I’ve had a minor obsession with Waymo’s autonomous vehicle depots recently. Over the past few months, I’ve flown a drone as part of a stakeout to understand how they work. And I’ve taken a deep dive into an apparent Waymo outage to find the company charging its electric vehicles from temporary diesel generators. The reason for my obsession? I believe depot buildouts will be one of the last hard problems in scaled autonomous driving. Long after the hardware, software, and AI have been perfected, real estate acquisition will remain a limiting factor in large-scale AV deployment. Waymo’s main depot at 201 Toland Street, San Francisco. Will self driving follow software scaling laws? In 2021, Elon Musk claimed that Tesla FSD’s release will be “one of the biggest asset value increases in history.” The day FSD goes to wide release will be one of the biggest asset value increases in history — Elon Musk (@elonmusk) October 20, 2021 Musk is arguing that, once autonomous driving has been solved, it can be instantly rolled out at the push of a button. Nearly all of Tesla’s fleet could be put to productive use without humans behind the wheel. While Musk’s viewpoint is on the extreme end, it’s a sentiment shared by many who have worked on or invested in autonomous driving over the years. Once you have hardware capable of supporting safe driverless operation, it’s just a matter of developing the right software. Software can be replicated infinitely at zero marginal cost. Could autonomous driving therefore scale as quickly as software platforms like Uber or DoorDash? The answer is not so simple. Self-driving cars are still cars — cars that exist in the physical world and need to be parked, fueled, cleaned, and repaired. Uber and other multi-sided marketplace platforms have been able to grow exponentially because they distribute these responsibilities to the individual drivers — many Uber drivers park at their own homes — allowing the platform provider to focus on developing the software pieces. So far, AV companies like Waymo and Cruise have taken a different approach. They’ve preferred to centralize these operational tasks in large depots staffed with their own personnel. This is because AV technology is still maturing and cannot be easily productized in the short term. Additionally, Timothy B. Lee notes in Understanding AI that “having hardware, software, and support services all under one roof makes it easier for Waymo to experiment with different technologies and business models.” When the kinks are still being worked out, it is more straightforward to vertically integrate everything in a single organization. The many jobs to be done of a robotaxi depot Depots for human-driven fleets, such as rental cars or delivery vans, only require a parking area with minimal additional infrastructure. This enables a fairly straightforward trade-off between location and cost: the fleet operator seeks a location close to customer demand while minimizing rent. For example, a logistics company participating in Amazon’s Delivery Service Partner program can run its depot from any sufficiently cheap parking lot near the local Amazon warehouse. The same constraints affect depot selection for autonomous vehicles. However, the depot also needs to be more than just a convenient parking lot to store off-duty cars. Because AVs are often also EVs, the ideal site also has electric vehicle charging. Because AVs need to upload driving logs to the cloud, it should have a high-speed Internet connection too. Let’s explore these constraints in detail. Location Depots should be placed close to customer demand to minimize deadheading (non-revenue driving), which would raise costs while degrading the customer experience with longer pickup times. Ideal depots are therefore located in desirable residential or commercial areas, where there is more competition among potential tenants. Placing depots in high-demand neighborhoods instead of industrial areas can also increase the probability of local opposition. Waymo has already encountered opposition during a proposed expansion of their main depot, even though it is located in an industrial neighborhood with many similar facilities. Again from Timothy B. Lee: Waymo sought a permit to convert the warehouse next door into some office space and a parking lot for Waymo employees. San Francisco’s Board of Supervisors unanimously rejected Waymo’s application. The rejection was partly based on fears that Waymo would eventually use the space to launch a delivery service in the city (Waymo hasn’t announced any plans to do this so far). But it also reflected city leaders’ frustration with their general lack of power over Waymo. Now consider the recent incident in which driverless Waymo vehicles honked at each other while entering a depot near residential buildings in San Francisco, often well into the early hours of the morning. While residents and the company resolved the situation amicably, it will surely be raised in future discussions of new Waymo depots in residential areas should they come before the Board of Supervisors or Planning Commission. Electric vehicle charging AV developers have preferred to run their services with electric vehicles. Although AV and EV technologies are not inherently coupled, running a fully electric fleet adds an environmental angle to the AV sales pitch, allowing the companies to claim that AV rides reduce emissions by displacing gas-powered driving. An EV fleet also lowers vehicle maintenance costs. Waymo and Cruise each have locations with DC fast charging capability. This approach avoids relying on public chargers. Taking Waymo’s primary San Francisco depot as an example, the company installed 38 chargers of approximately 60 kW each, implying a total site power of around 2.4 MW. Waymo vehicles charging in San Francisco. Approximately one-third of parking spots in the main depot have charging. Bringing in so many high-power chargers likely added significant complexity to Waymo’s depot construction. While we don’t Waymo’s process, we have a fairly good benchmark from the Tesla community, which tracks Tesla Supercharger installations closely. From Bruce Mah, a seasoned EV charging observer, the construction process is: As with any construction project, things usually start with selecting a site and permitting. There will often be some demolition / excavation of part of a parking lot (Superchargers are often built in existing parking lots). Tesla equipment such as charging cabinets, posts, etc. will usually be installed next (see T1 below). Eventually there will be some inspections from the local Authority Having Jurisdiction (AHJ). A utility transformer (from PG&E, SCE, etc.) is usually the last piece of equipment to be installed. Repaving, painting, and installation of parking stops will also usually happen late in the process, as well