IMU Types - ORB-SLAM3

Overview

The IMU namespace provides data structures and classes for handling inertial measurements in ORB-SLAM3’s visual-inertial mode. It includes raw IMU measurements, bias estimation, calibration, and preintegration. Header: ImuTypes.h Namespace: ORB_SLAM3::IMU

Constants

const float GRAVITY_VALUE = 9.81

Standard gravity magnitude in m/s².

IMU::Point

Represents a single IMU measurement with accelerometer and gyroscope data.

Structure

class Point {
public:
    Eigen::Vector3f a;  // Accelerometer (m/s²)
    Eigen::Vector3f w;  // Gyroscope (rad/s)
    double t;           // Timestamp (seconds)
}

Constructors

Point(
    const float &acc_x, const float &acc_y, const float &acc_z,
    const float &ang_vel_x, const float &ang_vel_y, const float &ang_vel_z,
    const double &timestamp
)

Construct from individual components. Parameters:

acc_x, acc_y, acc_z - Accelerometer readings in m/s²
ang_vel_x, ang_vel_y, ang_vel_z - Gyroscope readings in rad/s
timestamp - Measurement timestamp in seconds

Point(
    const cv::Point3f Acc,
    const cv::Point3f Gyro,
    const double &timestamp
)

Construct from OpenCV 3D points. Parameters:

Acc - Accelerometer vector
Gyro - Gyroscope vector
timestamp - Measurement timestamp

Usage Example

// Create IMU measurement
IMU::Point imu(ax, ay, az, wx, wy, wz, timestamp);

// Access data
Eigen::Vector3f accel = imu.a;
Eigen::Vector3f gyro = imu.w;
double time = imu.t;

IMU::Bias

Represents IMU biases for accelerometer and gyroscope.

Structure

class Bias {
public:
    float bax, bay, baz;  // Accelerometer bias (m/s²)
    float bwx, bwy, bwz;  // Gyroscope bias (rad/s)
}

Constructors

Bias()

Default constructor (all biases = 0).

Bias(
    const float &b_acc_x, const float &b_acc_y, const float &b_acc_z,
    const float &b_ang_vel_x, const float &b_ang_vel_y, const float &b_ang_vel_z
)

Construct with specific bias values. Parameters:

b_acc_x, b_acc_y, b_acc_z - Accelerometer bias components
b_ang_vel_x, b_ang_vel_y, b_ang_vel_z - Gyroscope bias components

Methods

void CopyFrom(Bias &b)

Copy bias values from another Bias object.

Stream Operator

friend std::ostream& operator<<(std::ostream &out, const Bias &b)

Output bias values to stream.

Usage Example

// Initialize bias
IMU::Bias bias(0.01, -0.005, 0.02, 0.001, -0.001, 0.0005);

// Access components
float acc_bias_x = bias.bax;
float gyro_bias_z = bias.bwz;

// Print
std::cout << "IMU Bias: " << bias << std::endl;

IMU::Calib

IMU calibration including extrinsic transform and noise parameters.

Structure

class Calib {
public:
    Sophus::SE3<float> mTcb;  // Transform: camera to IMU body
    Sophus::SE3<float> mTbc;  // Transform: IMU body to camera
    Eigen::DiagonalMatrix<float,6> Cov;      // Measurement covariance
    Eigen::DiagonalMatrix<float,6> CovWalk;  // Random walk covariance
    bool mbIsSet;  // Calibration validity flag
}

Constructors

Calib()

Default constructor (not set).

Calib(
    const Sophus::SE3<float> &Tbc,
    const float &ng,   // Gyroscope noise density
    const float &na,   // Accelerometer noise density
    const float &ngw,  // Gyroscope random walk
    const float &naw   // Accelerometer random walk
)

Construct with calibration parameters. Parameters:

Tbc - Transform from IMU body frame to camera frame
ng - Gyroscope noise density (rad/s/√Hz)
na - Accelerometer noise density (m/s²/√Hz)
ngw - Gyroscope random walk (rad/s²/√Hz)
naw - Accelerometer random walk (m/s³/√Hz)

Calib(const Calib &calib)

Copy constructor.

Methods

void Set(
    const Sophus::SE3<float> &sophTbc,
    const float &ng, const float &na,
    const float &ngw, const float &naw
)

Set calibration parameters.

Usage Example

// Define IMU-camera transform
Sophus::SE3f Tbc = ...; // From calibration

// Set noise parameters
float ng = 0.01;   // Gyro noise
float na = 0.1;    // Accel noise  
float ngw = 0.0001; // Gyro walk
float naw = 0.001;  // Accel walk

// Create calibration
IMU::Calib calib(Tbc, ng, na, ngw, naw);

IMU::IntegratedRotation

Integration of a single gyroscope measurement.

Structure

class IntegratedRotation {
public:
    float deltaT;              // Integration time
    Eigen::Matrix3f deltaR;    // Rotation increment
    Eigen::Matrix3f rightJ;    // Right Jacobian
}

Constructor

IntegratedRotation(
    const Eigen::Vector3f &angVel,
    const Bias &imuBias,
    const float &time
)

Integrate one gyroscope measurement. Parameters:

angVel - Angular velocity measurement
imuBias - Current IMU bias estimate
time - Integration time step

IMU::Preintegrated

Preintegrated IMU measurements between keyframes.

