Breaking Down Spatial Anchors in AR/MR Augmented Reality (AR) and Mixed Reality (MR) depend on accurate understanding of the physical environment to create realistic experiences, and they hit this target with the concept of spatial anchors. These anchors act like markers, either geometric or based on features, that help virtual objects stay in the same spot in the real world — even when users move around. Sounds simple, but the way spatial anchors are implemented varies a lot depending on the platform; for example, Apple’s ARKit, Google’s ARCore, and Microsoft’s Azure Spatial Anchors (ASA) all approach them differently. If you want to know how these anchors are used in practical scenarios or what challenges developers often face when working with them, this article dives into these insights too. What Are Spatial Anchors and Why They Matter A spatial anchor is like a marker in the real world, tied to a specific point or group of features. Once you create one, it allows for some important capabilities: Persistence. Virtual objects stay exactly where you placed them in the real-world, even if you close and restart the app. Multi-user synchronization. Multiple devices can share the same anchor, so everyone sees virtual objects aligned to the same physical space. Cross-session continuity. You can leave a space and come back later, and all the virtual elements will still be in the right place. In AR/MR, your device builds a point cloud or feature map by using the camera and built-in sensors like the IMU (inertial measurement unit). Spatial anchors are then tied to those features, and without them, virtual objects can drift or float around as you move, shattering the sense of immersion. Technical Mechanics of Spatial Anchors At a high level, creating and using spatial anchors involves a series of steps: Feature Detection & Mapping To start, the device needs to understand its surroundings: it scans the environment to identify stable visual features (e.g., corners, edges). Over time, these features are triangulated, forming a sparse map or mesh of the space. This feature map is what the system relies on to anchor virtual objects. Anchor Creation Next, anchors are placed at specific 3D locations in the environment in two possible ways: Hit-testing. The system casts a virtual ray from a camera to a user-tapped point, then drops an anchor on the detected surface. Manual placement. Sometimes, developers need precise control, so they manually specify the exact location of an anchor using known coordinates, like ensuring it perfectly fits on the floor or another predefined plane. Persistence & Serialization Anchors aren’t temporary — they can persist, and here’s how systems make that possible: Locally stored anchors. Frameworks save the anchor’s data, like feature descriptors and transforms, in a package called a “world map” or “anchor payload”. Cloud-based anchors. Cloud services like Azure Spatial Anchors (ASA) upload this anchor data to a remote server to let the same anchor be accessed across multiple devices. Synchronization & Restoration When you’re reopening the app or accessing the anchor on a different device, the system uses the saved data to restore the anchor’s location. It compares stored feature descriptors to what the camera sees in real time, and if there’s a good enough match, the system confidently snaps the anchor into position, and your virtual content shows up right where it’s supposed to. However, using spatial anchors isn’t perfect, like using any other technology, and there are some tricky issues to figure out: Low latency. Matching saved data to real-time visuals has to be quick; otherwise, the user experience feels clunky. Robustness in feature-scarce environments. Blank walls or textureless areas don’t give the system much to work with and make tracking tougher. Scale drift. Little errors in the system’s tracking add up over time to big discrepancies. When everything falls into place and the challenges are handled well, spatial anchors make augmented and virtual reality experiences feel seamless and truly real. ARKit’s Spatial Anchors (Apple) Apple’s ARKit, rolled out with iOS 11, brought powerful features to developers working on AR apps, and one of them is spatial anchoring, which allows virtual objects to stay fixed in the real world as if they belong there. To do this, ARKit provides two main APIs that developers rely on to achieve anchor-based persistence. ARAnchor & ARPlaneAnchor The simplest kind of anchor in ARKit is the ARAnchor, which represents a single 3D point in the real-world environment and acts as a kind of “pin” in space that ARKit can track. Building on this, ARPlaneAnchor identifies flat surfaces like tables, floors, and walls, allowing developers to tie virtual objects to these surfaces. ARWorldMap ARWorldMap makes ARKit robust for persistence and acts as a snapshot of the environment being tracked by ARKit. It captures the current session, including all detected anchors and their surrounding feature points, into a compact file. There are a few constraints developers need to keep in mind: World maps are iOS-only, which means they cannot be shared directly with Android. There must be enough overlapping features between the saved environment and the current physical space, and textured structures are especially valuable for this, as they help ARKit identify key points for alignment. Large world maps, especially those with many anchors or detailed environments, can be slow to serialize and deserialize, causing higher application latency when loading or saving. ARKit anchors are ideal for single-user persistence, but sharing AR experiences across multiple devices poses additional issues, and developers often employ custom server logic (uploading ARWorldMap data to a backend), enabling users to download and use the same map. However, this approach comes with caveats: it requires extra development work and doesn’t offer native support for sharing across platforms like iOS and Android. ARCore’s Spatial Anchors (Google) Google’s ARCore is a solid toolkit for building AR apps, and one of its best features is how it handles spatial anchors: Anchors & Hit-Testing ARCore offers two ways to create anchors. You can use Session.createAnchor(Pose) if you already know the anchor’s position, or…