as landscaping and lighting enhancements. Of these steps, permitting and utility work are not within the charging operator’s control. California municipalities, especially San Francisco, have a notoriously slow and political permitting process. With PG&E, the utility serving much of the state, electrical service upgrades involving a new distribution transformer can take months. Timelines aside, building out a charging site is also expensive. For example, an agreement between Tesla and the City of West Hollywood values an eight-plug location at $482,942 for both equipment and construction. Data offload The final piece of the puzzle is data offload. Autonomous vehicles log vast amounts of data as they drive, measured in hundreds of GBs to TBs per hour. Some of the data is subject to mandatory retention and must be uploaded for later review. At a minimum, all AV collisions in California must be reported to the DMV. Regulators at all levels of government expect the AV developer to present analyses of serious incidents, including recordings from the vehicle and explanations of the AV’s decisions. In addition to regulatory requirements, the AV developer often wants to return much more data for engineering purposes: near misses, stuck events, novel or interesting scenarios, and more. The upshot is that the AV operator needs to upload a substantial portion of the hundreds of GBs to TBs logged per hour of driving. Uploading over cellular networks would not be cost effective. These transfers must occur at a depot. Today, it’s likely that Waymo and Cruise use disk swapping for data offload. When a car fills up its internal logging disk, it notifies an operator to plug in a fresh one. The full disks may be uploaded directly from the depot or shipped to a datacenter. This whole process is labor intensive and, over time, may pose a reliability concern due to dust or water ingress. A Waymo operator performs a possible disk swap. Many AV developers are moving toward direct data transfer from the vehicle using Ethernet, Wi-Fi, or a private 5G network, which reduces the number of manual touch points and moving parts. Charging is a great time to perform these transfers. However, this imposes an additional requirement on the depot: a fast upload speed, probably a fiber connection of at least 10 Gbps. Where do we go from here? When we put all three requirements together (great location, high-power EV charging, and high-speed Internet), there may be few to none sites that fit the bill. This would require the AV operator to take on site-specific construction projects to add amenities like charging and Internet — a strategy that sits in direct opposition to rapid and cost-effective scaling. Another possibility is to engineer ways to relax the constraints. Decoupling the requirements Waymo and Cruise do not require all of their locations to have charging and data offload. For example, Waymo operates satellite lots in downtown San Francisco only for storing their off-hail vehicles. Every night, fleet management software instructs the cars to travel back to the main depot for charging and data offload. A Waymo satellite location in San Francisco with minimal staffing, no charging, and apparently no data offloading. This solution works as long as the total charging and data transfer capacity across all locations exceeds the average throughput required to keep the fleet in working order. However, the lack of redundancy can lead to cascading failures, such as the apparent power outage at Waymo’s main depot that led the company to shut down many vehicles during a Friday evening rush hour. Reducing charging power Waymo and Cruise currently use DC fast charging (DCFC) for their fleets. Level 2 (L2) or AC charging could reduce the cost of buildouts because the equipment is cheaper and can often be added without bringing a new utility transformer. This could enable overnight charging in satellite locations that do not currently have any charging capacity. Imagine an operator showing up to plug in all the cars at night when there is little demand, then returning in the morning to unplug them. There is an order of magnitude speed difference between L2 and DCFC. This is important for consumer charging, where the consumer cares about the time to get a single car back on the road. However, charging power for any individual car becomes less important when charging a large fleet. Fleet operators care about the total throughput of turning around cars, which is proportional to total power delivered across all chargers. In other words, assuming an autonomous ride hailing service will always have overnight lulls in demand and enough parking spots during those times, the most scalable strategy is to procure your desired total charging power at the lowest price. DCFC equipment costs disproportionately more per kW due to the additional complexity of the charging equipment — and that doesn’t include the additional maintenance complexity. The table below compares ChargePoint’s cheapest L2 and DCFC units: Charger Power (kW) Price ($) Unit Price ($/kW) ChargePoint CPF50 9.6 kW $1,299 $135/kW ChargePoint CPE250 62.5 kW $52,000 $832/kW In addition to more scalable depot buildouts, reduced charging power can also increase the longevity of the vehicle’s traction battery, which is an important factor in managing vehicle depreciation. Reducing data logging rate Most AV developers start out by logging and uploading all data generated on their vehicles. This makes development easy because the data is always there when you need it. These assumptions need to be broken when a growing fleet generates proportionally more logs, most of which contain routine driving and are not very interesting. We can split the data logged by AVs into two categories: Raw sensor data, such as lidar point clouds, camera images, radar returns, and audio. Derived data, such as detections from the perception system or motion plans from the behavior system. One approach is to keep only one category of data. Retaining only the derived data can still enable debugging of serious incidents, as long as the perception system can be trusted to provide a faithful representation of the raw sensor data. On the other hand, retaining only the raw sensor data makes the logs more useful for developing the mapping and perception system. Similar-looking derived data can be generated by running a replay simulator as needed, but it is challenging to reproduce the exact same outputs as those on the vehicle unless the AV software is fully deterministic. Data retention decisions can also be made temporally. The key challenge here is high-recall classification of which time ranges in the log must be retained. For example, if a DMV-reportable collision occurs, the associated log data must never be discarded. These decisions can happen either on-device or in the cloud, but they must be made without uploading the full log to the cloud, since our bottleneck is the connection from the vehicle to the Internet. Conclusion The current trajectory for scaled autonomous driving would require desirable depot locations to include charging and Internet, making real estate acquisition challenging. There exist opportunities to reduce the additional requirements over time with the goal of making the problem closer to “rent a bunch of conveniently located parking lots.” While these are not traditionally considered autonomous driving problems, solving them will be key to unlocking the next phase of scaling.

