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Description

This PR adds the MrIMU plugin, a Meshroom plugin that integrates IMU (Inertial Measurement Unit) data from accelerometers and gyroscopes into photogrammetry workflows. The plugin enables users to improve camera pose estimation by applying IMU-derived orientation constraints to StructureFromMotion results, particularly useful for maintaining vertical axis stability and correcting drift in camera tracking.

The plugin supports loading IMU data from:

• OpenCamera-Sensors CSV files: Standard format exported by OpenCamera Android app
• CAMM metadata from MP4 files: GoPro format (detection implemented, full extraction placeholder for future enhancement)

Key capabilities:

• Gravity vector extraction using low-pass Butterworth filtering
• IMU orientation estimation and Z-axis locking to gravity direction
• Configurable IMU influence weighting (0.0 to 1.0) for blending IMU and optical data
• Coordinate transformation from Android sensor frame to Meshroom's world frame

Features list

• LoadIMUData node: Load and preprocess IMU data from CSV files or MP4 metadata
• ApplyIMUConstraints node: Apply IMU-derived orientation constraints to SfM camera poses
• Gravity vector extraction using Butterworth filter
• Coordinate frame transformation (Android sensor to world frame)
• Configurable IMU influence weighting
• Z-axis locking to gravity direction option
• Complete plugin structure following Meshroom plugin architecture
• Documentation (README.md) with installation and usage instructions
• Code quality compliance (black, flake8, pylint, mypy, isort)
• Plugin integration in Meshroom README.md

Implementation remarks

Plugin Structure:

• Plugin located in plugins/MrIMU/ following Meshroom's plugin directory structure
• Nodes are in plugins/MrIMU/meshroom/nodes/ as required by Meshroom's plugin discovery mechanism
• Utility functions (imu_utils.py) placed in meshroom/ directory to avoid being treated as node classes
• Lazy imports used in node files to prevent blocking plugin discovery during module import

Key Design Decisions:

1. Lazy Import Pattern: The imu_utils module is imported lazily in processChunk() methods rather than at module level. This ensures plugin discovery works even if there are import issues, and errors are only raised when nodes are actually executed.

2. Import Compatibility: Multiple import fallback methods are implemented (sys.path, importlib.util, imp) to ensure compatibility across different Python versions and Meshroom environments.

3. Coordinate Transformations: Android sensor coordinate frame (X=East, Y=North, Z=Up) is transformed to Meshroom's world frame using the standard transformation: [Y, Z, -X].

4. Butterworth Filter: Gravity vector extraction uses scipy's Butterworth filter with configurable cutoff frequency (default 0.1 Hz) to separate gravity from dynamic acceleration.

5. IMU Weight Blending: Camera poses are corrected by blending optical SfM results with IMU orientation using a configurable weight parameter, allowing users to balance between pure optical tracking and IMU constraints.

Testing:

• Plugin structure verified against Meshroom plugin documentation
• Code quality tests passed (black, flake8, pylint, mypy, isort)
• Manual testing in development Meshroom environment confirmed plugin loads and nodes appear in palette
• Import tests verify all dependencies resolve correctly

Dependencies:

• numpy >= 1.19.0 (for array operations)
• scipy >= 1.5.0 (for Butterworth filter)
• pandas >= 2.0.0 (for CSV parsing)

Future Enhancements:

• Full CAMM metadata extraction from MP4 files (currently placeholder)
• Support for additional IMU data formats
• Real-time IMU data streaming
• Advanced sensor fusion algorithms

Note: The plugin has been tested in the development Meshroom environment. For the prebuilt Meshroom 2023.3.0, additional configuration may be required as that version appears to have different plugin loading behavior.