges due to the substantial memory requirements and computational demands of these models.

We address this challenge in our work titled “Speed Is All You Need: On-Device Acceleration of Large Diffusion Models via GPU-Aware Optimizations” (to be presented at the CVPR 2023 workshop for Efficient Deep Learning for Computer Vision) focusing on the optimized execution of a foundational LDM model on a mobile GPU. In this blog post, we summarize the core techniques we employed to successfully execute large diffusion models like Stable Diffusion at full resolution (512×512 pixels) and 20 iterations on modern smartphones with high-performing inference speed of the original model without distillation of under 12 seconds. As discussed in our previous blog post, GPU-accelerated ML inference is often limited by memory performance, and execution of LDMs is no exception. Therefore, the central theme of our optimization is efficient memory input/output (I/O) even if it means choosing memory-efficient algorithms over those that prioritize arithmetic logic unit efficiency. Ultimately, our primary objective is to reduce the overall latency of the ML inference.

A sample output of an LDM on Mobile GPU with the prompt text: “a photo realistic and high resolution image of a cute puppy with surrounding flowers”.

Enhanced attention module for memory efficiency

An ML inference engine typically provides a variety of optimized ML operations. Despite this, achieving optimal performance can still be challenging as there is a certain amount of overhead for executing individual neural net operators on a GPU. To mitigate this overhead, ML inference engines incorporate extensive operator fusion rules that consolidate multiple operators into a single operator, thereby reducing the number of iterations across tensor elements while maximizing compute per iteration. For instance, TensorFlow Lite utilizes operator fusion to combine computationally expensive operations, like convolutions, with subsequent activation functions, like rectified linear units, into one.

A clear opportunity for optimization is the heavily used attention block adopted in the denoiser model in the LDM. The attention blocks allow the model to focus on specific parts of the input by assigning higher weights to important regions. There are multiple ways one can optimize the attention modules, and we selectively employ one of the two optimizations explained below depending on which optimization performs better.

The first optimization, which we call partially fused softmax, removes the need for extensive memory writes and reads between the softmax and the matrix multiplication in the attention module. Let the attention block be just a simple matrix multiplication of the form Y = softmax(X) * W where X and W are 2D matrices of shape a×b and b×c, respectively (shown below in the top half).

For numerical stability, T = softmax(X) is typically calculated in three passes:

1. Determine the maximum value in the list, i.e., for each row in matrix X
2. Sum up the differences of the exponential of each list item and the maximum value (from pass 1)
3. Divide the exponential of the items minus the maximum value by the sum from pass 2

Carrying out these passes naïvely would result in a huge memory write for the temporary intermediate tensor T holding the output of the entire softmax function. We bypass this large memory write if we only store the results of passes 1 and 2, labeled m and s, respectively, which are small vectors, with a elements each, compared to T which has a·b elements. With this technique, we are able to reduce tens or even hundreds of megabytes of memory consumption by multiple orders of magnitude (shown below in the bottom half).

Attention modules. Top: A naïve attention block, composed of a SOFTMAX (with all three passes) and a MATMUL, requires a large memory write for the big intermediate tensor T. Bottom: Our memory-efficient attention block with partially fused softmax in MATMUL only needs to store two small intermediate tensors for m and s.

The other optimization involves employing FlashAttention, which is an I/O-aware, exact attention algorithm. This algorithm reduces the number of GPU high-bandwidth memory accesses, making it a good fit for our memory bandwidth–limited use case. However, we found this technique to only work for SRAM with certain sizes and to require a large number of registers. Therefore, we only leverage this technique for attention matrices with a certain size on a select set of GPUs.

Winograd fast convolution for 3×3 convolution layers

The backbone of common LDMs heavily relies on 3×3 convolution layers (convolutions with filter size 3×3), comprising over 90% of the layers in the decoder. Despite increased memory consumption and numerical errors, we found that Winograd fast convolution to be effective at speeding up the convolutions. Distinct from the filter size 3×3 used in convolutions, tile size refers to the size of a sub region of the input tensor that is processed at a time. Increasing the tile size enhances the efficiency of the convolution in terms of arithmetic logic unit (ALU) usage. However, this improvement comes at the expense of increased memory consumption. Our tests indicate that a tile size of 4×4 achieves the optimal trade-off between computational efficiency and memory utilization.

Specialized operator fusion for memory efficiency

We discovered that performantly inferring LDMs on a mobile GPU requires significantly larger fusion windows for commonly employed layers and units in LDMs than current off-the-shelf on-device GPU-accelerated ML inference engines provide. Consequently, we developed specialized implementations that could execute a larger range of neural operators than typical fusion rules would permit. Specifically, we focused on two specializations: the Gaussian Error Linear Unit (GELU) and the group normalization layer.

An approximation of GELU with the hyperbolic tangent function requires writing to and reading from seven auxiliary intermediate tensors (shown below as light orange rounded rectangles in the figure below), reading from the input tensor x three times, and writing to the output tensor y once across eight GPU programs implementing the labeled operation each (light blue rectangles). A custom GELU implementation that performs the eight operations in a single shader (shown below in the bottom) can bypass all the memory I/O for the intermediate tensors.