Recently, The Verge asked, “where are all the robot trucks?” It’s a good question. Trucking was supposed to be the ideal first application of autonomous driving. Freeways contain predictable, highly structured driving scenarios. An autonomous truck would not have to deal with the complexities of intersections and two-way traffic. It could easily drive hundreds of miles without encountering a single pedestrian. DALL-E 3 prompt: “Generate an artistic, landscape aspect ratio watercolor painting of a truck with a bright red cab, pulling a white trailer. The truck drives uphill on an empty, rural highway during wintertime, lined with evergreen trees and a snow bank on a foggy, cloudy day.” The trucks could also be commercially viable with only freeway driving capability, or freeways plus a short segment of surface streets needed to reach a transfer hub. The AV company would only need to deal with a limited set of businesses as customers, bypassing the messiness of supporting a large pool of consumers inherent to the B2C model. Autonomous trucks would not be subject to rest requirements. As The Verge notes, “truck operators are allowed to drive a maximum of 11 hours a day and have to take a 30-minute rest after eight consecutive hours behind the wheel. Autonomous trucks would face no such restrictions,” enabling them to provide a service that would be literally unbeatable by a human driver. If you had asked me in 2018, when I first started working in the AV industry, I would’ve bet that driverless trucks would be the first vehicle type to achieve a million-mile driverless deployment. Aurora even pivoted their entire company to trucking in 2020, believing it to be easier than city driving. Yet sitting here in 2024, we know that both Waymo and Cruise have driven millions of miles on city streets — a large portion in the dense urban environment of San Francisco — and there are no driverless truck deployments. What happened? I think the problem is that driverless autonomous trucking is simply harder than driverless rideshare. The trucking problem appears easier at the outset, and indeed many AV developers quickly reach their initial milestones, giving them false confidence. But the difficulty ramps up sharply when the developer starts working on the last bit of polish. They encounter thorny problems related to the high speeds on freeways and trucks’ size, which must be solved before taking the human out of the driver’s seat. What is the driverless bar? Here’s a simplistic framework: No driver in the vehicle. No guarantee of a timely response from remote operators or backend services. Therefore, all safety-critical decisions must be made by the onboard computer alone. Under these constraints, the system still meets or exceeds human safety level. This is a really, really high bar. For example, on surface streets, this means the system on its own is capable of driving at least 100k miles without property damage and 40M miles without fatality.1 The system can still have flaws, but virtually all of those problems must result in a lack of progress, rather than collision or injury. In short, while the system may not know the right thing to do in every scenario, it should never do the wrong thing. (There are several high quality safety frameworks for those interested in a rigorous definition.23 It’s beyond the scope of this post.) Now, let’s look at each aspect of trucking to see how it exacerbates these challenges. Truck-specific challenges Stopping distance vs. sensing range The required sensor capability for an autonomous vehicle is determined by the most challenging scenario that the vehicle needs to handle. A major challenge in trucking is stopping behind a stalled vehicle or large debris in a travel lane. To avoid collision, the autonomous vehicle would need a sensing range greater than or equal to its stopping distance. We’ll make a simplifying assumption that stopping distance defines the minimum detection range requirements. A driverless-quality perception system needs perfect recall on other vehicles within the vehicle’s worst-case stopping distance. Passenger vehicles can decelerate up to –8 m/s². Trucks can only achieve around –4 m/s², which increases the stopping distance and puts the sensing range requirement right at the edge of what today’s sensors can deliver. Here are the sight stopping distances for an empty truck in dry conditions on roads of varying grade:4 Speed (mph) 0% Grade (m) –3% Grade (m) –6% Grade (m) 50 115–141 124–150 136–162 70 122–178 136–162 236–305 Sight stopping distances defined as the distance needed to stop assuming a 2.5-second reaction time with no braking, followed by maximum braking. The distance is computed for an empty truck in dry conditions on roads of varying grade. Stopping distance increases in wet weather or when driving downhill with a load (not shown). Now let’s compare these distances with the capabilities of various sensors: Lidar sensors provide trustworthy 3D data because they take direct measurements based on physical principles. They have a usable range of around 200–250 meters, plenty for city driving but not enough for every truck use case. Lidar detection models may also need to accumulate multiple scans/frames over time to detect faraway objects reliably, especially for smaller items like debris, further decreasing the usable detection range. Note that some solid-state lidars claim significantly more range than 250 meters. These numbers are collected under ideal conditions; for computing minimum sensing capability, we are interested in the range that can provide perfect recall and really great precision. For example, the lidar may be unable to reach its maximum range over the entire field of view, or may require undesirable trade-offs like a scan pattern that reduces point density and field of view to achieve more range. Radar can see farther than lidar. For example, this high-end ZF radar claims vehicle detections up to 350 meters away. Radar is great for tracking moving vehicles, but has trouble distinguishing between stationary vehicles and other background objects. Tesla Autopilot has infamously shown this problem by braking for overpasses and running into stalled vehicles. “Imaging” radars