> ## Documentation Index > Fetch the complete documentation index at: https://docs.altnautica.com/llms.txt > Use this file to discover all available pages before exploring further. # Visual Inertial Odometry > Full 6-degree-of-freedom pose estimation by fusing a forward or downward camera with the FC's IMU. Two engines (OpenVINS, VINS-Fusion) ship as bundled vendor binaries. # Visual Inertial Odometry Visual Inertial Odometry (VIO) is the bigger sibling of optical flow. Where optical flow gives you horizontal velocity from a downward camera plus a rangefinder, VIO gives you a full 6-degree-of-freedom pose (position and orientation in three dimensions) from a forward or downward camera fused with the FC's IMU. VIO ships in the plugin. Two engines (OpenVINS and VINS-Fusion) ride inside the signed plugin archive as vendor binaries. They stay on disk and never run until you select a VIO mode. VIO needs a one-time camera-IMU calibration and a board with an NPU or a strong multicore CPU; on a weak board the estimator cannot keep up. For low-altitude GPS-denied flight on light hardware, the optical-flow modes are the simpler path. ## What it adds Optical flow tells the EKF "the drone is moving north at 2.1 m/s." VIO tells the EKF "the drone is at (12.3, 4.7, 1.8) meters from the origin, rolled 2 degrees, pitched 5 degrees, yawed 18 degrees, and moving at (1.4, 1.0, 0.0) m/s." The richer state lets the FC do things optical flow can't: * Fly at altitudes where the rangefinder can't see the ground (above 10 m on most sensors). * Follow corridors and tunnels where the downward camera has nothing to track. * Maintain heading-relative position over long flights without GPS or compass drift. * Land on uneven terrain by combining VIO altitude with the IMU's gravity vector. The trade-off is compute. A monocular VIO at 30 Hz on a 640x480 camera needs an NPU or a strong CPU. VIO modes are intended for capable boards; a weak board cannot run them at the rate the EKF expects. ## Two engines, side by side The plugin ships two VIO modes: * **`vio_openvins`** uses OpenVINS, a filter-based (MSCKF) estimator. Lower CPU cost, strong on fast motion, slightly looser bound on long-flight drift. * **`vio_vins_fusion`** uses VINS-Fusion, a sliding-window bundle-adjustment estimator. Higher CPU cost, tighter drift bound in feature-rich scenes, stronger in low-feature or texture-poor environments. OpenVINS is the default. VINS-Fusion is the advanced option for boards with the headroom. ## Hardware requirements The plugin's VIO modes target a monocular forward-facing or downward-facing camera plus the FC IMU. Stereo and full-SLAM (loop-closure) engines are not in this release; the architecture leaves the seam open. | Board | OpenVINS | VINS-Fusion | | --------------------------- | --------------------- | ---------------- | | Radxa ROCK 5C Lite (RK3582) | Yes, primary target | Yes | | Radxa CM4 (RK3588S2) | Yes | Yes | | Rockchip RK3576 | Yes | Yes | | Raspberry Pi 5 (BCM2712) | Likely, NEON fallback | Marginal | | Raspberry Pi CM5 | Likely, NEON fallback | Marginal | | Raspberry Pi 4B, CM4 | Marginal | No (over budget) | | Raspberry Pi Zero 2 W | No | No | Camera: a forward-facing or downward-facing camera at 30 Hz, 640x480 or higher. A rolling-shutter camera works but is more sensitive to fast motion. Global shutter is strongly preferred. IMU: the FC's IMU, sampled through MAVLink `RAW_IMU` (the universal path every FC publishes). A higher-rate direct IMU path (a direct-I2C BMI088 at around 400 Hz) is a planned addition; until it lands the MAVLink path is the only source. ## How it plugs in The VIO engine runs as a vendor binary inside the plugin's process sandbox, started by the plugin and watched by a heartbeat watchdog. The Rust agent reads frames from the shared vision frame bus and bridges them into the engine's shared-memory ring (zero-copy); IMU samples and control messages flow through a Unix-domain socket encoded as length-prefixed msgpack. Pose estimates flow back over the same socket. The plugin then emits `VISION_POSITION_ESTIMATE` on MAVLink component 197 (`MAV_COMP_ID_VISUAL_INERTIAL_ODOMETRY`) at the camera frame rate (around 30 Hz). ArduPilot's EKF3 fuses these messages when `EK3_SRC1_POSXY`, `EK3_SRC1_VELXY`, and `EK3_SRC1_YAW` are set to `ExternalNav` (6). PX4's EKF2 fuses them when `EKF2_EV_CTRL` enables external vision. The IPC protocol is documented in the [Architecture page](/drone-agent/vision-nav-architecture). Adding a third engine (a future home-grown estimator) is a new VIO engine variant plus a vendor binary that speaks the same protocol. ## Calibration VIO needs calibrated intrinsics and known camera-IMU extrinsics including the static time offset between the two clocks. The plugin accepts Kalibr-compatible `camchain.yaml` files directly, or you can run the guided in-app wizard. See [Calibration](/drone-agent/vision-nav-calibration) for the full workflow. The pre-arm gate refuses to arm a VIO mode without loaded calibration files. The GCS sensors card surfaces a Calibrate prompt when intrinsics are not yet loaded. ## License compliance Both OpenVINS and VINS-Fusion are GPL-3.0-licensed. Shipping a pre-compiled binary inside the plugin archive carries the GPL source-offer obligation. The plugin's manifest declares a `vendor_attribution` block for each engine with the upstream repository, the license, the upstream version, and a notice. The install dialog surfaces these blocks during the summary stage so operators see what they are installing. ## When to use VIO vs optical flow If you fly close to the ground (under 8 m), have a downward camera, and need stable hover and waypoint flight, the optical-flow modes are the simpler path. See [Configure optical flow](/drone-agent/vision-nav-optical-flow) for the setup. If you fly at altitude, in long corridors, or in environments where the downward camera has nothing to track, VIO is the mode you want, on a board with the compute to run it. ## Stay in the loop Plugin updates are announced through the standard channels: * The ADOSExtensions GitHub repository's release feed. * The Mission Control update notifier (in-app banner when a new plugin version is published). * Community channels listed in [Open source](/getting-started/open-source).