GELU implementations. Top: A naïve implementation with built-in operations would require 8 memory writes and 10 reads. Bottom: Our custom GELU only requires 1 memory read (for x) and 1 write (for y).

Results

After applying all of these optimizations, we conducted tests of Stable Diffusion 1.5 (image resolution 512×512, 20 iterations) on high-end mobile devices. Running Stable Diffusion with our GPU-accelerated ML inference model uses 2,093MB for the weights and 84MB for the intermediate tensors. With latest high-end smartphones, Stable Diffusion can be run in under 12 seconds.

Stable Diffusion runs on modern smartphones in under 12 seconds. Note that running the decoder after each iteration for displaying the intermediate output in this animated GIF results in a ~2× slowdown.

Conclusion

Performing on-device ML inference of large models has proven to be a substantial challenge, encompassing limitations in model file size, extensive runtime memory requirements, and protracted inference latency. By recognizing memory bandwidth usage as the primary bottleneck, we directed our efforts towards optimizing memory bandwidth utilization and striking a delicate balance between ALU efficiency and memory efficiency. As a result, we achieved state-of-the-art inference latency for large diffusion models. You can learn more about this work in the paper.

Acknowledgments

We’d like to thank Yu-Hui Chen, Jiuqiang Tang, Frank Barchard, Yang Zhao, Joe Zou, Khanh LeViet, Chuo-Ling Chang, Andrei Kulik, Lu Wang, and Matthias Grundmann.

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Lens makes it easy to search what you see and explore the world around you — including the new ability to search for skin conditions.Read More

Marcos Seefelder, Software Engineer, and Daniel Duckworth, Research Software Engineer, Google Research

When choosing a venue, we often find ourselves with questions like the following: Does this restaurant have the right vibe for a date? Is there good outdoor seating? Are there enough screens to watch the game? While photos and videos may partially answer questions like these, they are no substitute for feeling like you’re there, even when visiting in person isn’t an option.

Immersive experiences that are interactive, photorealistic, and multi-dimensional stand to bridge this gap and recreate the feel and vibe of a space, empowering users to naturally and intuitively find the information they need. To help with this, Google Maps launched Immersive View, which uses advances in machine learning (ML) and computer vision to fuse billions of Street View and aerial images to create a rich, digital model of the world. Beyond that, it layers helpful information on top, like the weather, traffic, and how busy a place is. Immersive View provides indoor views of restaurants, cafes, and other venues to give users a virtual up-close look that can help them confidently decide where to go.

Today we describe the work put into delivering these indoor views in Immersive View. We build on neural radiance fields (NeRF), a state-of-the-art approach for fusing photos to produce a realistic, multi-dimensional reconstruction within a neural network. We describe our pipeline for creation of NeRFs, which includes custom photo capture of the space using DSLR cameras, image processing and scene reproduction. We take advantage of Alphabet’s recent advances in the field to design a method matching or outperforming the prior state-of-the-art in visual fidelity. These models are then embedded as interactive 360° videos following curated flight paths, enabling them to be available on smartphones.

From photos to NeRFs

At the core of our work is NeRF, a recently-developed method for 3D reconstruction and novel view synthesis. Given a collection of photos describing a scene, NeRF distills these photos into a neural field, which can then be used to render photos from viewpoints not present in the original collection.

While NeRF largely solves the challenge of reconstruction, a user-facing product based on real-world data brings a wide variety of challenges to the table. For example, reconstruction quality and user experience should remain consistent across venues, from dimly-lit bars to sidewalk cafes to hotel restaurants. At the same time, privacy should be respected and any potentially personally identifiable information should be removed. Importantly, scenes should be captured consistently and efficiently, reliably resulting in high-quality reconstructions while minimizing the effort needed to capture the necessary photographs. Finally, the same natural experience should be available to all mobile users, regardless of the device on hand.

Capture & preprocessing

The first step to producing a high-quality NeRF is the careful capture of a scene: a dense collection of photos from which 3D geometry and color can be derived. To obtain the best possible reconstruction quality, every surface should be observed from multiple different directions. The more information a model has about an object’s surface, the better it will be in discovering the object’s shape and the way it interacts with lights.

In addition, NeRF models place further assumptions on the camera and the scene itself. For example, most of the camera’s properties, such as white balance and aperture, are assumed to be fixed throughout the capture. Likewise, the scene itself is assumed to be frozen in time: lighting changes and movement should be avoided. This must be balanced with practical concerns, including the time needed for the capture, available lighting, equipment weight, and privacy. In partnership with professional photographers, we developed a strategy for quickly and reliably capturing venue photos using DSLR cameras within only an hour timeframe. This approach has been used for all of our NeRF reconstructions to date.