like the ZF device will do better than the radars on production vehicles. They still do not have the azimuth resolution to separate objects beyond 200 meters, where radar input is most needed. Cameras can detect faraway objects as long as there are enough pixels on the object, which leads to the selection of cameras with high resolution and a narrow field of view (telephoto lens). A vehicle will carry multiple narrow cameras for full coverage during turns. However, cameras cannot measure distance or speed directly. A combined camera + radar system using machine learning probably has the best chance here, especially with recent advances in ML-based early fusion, but would need to perform well enough to serve as the primary detection source beyond 200 meters. Training such a model is closer to an open problem than simply receiving that data from a lidar. In summary, we don’t appear to have any sensing solutions with the performance needed for trucks to meet the driverless bar. Controls Controlling a passenger vehicle — determining the amount of steering and throttle input to make the vehicle follow a trajectory — is a simpler problem than controlling a truck. For example, passenger vehicles are generally modeled as a single rigid body, while a truck and its trailer can move separately. The planner and controller need to account for this when making sharp turns and, in extreme low-friction conditions, to avoid jackknifing. These features come in addition to all the usual controls challenges that also apply to passenger vehicles. They can be built but require additional development and validation time. Freeway-specific challenges OK, so trucks are hard, but what about the freeway part? It may now sound appealing to build L4 freeway autonomy for passenger vehicles. However, driving on freeways also brings additional challenges on top of what is needed for city streets. Achieving the minimal risk condition on freeways Autonomous vehicles are supposed to stop when they detect an internal fault or driving situation that they can’t handle. This is called the minimal risk condition (MRC). For example, an autonomous passenger vehicle that detects an error in the HD map or a sensor failure might be programmed to execute a pullover or stop in lane depending on the problem severity. While MRC behaviors are annoying for other road users and embarrassing for the AV developer, they do not add undue risk on surface streets given the low speeds and already chaotic nature of city driving. This gives the AV developer more breathing room (within reason) to deploy a system that does not know how to handle every driving scenario perfectly, but knows enough to stay out of trouble. It’s a different story on the freeway. Stopping in lane becomes much more dangerous with the possibility of a rear-end collision at high speed. All stopping should be planned well in advance, ideally exiting at the next ramp, or at least driving to the closest shoulder with enough room to park. This greatly increases the scope of edge cases that need to be handled autonomously and at freeway speeds. For example: Scene understanding: If the vehicle encounters an unexpected construction zone, crash site, or other non-nominal driving scenario, it’s not enough to detect and stop. Rerouting, while a viable option on surface streets, usually isn’t an option on freeways because it may be difficult or illegal to make a u-turn by the time the vehicle can see the construction. A freeway under construction is also more likely to be the only path to the destination, especially if the autonomous vehicle in question is not designed to drive on city streets. Operational solutions are also not enough for a scaled deployment. AV developers often disallow their vehicles from routing through known problem areas gathered from manually driven scouting vehicles or announcements made by authorities. For a scaled deployment, however, it’s not reasonable to know the status of every mile of road at all times. Therefore, the system needs to find the right path through unstructured scenarios, possibly following instructions from police directing traffic, even if it involves traffic violations such as driving on the wrong side of the road. We know that current state-of-the-art autonomous vehicles still occasionally drive into wet concrete and trenches, which shows it is nontrivial to make a correct decision. Mapping: If the lane lines have been repainted, and the system normally uses an HD map, it needs to ignore the map and build a new one on-the-fly from the perception system’s output. It needs to distinguish between mapping and perception errors. Uptime: Sensor, computer, and software failures need to be virtually eliminated through redundancy and/or engineering elbow grease. The system needs almost perfect uptime. For example, it’s fine to enter a max-braking MRC when losing a sensor or restarting a software module on surface streets, provided those failures are rare. The same maneuver would be dangerous on the freeway, so the failure must be eliminated, or a fallback/redundancy developed. These problems are not impossible to overcome. Every autonomous passenger vehicle has solved them to some extent, with the remaining edge cases punted to some combination of MRC and remote operators. The difference is that, on freeways, they need to be solved with a very high level of reliability to meet the driverless bar. Freeways are boring The features that make freeways simpler — controlled access, no intersections, one-way traffic — also make “interesting” events more rare. This is a double-edged sword. While the simpler environment reduces the number of software features to be developed, it also increases the iteration time and cost. During development, “interesting” events are needed to train data-hungry ML models. For validation, each new software version to be qualified for driverless operation needs to encounter a minimum number of “interesting” events before comparisons to a human safety level can have statistical significance. Overall, iteration becomes more expensive when it takes more vehicle-hours to collect each event. AV developers can only respond by increasing the size of their operations teams or accepting more time between software releases. (Note that simulation is not a perfect solution either. The rarity of events increases vehicle-hours run in simulation, and so far, nobody has shown a substitute for real-world miles in the context of driverless software validation.) Is it ever going to happen? Trucking requires longer range sensing and more complex controls, increasing system complexity and pushing the problem to the bleeding edge of current sensing capabilities. At the same time, driving on freeways brings additional reliability requirements, raising the quality bar on every software component from mapping to scene