# OpenVR Library Modifications for Raw IMU Data Access

This document outlines the necessary modifications to the OpenVR library to enable continuous access to raw IMU data, even when optical tracking is lost.

## Problem Statement

The current OpenVR API only provides IMU data through the `TrackedDevicePose_t` structure, which becomes invalid when optical tracking is lost. This means applications cannot access raw IMU data during tracking loss, which is problematic for applications that need continuous IMU data.

## Solution Overview

We need to modify the OpenVR library to provide direct access to the driver's IMU data. This requires:

1. Exposing the driver context to applications
2. Adding interfaces for accessing raw IMU data
3. Implementing these interfaces in the driver

## Detailed Modifications

### 1. Expose the Driver Context

In `src/public/openvr.h`, add a function to get the driver context:

```cpp
/** Returns the driver context interface */
VR_INTERFACE void* VR_CALLTYPE VR_GetDriverContext();
```

In `src/vrcommon/vrpathregistry_public.cpp`, implement this function:

```cpp
VR_INTERFACE void* VR_CALLTYPE VR_GetDriverContext()
{
    return VRDriverManager()->GetDriverContext();
}
```

### 2. Add IMU Data Interfaces

In `src/public/openvr_driver.h`, add the following interfaces:

```cpp
/** Interface for accessing raw IMU data */
class IVRIMUDevice
{
public:
    /** Get raw gyroscope data in radians per second */
    virtual bool GetRawGyroscopeData(float* x, float* y, float* z) = 0;
    
    /** Get raw accelerometer data in meters per second squared */
    virtual bool GetRawAccelerometerData(float* x, float* y, float* z) = 0;
    
    /** Get raw magnetometer data (if available) in arbitrary units */
    virtual bool GetRawMagnetometerData(float* x, float* y, float* z) = 0;
    
    /** Get the timestamp of the last IMU sample in microseconds */
    virtual bool GetIMUSampleTimestamp(uint64_t* timestamp) = 0;
};

/** Extension to IVRDevice to support IMU access */
class IVRDevice
{
public:
    // Existing methods...
    
    /** Get the IMU interface for this device */
    virtual IVRIMUDevice* GetIMUInterface() = 0;
};
```

### 3. Modify the Driver Host Interface

In `src/public/openvr_driver.h`, modify the `IVRServerDriverHost` interface to add a method for getting a device:

```cpp
class IVRServerDriverHost
{
public:
    // Existing methods...
    
    /** Get a device by its index */
    virtual IVRDevice* GetDevice(uint32_t unDeviceIndex) = 0;
};
```

### 4. Implement the Interfaces in the Driver

In the driver implementation (e.g., `src/vrdriver/cvrdevice.cpp`), implement the IMU interface:

```cpp
class CVRIMUDevice : public vr::IVRIMUDevice
{
public:
    CVRIMUDevice(vr::CVRDevice* pDevice)
        : m_pDevice(pDevice)
    {
    }
    
    virtual bool GetRawGyroscopeData(float* x, float* y, float* z) override
    {
        // Access the raw gyroscope data from the device
        // This will depend on the specific device implementation
        
        // For example, for the HTC Vive:
        vr::CViveDevice* pViveDevice = dynamic_cast<vr::CViveDevice*>(m_pDevice);
        if (pViveDevice)
        {
            // Access the Vive's IMU data
            // This would require knowledge of the Vive driver implementation
            return pViveDevice->GetRawGyroscopeData(x, y, z);
        }
        
        return false;
    }
    
    // Implement the other methods similarly
    
private:
    vr::CVRDevice* m_pDevice;
};

// In CVRDevice, implement the GetIMUInterface method:
IVRIMUDevice* CVRDevice::GetIMUInterface()
{
    if (!m_pIMUDevice)
    {
        m_pIMUDevice = new CVRIMUDevice(this);
    }
    
    return m_pIMUDevice;
}
```

### 5. Implement the GetDevice Method in the Driver Host

In `src/vrdriver/cvrserverdriverhost.cpp`, implement the GetDevice method:

```cpp
IVRDevice* CVRServerDriverHost::GetDevice(uint32_t unDeviceIndex)
{
    if (unDeviceIndex < m_vecDevices.size())
    {
        return m_vecDevices[unDeviceIndex];
    }
    
    return nullptr;
}
```

## Building the Modified OpenVR Library

1. Clone the OpenVR repository: `git clone https://github.com/ValveSoftware/openvr.git`
2. Make the modifications described above
3. Build the library following the standard build instructions
4. Replace the existing OpenVR library with your modified version

## Using the Modified Library

Once the modified library is in place, you can access the raw IMU data using the code in our `OpenVRNativePlugin.cpp` file, which tries to access the IMU data directly through the driver.

## Alternative Approaches

If modifying the OpenVR library is not feasible, consider these alternatives:

### 1. Create a Custom Driver

Create a custom OpenVR driver that wraps the existing driver and exposes the IMU data through a custom interface.

### 2. Use Reverse Engineering

Use reverse engineering techniques to access the driver's memory directly. This is more complex but doesn't require modifying OpenVR.

### 3. Use a Different VR SDK

Consider using a different VR SDK that provides direct access to IMU data, such as the Oculus SDK or the SteamVR Unity Plugin.

## Notes

- The exact implementation details may vary depending on the specific driver you're using (HTC Vive, Oculus, etc.)
- You may need to study the driver code to understand how to access the raw IMU data
- Some drivers may already have internal access to raw IMU data, but don't expose it through the public API