Once the capture is uploaded to our system, processing begins. As photos may inadvertently contain sensitive information, we automatically scan and blur personally identifiable content. We then apply a structure-from-motion pipeline to solve for each photo’s camera parameters: its position and orientation relative to other photos, along with lens properties like focal length. These parameters associate each pixel with a point and a direction in 3D space and constitute a key signal in the NeRF reconstruction process.

NeRF reconstruction

Unlike many ML models, a new NeRF model is trained from scratch on each captured location. To obtain the best possible reconstruction quality within a target compute budget, we incorporate features from a variety of published works on NeRF developed at Alphabet. Some of these include:

• We build on mip-NeRF 360, one of the best-performing NeRF models to date. While more computationally intensive than Nvidia’s widely-used Instant NGP, we find the mip-NeRF 360 consistently produces fewer artifacts and higher reconstruction quality.
• We incorporate the low-dimensional generative latent optimization (GLO) vectors introduced in NeRF in the Wild as an auxiliary input to the model’s radiance network. These are learned real-valued latent vectors that embed appearance information for each image. By assigning each image in its own latent vector, the model can capture phenomena such as lighting changes without resorting to cloudy geometry, a common artifact in casual NeRF captures.
• We also incorporate exposure conditioning as introduced in Block-NeRF. Unlike GLO vectors, which are uninterpretable model parameters, exposure is directly derived from a photo’s metadata and fed as an additional input to the model’s radiance network. This offers two major benefits: it opens up the possibility of varying ISO and provides a method for controlling an image’s brightness at inference time. We find both properties invaluable for capturing and reconstructing dimly-lit venues.

We train each NeRF model on TPU or GPU accelerators, which provide different trade-off points. As with all Google products, we continue to search for new ways to improve, from reducing compute requirements to improving reconstruction quality.

A scalable user experience

Once a NeRF is trained, we have the ability to produce new photos of a scene from any viewpoint and camera lens we choose. Our goal is to deliver a meaningful and helpful user experience: not only the reconstructions themselves, but guided, interactive tours that give users the freedom to naturally explore spaces from the comfort of their smartphones.

To this end, we designed a controllable 360° video player that emulates flying through an indoor space along a predefined path, allowing the user to freely look around and travel forward or backwards. As the first Google product exploring this new technology, 360° videos were chosen as the format to deliver the generated content for a few reasons.

On the technical side, real-time inference and baked representations are still resource intensive on a per-client basis (either on device or cloud computed), and relying on them would limit the number of users able to access this experience. By using videos, we are able to scale the storage and delivery of videos to all users by taking advantage of the same video management and serving infrastructure used by YouTube. On the operations side, videos give us clearer editorial control over the exploration experience and are easier to inspect for quality in large volumes.

While we had considered capturing the space with a 360° camera directly, using a NeRF to reconstruct and render the space has several advantages. A virtual camera can fly anywhere in space, including over obstacles and through windows, and can use any desired camera lens. The camera path can also be edited post-hoc for smoothness and speed, unlike a live recording. A NeRF capture also does not require the use of specialized camera hardware.

Our 360° videos are rendered by ray casting through each pixel of a virtual, spherical camera and compositing the visible elements of the scene. Each video follows a smooth path defined by a sequence of keyframe photos taken by the photographer during capture. The position of the camera for each picture is computed during structure-from-motion, and the sequence of pictures is smoothly interpolated into a flight path.

To keep speed consistent across different venues, we calibrate the distances for each by capturing pairs of images, each of which is 3 meters apart. By knowing measurements in the space, we scale the generated model, and render all videos at a natural velocity.

The final experience is surfaced to the user within Immersive View: the user can seamlessly fly into restaurants and other indoor venues and discover the space by flying through the photorealistic 360° videos.

Open research questions

We believe that this feature is the first step of many in a journey towards universally accessible, AI-powered, immersive experiences. From a NeRF research perspective, more questions remain open. Some of these include:

1. Enhancing reconstructions with scene segmentation, adding semantic information to the scenes that could make scenes, for example, searchable and easier to navigate.
2. Adapting NeRF to outdoor photo collections, in addition to indoor. In doing so, we’d unlock similar experiences to every corner of the world and change how users could experience the outdoor world.
3. Enabling real-time, interactive 3D exploration through neural-rendering on-device.

As we continue to grow, we look forward to engaging with and contributing to the community to build the next generation of immersive experiences.

Acknowledgments

This work is a collaboration across multiple teams at Google. Contributors to the project include Jon Barron, Julius Beres, Daniel Duckworth, Roman Dudko, Magdalena Filak, Mike Harm, Peter Hedman, Claudio Martella, Ben Mildenhall, Cardin Moffett, Etienne Pot, Konstantinos Rematas, Yves Sallat, Marcos Seefelder, Lilyana Sirakovat, Sven Tresp and Peter Zhizhin.

Also, we’d like to extend our thanks to Luke Barrington, Daniel Filip, Tom Funkhouser, Charles Goran, Pramod Gupta, Mario Lučić, Isalo Montacute and Dan Thomasset for valuable feedback and suggestions.

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