understanding. If both the truck form factor and the freeway domain increase the level of difficulty, then driverless trucking might be the hardest application of autonomous driving: City Freeway Cars Baseline Harder Trucks Harder Hardest Now that scaled rideshare is mostly working in cities, I expect to see scaled freeway rideshare next. Does this mean driverless trucking will never happen? No, I still believe AV developers will overcome these challenges eventually. Aurora, Kodiak, and Gatik have all promised some form of driverless deployment by the end of the year. We probably won’t see anything close to a million-mile deployment in 2024 though. Getting there will require advances in sensing, machine learning, and a lot of hard work. Thanks to Steven W. and others for the discussions and feedback. This should be considered a bare minimum because humans perform much better on freeways, raising the bar for AVs. Rough numbers taken from Table 3, passenger vehicle national average on surface streets: Scanlon, J. M., Kusano, K. D., Fraade-Blanar, L. A., McMurry, T. L., Chen, Y. H., & Victor, T. (2023). Benchmarks for Retrospective Automated Driving System Crash Rate Analysis Using Police-Reported Crash Data. arXiv preprint arXiv:2312.13228. (blog) ↩ Kalra, N., & Paddock, S. M. (2016). Driving to safety: How many miles of driving would it take to demonstrate autonomous vehicle reliability? Transportation Research Part A: Policy and Practice, 94, 182-193. ↩ Favaro, F., Fraade-Blanar, L., Schnelle, S., Victor, T., Peña, M., Engstrom, J., … & Smith, D. (2023). Building a Credible Case for Safety: Waymo’s Approach for the Determination of Absence of Unreasonable Risk. arXiv preprint arXiv:2306.01917. (blog) ↩ Computed from tables 1 and 2: Harwood, D. W., Glauz, W. D., & Mason, J. M. (1989). Stopping sight distance design for large trucks. Transportation Research Record, 1208, 36-46. ↩

Background Last month, Kyle Vogt (CEO of Cruise) tweeted a screenshot of Cruise’s fleet management dashboard: The press: AVs are over-hyped and are still 5-10 years away. Us: At this moment there are 100 @Cruise AVs in driverless mode in SF, and many are currently carrying passengers. (from two nights ago) pic.twitter.com/rV6gWRgK0q — Kyle Vogt (@kvogt) December 1, 2022 Eagle-eyed commenters noticed there were several types of vehicle icons and wondered whether they had any meaning. Full-resolution image from Kyle’s tweet. Cruise’s letter to the CPUC The answer has been hiding in plain sight. Two weeks later, on December 16, 2022, Cruise submitted an Advice Letter (PDF, 23 MB) to the California Public Utilities Commission. They were requesting an expansion of Cruise’s driverless deployment permit. Page 48 contains a screenshot of Cruise’s fleet monitoring dashboard: 6.3. Fleet Monitoring and Learning Cruise continuously monitors its driverless fleet while it is in operation. Cruise uses a suite of internal tools to oversee its fleet of Cruise AVs, including information about each Cruise AV on the road, such as their current location, operating condition and passenger states. Figure 24: Cruise internal fleet monitoring tool (provided as example; actual may vary) The key shows: Failed: red Healthy: green Recovery: blue Occupied: filled Vacant: outline I’m assuming occupied vs. vacant indicates whether the vehicle currently carries a passenger. Note that the numbers in this image don’t add up, suggesting it might be a mockup rather than the live app. For example: 12 failed + 160 driverless + 4 recovery = 24 manual = 200 vehicles total 56 awaiting ride assignment + 13 assigned a ride + 131 assignment in progress + 20 returning to facility + 30 unavailable = 250 vehicles total Interpreting Kyle’s screenshot We now have a snapshot of Cruise’s ridership at some unknown time on the night of November 29, 2022. Healthy, occupied: 2 vehicles Healthy, vacant: 85 vehicles Total: 87 vehicles Note that the number of vehicles may be undercounted because: The viewport doesn’t show Cruise’s entire service area. Some map icons overlap. We don’t know whether the filter by AV mode has been set to show only healthy (green) vehicles.

Rewind is a Mac app that records your computer’s screen and audio, allowing the user to scroll through a timeline of past screen recordings. Rewind also recognizes text, including text in videos and Zoom calls, allowing the user to perform full-text search on anything that has been displayed. Rewind is developed by the same team as Scribe. Rewind also records Zoom meetings as a first-class feature. Whenever the user enters a meeting, Rewind asks to record and transcribe audio from all participants. A Zoom call shown in the Rewind app’s history browser. All indexing, including OCR and speech-to-text, happens locally. The developers claim that Rewind “doesn’t tax system resources, like CPU and memory, while recording” by taking advantage of accelerators built into the Apple M1 and M2. Given these claims, I was eager to sign up for the beta ($20/month, first month free) to find out how they pulled this off. Contents How it works: Overview Analyzing the Rewind app Application Bundle Frameworks Permissions Excluded Applications & Private Browsing Storage Format (1) chunks: H.264 videos (2) temp: PNG screenshots (3) db.sqlite3: Metadata Resource Usage & Battery Life Ideas for Improvement Battery Life Storage Format Security Privacy Conclusion Appendix Rewind App Database Schema How it works: Overview Use accessibility APIs to identify the frontmost window. Store the timestamps to a SQLite database in the user’s Library folder. Take a screenshot of the screen that contains the frontmost window. If there are multiple screens, only the currently focused screen will be captured. Use ScreenCaptureKit to hide disallowed windows, including private browser windows and a user-defined exclusion list. OCR the screenshot on-device using Apple’s Vision framework, the same pipeline that powers Live Text. Store the inference results to a SQLite database. Compress the screenshot sequence to an H.264 video with FFmpeg. Store videos in the user’s Library folder. Additionally, if the user joins a Zoom call and enables transcription through Rewind: Transcribe the audio on-device using the OpenAI Whisper model. Store the transcripts and speaker information in a SQLite database. In the following sections, I’ll provide details on how I came to these conclusions. Analyzing the Rewind app Application Bundle After installation, I poked through the Rewind.app bundle. Executables: Rewind: the Cocoa application Rewind Helper: a non-UI binary Other notable files include: Resources/ggml-base.en.bin: The model weights for the OpenAI Whisper transcription model in the whisper.cpp, likely for Zoom call transcription. The SHA-1 taken on my local machine (137c40403d78fd54d454da0f9bd998f78703390c) matches the file