Key Members

class Preintegrated {
public:
    float dT;  // Total integration time
    
    // Preintegrated measurements
    Eigen::Matrix3f dR;  // Rotation
    Eigen::Vector3f dV;  // Velocity  
    Eigen::Vector3f dP;  // Position
    
    // Jacobians w.r.t. bias
    Eigen::Matrix3f JRg, JVg, JVa, JPg, JPa;
    
    // Average measurements
    Eigen::Vector3f avgA;  // Average acceleration
    Eigen::Vector3f avgW;  // Average angular velocity
    
    // Covariance matrices
    Eigen::Matrix<float,15,15> C;     // Covariance
    Eigen::Matrix<float,15,15> Info;  // Information matrix
    
    // Noise parameters
    Eigen::DiagonalMatrix<float,6> Nga, NgaWalk;
    
    // Bias
    Bias b;   // Original bias
}

Constructors

Preintegrated(const Bias &b_, const Calib &calib)

Construct preintegration with initial bias and calibration.

Preintegrated(Preintegrated *pImuPre)

Copy constructor.

Core Methods

Initialize

void Initialize(const Bias &b_)

Initialize/reset preintegration with new bias.

Integrate Measurement

void IntegrateNewMeasurement(
    const Eigen::Vector3f &acceleration,
    const Eigen::Vector3f &angVel,
    const float &dt
)

Add new IMU measurement to preintegration. Parameters:

acceleration - Accelerometer reading (m/s²)
angVel - Gyroscope reading (rad/s)
dt - Time step since last measurement

Reintegrate

void Reintegrate()

Recompute preintegration with updated bias (after optimization).

Merge

void MergePrevious(Preintegrated *pPrev)

Merge with previous preintegration (when removing keyframe).

Update Bias

void SetNewBias(const Bias &bu_)

Update bias estimate and recompute deltas.

Delta Computation

Get preintegrated values with bias correction:

Eigen::Matrix3f GetDeltaRotation(const Bias &b_)

Eigen::Vector3f GetDeltaVelocity(const Bias &b_)

Eigen::Vector3f GetDeltaPosition(const Bias &b_)

Parameters:

b_ - Bias to use for correction

Returns: Corrected preintegrated rotation/velocity/position

Original Values

Get values computed with original bias:

Eigen::Matrix3f GetOriginalDeltaRotation()
Eigen::Vector3f GetOriginalDeltaVelocity()
Eigen::Vector3f GetOriginalDeltaPosition()

Updated Values

Get values with current updated bias:

Eigen::Matrix3f GetUpdatedDeltaRotation()
Eigen::Vector3f GetUpdatedDeltaVelocity()
Eigen::Vector3f GetUpdatedDeltaPosition()

Bias Access

Bias GetOriginalBias()

Get bias used for original integration.

Bias GetUpdatedBias()

Get current updated bias.

IMU::Bias GetDeltaBias(const Bias &b_)

Compute difference between given bias and original.

Eigen::Matrix<float,6,1> GetDeltaBias()

Get current bias correction as 6D vector.

Copy

void CopyFrom(Preintegrated *pImuPre)

Copy all preintegration data from another object.

Usage Example

// Create preintegration
IMU::Bias initialBias;
IMU::Calib calib(Tbc, ng, na, ngw, naw);
IMU::Preintegrated preint(initialBias, calib);

// Add measurements
for (auto& imu : vImuMeasurements) {
    preint.IntegrateNewMeasurement(imu.a, imu.w, dt);
}

// Get preintegrated values
Eigen::Matrix3f dR = preint.GetDeltaRotation(currentBias);
Eigen::Vector3f dV = preint.GetDeltaVelocity(currentBias);
Eigen::Vector3f dP = preint.GetDeltaPosition(currentBias);

// After bias optimization
preint.SetNewBias(optimizedBias);

Lie Algebra Functions

Utility functions for SO(3) operations:

Eigen::Matrix3f RightJacobianSO3(const Eigen::Vector3f &v)

Compute right Jacobian of SO(3).

Eigen::Matrix3f InverseRightJacobianSO3(const Eigen::Vector3f &v)

Compute inverse right Jacobian of SO(3).

Eigen::Matrix3f NormalizeRotation(const Eigen::Matrix3f &R)

Project matrix to SO(3) (nearest rotation matrix).

Integration in SLAM Pipeline

Initialization: Create Preintegrated with initial bias from previous keyframe
Tracking: Integrate IMU measurements between frames using IntegrateNewMeasurement
Keyframe Creation: Store preintegrated measurements in keyframe
Optimization: Update bias estimates, recompute using SetNewBias
Prediction: Use deltas to predict next frame pose

Core API

Data Structures

Camera Models

IMU & Sensors

Utilities

​Overview

​Constants

​IMU::Point

​Structure

​Constructors

​Usage Example

​IMU::Bias

​Structure

​Constructors

​Methods

​Stream Operator

​Usage Example

​IMU::Calib

​Structure

​Constructors

​Methods

​Usage Example

​IMU::IntegratedRotation

​Structure

​Constructor

​IMU::Preintegrated

​Key Members

​Constructors

​Core Methods

​Initialize

​Integrate Measurement

​Reintegrate

​Merge

​Update Bias

​Delta Computation

​Original Values

​Updated Values

​Bias Access

​Copy

​Usage Example

​Lie Algebra Functions

​Integration in SLAM Pipeline

​See Also

Build docs developers (and LLMs) love

Overview

Constants

IMU::Point

Structure

Constructors

Usage Example

IMU::Bias

Structure

Constructors

Methods

Stream Operator

Usage Example

IMU::Calib

Structure

Constructors

Methods

Usage Example

IMU::IntegratedRotation

Structure

Constructor

IMU::Preintegrated

Key Members

Constructors

Core Methods

Initialize

Integrate Measurement

Reintegrate

Merge

Update Bias

Delta Computation

Original Values

Updated Values

Bias Access

Copy

Usage Example

Lie Algebra Functions

Integration in SLAM Pipeline

See Also