from the weights repo. Resources/favicons: A directory of 912 favicons for popular websites, stored as PNG images. Examples include amazon_com.png, dropbox_com.png, youtu_be.png, etc. These are used in the timeline view when the frontmost app is a web browser, instead of showing the browser’s app icon. Frameworks/Sparkle.framework: Despite installing through a .pkg, Rewind uses Sparkle for updates. Frameworks After loading into a disassembler, the Rewind binary contains references to: whisper.cpp VisionKit ImageAnalyzer — API for running Live Text inference and accessing results ImageAnalyzerOverlayView — displays Live Text results and allows the user to copy text Sentry — telemetry The Rewind Helper contains a statically linked FFmpeg. Permissions Upon launch, Rewind requests permissions to: record the screen record the microphone control other applications (accessibility). Recording the microphone is optional if the user doesn’t want to transcribe Zoom meeting audio. Excluded Applications & Private Browsing Rewind allows the user to exclude apps from its recording. By default, Rewind also excludes private windows from several popular browsers. In this example, I’ve opened three windows (listed from back to front): TextEdit Safari (regular) Safari (Private Browsing) Top: My actual desktop. Bottom: What Rewind sees. Rewind’s screenshot excludes the private browsing window and the menu bar — and reveals additional content previously occluded by the private window. This suggests that Rewind uses Apple’s ScreenCaptureKit, which allows filtering by window and can recomposite the desktop based on the input parameters. (The legacy CGDisplayCapture API can only capture the entire screen or individual windows. The app developer would have to perform their own compositing.) Storage Format Rewind stores all screen recordings and transcripts to: ~/Library/Application Support/com.memoryvault.MemoryVault There are three items in this directory: (1) chunks: H.264 videos A directory of timestamped movie files. Surprisingly, the date format contains colons. The chunks directory. Each chunk file is an MP4 container: $ cd ~/Library/Application\ Support/com.memoryvault.MemoryVault/chunks/2022-12-19T00:57:32 $ file chunk chunk: ISO Media, MP4 Base Media v1 [ISO 14496-12:2003] Inspecting the file with VLC, we find that it contains a single H.264 stream, about 5 minutes long at 0.5 fps. VLC inspector on a chunk file. (2) temp: PNG screenshots This directory contains a series of screenshots. A new screenshot is created every two seconds. With a screen resolution of 3024 × 1964 (14-inch MacBook Pro), the app writes about 1–2 MB/s of PNGs when displaying text. The exact throughput depends on the total screen size and content being shown. In the worst case, such as when playing back a full-screen movie, each screenshot might exceed 10 MB. Download A PNG file gets created every two seconds. (3) db.sqlite3: Metadata Rewind uses this SQLite database to store video file metadata, focused app metadata, OCR results, and call transcripts. Here are the key tables: frame contains a row for each video frame. SELECT * FROM frame LIMIT 10 Query Results id createdAt imageFileName segmentId videoId videoFrameIndex isStarred encodingStatus 1 2022-12-19T00:57:32.890 2022-12-19T00:57:32.788 1 1 0 0 success 2 2022-12-19T00:57:36.845 2022-12-19T00:57:36.742 1 1 1 0 success 3 2022-12-19T00:57:38.827 2022-12-19T00:57:38.738 2 1 2 0 success 4 2022-12-19T00:57:40.833 2022-12-19T00:57:40.741 2 1 3 0 success 5 2022-12-19T00:57:43.573 2022-12-19T00:57:43.494 3 1 4 0 success 6 2022-12-19T00:57:44.807 2022-12-19T00:57:44.723 4 1 5 0 success 7 2022-12-19T00:57:46.794 2022-12-19T00:57:46.722 4 1 6 0 success 8 2022-12-19T00:57:48.854 2022-12-19T00:57:48.763 4 1 7 0 success 9 2022-12-19T00:57:50.848 2022-12-19T00:57:50.759 4 1 8 0 success 10 2022-12-19T00:57:52.836 2022-12-19T00:57:52.744 4 1 9 0 success search_content contains raw OCR results, and node maps them to positions on frame. There can be multiple node per frame. SELECT * FROM search_content LIMIT 1 Query Results docid c0text c1otherText 1 ? Advanced… 000 ###: Security & Privacy Q Search Click the lock to make changes… 4:54 PM 4:54 PM y Access! Q ock Auctions 12/15/22 )23 12/15/22 Date Received out… SELECT * FROM node LIMIT 10 Query Results id frameId nodeOrder textOffset textLength leftX topY width height windowIndex 1 1 0 0 1 0.467398302591349 0.597970651366107 0.00672236248460967 0.013157853170445 0 2 1 1 2 9 0.399312973022461 0.59765262753272 0.0522676955821902 0.0116193030208916 0 3 1 2 12 3 0.0610130221344704 0.0570757653384834 0.0364272095436274 0.0161139305601729 0 4 1 3 16 23 0.163880813953488 0.0565509518477043 0.113372092912819 0.0179171332422108 0 5 1 4 40 8 0.381177325581395 0.0565509518477043 0.0414244182655039 0.0156774914800299 0 6 1 5 49 31 0.0875726744186047 0.598544232922732 0.127906976671512 0.0145576707649976 0 7 1 6 81 13 0.107422983923624 0.499536426710789 0.0514842521312625 0.014643243018617 0 8 1 7 95 18 0.10719476744186 0.458566629339306 0.0821220929505814 0.0145576707606783 0 9 1 8 114 10 0.106081352677456 0.417901666006876 0.0499914524167083 0.0125384870061416 0 10 1 9 125 7 0.287038004675577 0.376678364274216 0.0357664684916651 0.0107925261255073 0 search is a virtual table constructed from search_content using the SQLite FTS (full-text search) extension. sqlite> .schema search CREATE VIRTUAL TABLE "search" USING fts4("text", "otherText", tokenize=porter) /* search(text,otherText) */; segment contains a row for each instance of the focused application changing. This data appears to be gathered from the accessibility API, because the timestamps and window titles are exact. Additionally, there’s an optional column for the browser URL when the focused application is Safari, Chrome, or Arc. I’m not sure how this information is captured. SELECT * FROM segment WHERE id > 85 LIMIT 10 Query Results id appId startTime endTime windowName browserUrl browserProfile type 86 com.apple.Safari 2022-12-19T01:07:16.816 2022-12-19T01:07:18.815 Storage | Rewind Help Center https://help.rewind.ai/en/collections/3698681-storage   screenshot 87 com.apple.Safari 2022-12-19T01:07:18.815 2022-12-19T01:07:42.808 How does Rewind compression work? | Rewind Help Center https://help.rewind.ai/en/articles/6706118-how-does-rewind-compression-work   screenshot 88 com.apple.finder 2022-12-19T01:07:42.808 2022-12-19T01:07:44.793 Desktop — Local     screenshot 89 com.apple.ActivityMonitor 2022-12-19T01:07:44.793 2022-12-19T01:08:06.847 Activity Monitor     screenshot 90 com.facebook.archon 2022-12-19T01:08:06.847 2022-12-19T01:08:32.845 Messenger     screenshot 91 com.apple.ActivityMonitor 2022-12-19T01:08:32.845 2022-12-19T01:08:36.840 Activity Monitor     screenshot 92 com.apple.finder 2022-12-19T01:08:36.840 2022-12-19T01:08:40.844 Desktop — Local     screenshot 93 com.apple.finder 2022-12-19T01:08:40.844 2022-12-19T01:08:44.844 Library     screenshot 94 com.apple.finder 2022-12-19T01:08:44.844 2022-12-19T01:08:50.839 Application Support     screenshot 95 com.apple.finder 2022-12-19T01:08:50.839 2022-12-19T01:08:54.833       screenshot SELECT * FROM segment WHERE type != "screenshot" Query Results id appId startTime endTime windowName browserUrl browserProfile type 349 ai.rewind.audiorecorder 2022-12-19T01:47:37.511 2022-12-19T02:03:54.695       audio transcript_word contains a row for each word of Zoom call transcripts. This storage is rather inefficient given the app’s current functionality, but appears to be setting up for a future UI that can match the transcript to the call audio. SELECT * FROM transcript_word LIMIT 20 OFFSET 24 Query Results id segmentId speechSource word timeOffset fullTextOffset duration 25 349 me how’s 51000 93 1000 26 349 me it 52000 99 1000 27 349 me going? 53000 102 1000 28 349 me Yo, 54000 109 250 29 349 me so 54250 113 250 30 349 me this 54500 116 250 31 349 me is 54750 121 250 32 349 me going 55000 124 250 33 349 me to 55250 130 250 34 349 me transcribe 55500 133 250 35 349 me us? 55750 144 250 36 349 me Is 56000 148 400 37 349 me that 56400 151 400 38 349 me what 56800 156 400 39 349 me I’m 57200 161 400 40 349 me asking? 57600 165 400 41 349 me Yeah, 58000 173 249 42 349 me this 58249 179 249 43 349 me meeting 58499 184 249 44 349 me is 58749 192 249 Finally, clip deletion is temporarly enqueued in the purge table. It appears to contain a row per deleted frame or chunk. The referenced rows in frame, rows in video, and the chunk files get deleted soon after user request. But the purge table only gets cleared the next time the app starts up. If the user deletes all clips contained in a chunk file, the chunk file is also deleted. In other cases, the app implements soft-deletion by removing the SQLite metadata only. SELECT * FROM purge WHERE path >= '2022-12-25T00:19:26' ORDER BY path LIMIT 5 Query Results path fileType 2022-12-25T00:19:26.962 image 2022-12-25T00:19:26/chunk video 2022-12-25T00:19:28.943 image 2022-12-25T00:19:30.951 image 2022-12-25T00:19:32.931 image Resource Usage & Battery Life CPU usage while recording on my 14-inch MacBook Pro with M1 Pro: Rewind uses about 20% CPU continuously Rewind Helper spikes over 200% CPU every time the temporary PNG images are compressed to a H.264 video I suspect that Rewind also indirectly consumes resources through WindowServer because Rewind requests filtered windows. This may require WindowServer to perform additional compositing just for Rewind. However, I haven’t been able to measure this conclusively. Storage usage: Screen recordings (chunks): 180 MB / hour Metadata, OCR results, and call transcripts (db.sqlite3): 26 MB / hour Console logs: 4 MB / hour These numbers were calculated after the first 11 hours of using Rewind. My workload is a mix of text-heavy (reading, writing) and image-heavy (video editing) tasks. The storage used will vary depending on workload: for example, a text- and Zoom-heavy workload will generate a larger SQLite database. Overall, running Rewind reduces my battery life by about 20–40 percent. Ideas for Improvement Below are some areas the Rewind app’s architecture could be improved. It’s understandable that the developers prioritized shipping an MVP over polishing these details. However, if the pricing remains $20 per month, users will expect a lot of polish in the released app. Battery Life Reduced battery life is my primary pain point when using Rewind. PNG encoding. Rewind could encode the screenshots directly to H.264, instead of temporarily writing PNG images. This would eliminate throwaway work to encode and decode PNGs. It would also eliminate 0.6–3.0 GB / hour of writes, or 4–19 TB / year assuming continuous operation. From a NAND wear perspective, this is unlikely to be a problem: modern SSDs can handle well over 500 TB written per TB of capacity. However, generating a large amount of I/O remains a battery life issue. Video encoding. Rewind encodes with FFmpeg. I wasn’t able to determine whether FFmpeg was using the M1’s Media Engine (video encode/decode accelerator). Update (February 2, 2023): Rewind appears to encode video on the CPU (libx264 via FFmpeg). Matteo Contrini and Andy Xu pointed out that it’s possible to read encoder settings from an ffmpeg-encoded movie. Matteo writes: Regarding the ffmpeg part, if they’re not overriding metadata you can find which encoder was used with ffmpeg itself, or ffprobe. For example, if you ffprobe a video file encoded by VideoToolbox (hw encoding) you would find h264_videotoolbox in the encoder field of the metadata. If it’s x264, you’d find libx264. Rewind appears to use software encoding — see libx264 below. This appears consistently on files generated by Rewind version 0.6309 (the version originally tested) through 0.7312 (the current version as of February 2, 2023). $ ffprobe ~/Library/Application\ Support/com.memoryvault.MemoryVault/chunks/2023-01-31T22\:33\:11/chunk ffprobe version 5.1.2 Copyright (c) 2007-2022 the FFmpeg developers [...] Input #0, mov,mp4,m4a,3gp,3g2,mj2, from '/Users/Kevin/Library/Application Support/com.memoryvault.MemoryVault/chunks/2023-01-31T22:33:11/chunk': Metadata: major_brand : isom minor_version : 512 compatible_brands: isomiso2avc1mp41 encoder : Lavf59.30.100 Duration: 00:05:00.00, start: 0.000000, bitrate: 497 kb/s Stream #0:0[0x1](und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(progressive), 3024x1964, 496 kb/s, 0.50 fps, 0.50 tbr, 16384 tbn (default) Metadata: handler_name : VideoHandler vendor_id : [0][0][0][0] encoder : Lavc59.42.103 libx264 As of Rewind 0.7168 (released on January 27, 2023), the app now defers H.264 encoding when the user’s device is running on battery power. This resolves the battery impact of video encoding. However, this approach still strikes me as wasteful. Rewind might compete with the user for CPU time in some cases, and at a minimum, it heats up the user’s machine unnecessarily. Rewind already requires Apple Silicon. This allows them to assume that hardware-accelerated encoding is always available. Perhaps they have other reasons not to use it, such as needing to support a wider range of encoder settings. On-device inference frequency. OCR currently runs on every image (0.5 Hz). It’s possible that OCR can be subsampled or deferred entirely when running on battery without compromising the search experience. Any deferred images could be scanned the next time the user plugs in their computer or opens the Rewind UI. Storage Format The current directory structure and schemas are performant enough for a user base that only has a few months of recordings at most. Although Rewind can set a retention period, the default setting retains recordings forever, and it’s clear from the marketing materials that indefinite retention is the intended use case. The storage formats could be changed slightly to account for an ever-growing archive: Database. SQLite is quite performant; however, the text search table also grows quickly (26 MB / hour on my machine). Putting all text into a single table may cause problems in the future. For example, backup software may have trouble with a large and constantly changing file, especially Time Machine on HFS+ and others that don’t support block-level deduplication. For users with retention enabled, performing a SQLite VACUUM after deleting a large number of records may take a long time and require lots of scratch space. Directory structure. Currently, each video clip receives its own subdirectory in the chunks directory. Filesystems become less performant when listing and traversing directories as the number of children grows, even modern filesystems that use hash tables. A common solution is to create multiple levels of subdirectories using the prefix of the desired directory name. For Rewind’s timestamp directories (such as 2022-12-19T00:57:32), the prefixes could be some concatenation of the year, month, and day. Security Rewind currently doesn’t encrypt data at rest. Any app with full disk access and any attacker who encounters an unlocked computer has the ability to read recordings from all time, including soft-deleted clips. Rewind could encrypt files on disk using a shared secret stored in the macOS Keychain. Access to Keychain secrets can be configured on a per-app, per-secret basis. For increased security, Rewind could implement public-key cryptography, where the public key is used to append recordings and the private key is required to search them. Users would need to authenticate to unlock the search UI. Privacy Rewind currently allows users to exclude apps from recording and to delete specific past recordings. It would be great to combine these features by offering bulk deletion by app, time range, URL, etc. Ideally, deletions would always remove the underlying imagery, even if it requires an expensive re-encode of the chunk files to support partial deletion. Conclusion The Rewind app is a clever and helpful user interface built on top of components such as Apple’s VisionKit and OpenAI’s Whisper. Imagery is compressed with H.264 and text search uses SQLite FTS. As a developer, it’s cool to see that such advanced features can be built without training any ML models. The ever-growing collection of powerful, pretrained models continues to lower the barrier to entry for building ML-powered apps. On the other hand, I would also feel concerned that “fast follower” type competitors can easily clone apps like Rewind. As a user, I’m excited to be served by increasingly intelligent software that, in addition to competing on the quality of machine learning, must also compete on user experience. Appendix Rewind App Version: 0.6309 Bundle ID: com.memoryvault.MemoryVault Database Schema Table names: doc_segment frame node purge search search_content search_docsize search_segdir search_segments search_stat segment segment_video tokenizer transcript_word video Full schema: CREATE TABLE IF NOT EXISTS "frame" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "createdAt" TEXT NOT NULL, "imageFileName" TEXT NOT NULL, "segmentId" INTEGER REFERENCES "segment" ("id"), "videoId" INTEGER REFERENCES "video" ("id"), "videoFrameIndex" INTEGER, "isStarred" INTEGER NOT NULL DEFAULT (0), "encodingStatus" TEXT); CREATE TABLE sqlite_sequence(name,seq); CREATE TABLE IF NOT EXISTS "node" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "frameId" INTEGER NOT NULL REFERENCES "frame" ("id"), "nodeOrder" INTEGER NOT NULL, "textOffset" INTEGER NOT NULL, "textLength" INTEGER NOT NULL, "leftX" REAL NOT NULL, "topY" REAL NOT NULL, "width" REAL NOT NULL, "height" REAL NOT NULL, "windowIndex" INTEGER); CREATE TABLE IF NOT EXISTS "segment" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "appId" TEXT, "startTime" TEXT NOT NULL, "endTime" TEXT NOT NULL, "windowName" TEXT, "browserUrl" TEXT, "browserProfile" TEXT, "type" TEXT NOT NULL DEFAULT ('screenshot')); CREATE TABLE IF NOT EXISTS "video" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "frameDuration" REAL NOT NULL, "height" INTEGER NOT NULL, "width" INTEGER NOT NULL, "path" TEXT NOT NULL DEFAULT (''), "captureType" TEXT, "fileSize" INTEGER); CREATE INDEX "index_frame_on_segmentid_createdat" ON "frame" ("segmentId", "createdAt"); CREATE INDEX "index_node_on_frameid" ON "node" ("frameId"); CREATE INDEX "index_frame_on_createdat" ON "frame" ("createdAt"); CREATE VIRTUAL TABLE "search" USING fts4("text", "otherText", tokenize=porter) /* search(text,otherText) */; CREATE TABLE IF NOT EXISTS 'search_content'(docid INTEGER PRIMARY KEY, 'c0text', 'c1otherText'); CREATE TABLE IF NOT EXISTS 'search_segments'(blockid INTEGER PRIMARY KEY, block BLOB); CREATE TABLE IF NOT EXISTS 'search_segdir'(level INTEGER,idx INTEGER,start_block INTEGER,leaves_end_block INTEGER,end_block INTEGER,root BLOB,PRIMARY KEY(level, idx)); CREATE TABLE IF NOT EXISTS 'search_docsize'(docid INTEGER PRIMARY KEY, size BLOB); CREATE TABLE IF NOT EXISTS 'search_stat'(id INTEGER PRIMARY KEY, value BLOB); CREATE VIRTUAL TABLE tokenizer USING fts3tokenize(porter) /* tokenizer(input,token,start,"end",position) */; CREATE INDEX "index_frame_on_isstarred_createdat" ON "frame" ("isStarred", "createdAt"); CREATE INDEX "index_frame_on_videoid" ON "frame" ("videoId"); CREATE INDEX "index_segment_on_starttime" ON "segment" ("startTime"); CREATE TABLE IF NOT EXISTS "doc_segment" ("docid" INTEGER NOT NULL UNIQUE REFERENCES "search" ("docid"), "segmentId" INTEGER NOT NULL REFERENCES "segment" ("id"), "frameId" INTEGER REFERENCES "frame" ("id")); CREATE INDEX "index_doc_segment_on_segmentid_docid" ON "doc_segment" ("segmentId", "docid"); CREATE INDEX "index_doc_segment_on_frameid_docid" ON "doc_segment" ("frameId", "docid"); CREATE TABLE IF NOT EXISTS "segment_video" ("segmentId" INTEGER NOT NULL REFERENCES "segment" ("id"), "videoId" INTEGER NOT NULL REFERENCES "video" ("id"), "startTime" TEXT NOT NULL, "endTime" TEXT NOT NULL); CREATE INDEX "index_segment_video_on_segmentid_starttime_endtime" ON "segment_video" ("segmentId", "startTime", "endTime"); CREATE TABLE IF NOT EXISTS "transcript_word" ("id" INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, "segmentId" INTEGER NOT NULL REFERENCES "segment" ("id"), "speechSource" TEXT NOT NULL, "word" TEXT NOT NULL, "timeOffset" INTEGER NOT NULL, "fullTextOffset" INTEGER, "duration" INTEGER NOT NULL); CREATE INDEX "index_transcript_word_on_segmentid_fulltextoffset" ON "transcript_word" ("segmentId", "fullTextOffset"); CREATE INDEX "index_segment_on_appid" ON "segment" ("appId"); CREATE TABLE IF NOT EXISTS "purge" ("path" TEXT NOT NULL, "fileType" TEXT NOT NULL, UNIQUE ("path", "fileType"));